[PATCH v5 for-5.0? 0/7] Tighten qemu-img rules on missing backing format

[PATCH v5 for-5.0? 0/7] Tighten qemu-img rules on missing backing format

[Eric Blake]

v4 was here:
https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg03775.html
In v5:
- fix 'qemu-img convert -B' to actually warn [Kashyap]
- squash in followups
- a couple more iotest improvements

If we decide this is not 5.0 material, then patches 4 and 7 need a
tweak to s/5.0/5.1/ as the start of the deprecation clock.

Eric Blake (7):
  sheepdog: Add trivial backing_fmt support
  vmdk: Add trivial backing_fmt support
  qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw
  qcow2: Deprecate use of qemu-img amend to change backing file
  iotests: Specify explicit backing format where sensible
  block: Add support to warn on backing file change without format
  qemu-img: Deprecate use of -b without -F

 docs/system/deprecated.rst    | 32 ++++++++++++++++
 docs/tools/qemu-img.rst       |  4 ++
 include/block/block.h         |  4 +-
 block.c                       | 34 +++++++++++++++--
 block/qcow.c                  | 16 +++++++-
 block/qcow2.c                 |  7 +++-
 block/sheepdog.c              | 18 ++++++++-
 block/stream.c                |  2 +-
 block/vmdk.c                  | 14 +++++++
 blockdev.c                    |  3 +-
 qemu-img.c                    | 11 +++++-
 tests/qemu-iotests/017        |  2 +-
 tests/qemu-iotests/017.out    |  2 +-
 tests/qemu-iotests/018        |  2 +-
 tests/qemu-iotests/018.out    |  2 +-
 tests/qemu-iotests/019        |  5 ++-
 tests/qemu-iotests/019.out    |  2 +-
 tests/qemu-iotests/020        |  4 +-
 tests/qemu-iotests/020.out    |  4 +-
 tests/qemu-iotests/024        |  8 ++--
 tests/qemu-iotests/024.out    |  5 ++-
 tests/qemu-iotests/028        |  4 +-
 tests/qemu-iotests/028.out    |  2 +-
 tests/qemu-iotests/030        | 26 +++++++++----
 tests/qemu-iotests/034        |  2 +-
 tests/qemu-iotests/034.out    |  2 +-
 tests/qemu-iotests/037        |  2 +-
 tests/qemu-iotests/037.out    |  2 +-
 tests/qemu-iotests/038        |  2 +-
 tests/qemu-iotests/038.out    |  2 +-
 tests/qemu-iotests/039        |  3 +-
 tests/qemu-iotests/039.out    |  2 +-
 tests/qemu-iotests/040        | 47 ++++++++++++++++-------
 tests/qemu-iotests/041        | 37 ++++++++++++------
 tests/qemu-iotests/042        |  4 +-
 tests/qemu-iotests/043        | 18 ++++-----
 tests/qemu-iotests/043.out    | 16 +++++---
 tests/qemu-iotests/046        |  2 +-
 tests/qemu-iotests/046.out    |  2 +-
 tests/qemu-iotests/050        |  4 +-
 tests/qemu-iotests/050.out    |  2 +-
 tests/qemu-iotests/051        |  2 +-
 tests/qemu-iotests/051.out    |  2 +-
 tests/qemu-iotests/051.pc.out |  2 +-
 tests/qemu-iotests/056        |  3 +-
 tests/qemu-iotests/060        |  2 +-
 tests/qemu-iotests/060.out    |  2 +-
 tests/qemu-iotests/061        | 10 ++---
 tests/qemu-iotests/061.out    | 11 +++---
 tests/qemu-iotests/069        |  2 +-
 tests/qemu-iotests/069.out    |  2 +-
 tests/qemu-iotests/073        |  2 +-
 tests/qemu-iotests/073.out    |  2 +-
 tests/qemu-iotests/082        | 10 +++--
 tests/qemu-iotests/082.out    | 14 ++++---
 tests/qemu-iotests/085        |  4 +-
 tests/qemu-iotests/085.out    |  6 +--
 tests/qemu-iotests/089        |  2 +-
 tests/qemu-iotests/089.out    |  2 +-
 tests/qemu-iotests/095        |  4 +-
 tests/qemu-iotests/095.out    |  4 +-
 tests/qemu-iotests/097        |  4 +-
 tests/qemu-iotests/097.out    | 16 ++++----
 tests/qemu-iotests/098        |  2 +-
 tests/qemu-iotests/098.out    |  8 ++--
 tests/qemu-iotests/110        |  4 +-
 tests/qemu-iotests/110.out    |  4 +-
 tests/qemu-iotests/114        | 12 ++++++
 tests/qemu-iotests/114.out    |  9 +++++
 tests/qemu-iotests/122        | 27 +++++++------
 tests/qemu-iotests/122.out    |  8 ++--
 tests/qemu-iotests/126        |  4 +-
 tests/qemu-iotests/126.out    |  4 +-
 tests/qemu-iotests/127        |  4 +-
 tests/qemu-iotests/127.out    |  4 +-
 tests/qemu-iotests/129        |  3 +-
 tests/qemu-iotests/133        |  2 +-
 tests/qemu-iotests/133.out    |  2 +-
 tests/qemu-iotests/139        |  2 +-
 tests/qemu-iotests/141        |  4 +-
 tests/qemu-iotests/141.out    |  4 +-
 tests/qemu-iotests/142        |  2 +-
 tests/qemu-iotests/142.out    |  2 +-
 tests/qemu-iotests/153        | 14 +++----
 tests/qemu-iotests/153.out    | 35 +++++++++--------
 tests/qemu-iotests/154        | 42 ++++++++++----------
 tests/qemu-iotests/154.out    | 42 ++++++++++----------
 tests/qemu-iotests/155        | 12 ++++--
 tests/qemu-iotests/156        |  9 +++--
 tests/qemu-iotests/156.out    |  6 +--
 tests/qemu-iotests/158        |  2 +-
 tests/qemu-iotests/158.out    |  2 +-
 tests/qemu-iotests/161        |  8 ++--
 tests/qemu-iotests/161.out    |  8 ++--
 tests/qemu-iotests/176        |  4 +-
 tests/qemu-iotests/176.out    | 32 ++++++++--------
 tests/qemu-iotests/177        |  2 +-
 tests/qemu-iotests/177.out    |  2 +-
 tests/qemu-iotests/179        |  2 +-
 tests/qemu-iotests/179.out    |  2 +-
 tests/qemu-iotests/189        |  2 +-
 tests/qemu-iotests/189.out    |  2 +-
 tests/qemu-iotests/191        | 12 +++---
 tests/qemu-iotests/191.out    | 12 +++---
 tests/qemu-iotests/195        |  6 +--
 tests/qemu-iotests/195.out    |  6 +--
 tests/qemu-iotests/198        |  2 +-
 tests/qemu-iotests/198.out    |  3 +-
 tests/qemu-iotests/204        |  2 +-
 tests/qemu-iotests/204.out    |  2 +-
 tests/qemu-iotests/216        |  2 +-
 tests/qemu-iotests/224        |  4 +-
 tests/qemu-iotests/225        |  2 +-
 tests/qemu-iotests/225.out    |  2 +-
 tests/qemu-iotests/228        |  5 ++-
 tests/qemu-iotests/245        |  3 +-
 tests/qemu-iotests/249        |  4 +-
 tests/qemu-iotests/249.out    |  4 +-
 tests/qemu-iotests/252        |  2 +-
 tests/qemu-iotests/257        |  3 +-
 tests/qemu-iotests/267        |  4 +-
 tests/qemu-iotests/267.out    |  6 +--
 tests/qemu-iotests/270        |  2 +-
 tests/qemu-iotests/270.out    |  2 +-
 tests/qemu-iotests/273        |  4 +-
 tests/qemu-iotests/273.out    |  4 +-
 tests/qemu-iotests/279        |  4 +-
 tests/qemu-iotests/279.out    |  4 +-
 tests/qemu-iotests/290        | 72 +++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/290.out    | 45 ++++++++++++++++++++++
 tests/qemu-iotests/group      |  1 +
 131 files changed, 683 insertions(+), 350 deletions(-)
 create mode 100755 tests/qemu-iotests/290
 create mode 100644 tests/qemu-iotests/290.out

-- 
2.26.0.rc2

[PATCH v5 1/7] sheepdog: Add trivial backing_fmt support

[Eric Blake]

Sheepdog already requires that if backing_file is present, that it be
another sheepdog image (see sd_co_create).  Meanwhile, we want to move
towards always being explicit about the backing format for other
drivers where it matters.  So for convenience, make qemu-img create -F
sheepdog work, while rejecting all other explicit formats (note that
this is only for QemuOpts usage; there is no change to the QAPI to
allow a format through -blockdev).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/sheepdog.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/block/sheepdog.c b/block/sheepdog.c
index 59f7ebb1710f..8f558367d2d5 100644
--- a/block/sheepdog.c
+++ b/block/sheepdog.c
@@ -2163,13 +2163,21 @@ static int coroutine_fn sd_co_create_opts(BlockDriver *drv,
                                           Error **errp)
 {
     BlockdevCreateOptions *create_options = NULL;
-    QDict *qdict, *location_qdict;
+    QDict *qdict = NULL, *location_qdict;
     Visitor *v;
-    char *redundancy;
+    char *redundancy = NULL;
     Error *local_err = NULL;
     int ret;
+    char *backing_fmt = NULL;

     redundancy = qemu_opt_get_del(opts, BLOCK_OPT_REDUNDANCY);
+    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
+
+    if (backing_fmt && strcmp(backing_fmt, "sheepdog") != 0) {
+        error_setg(errp, "backing_file must be a sheepdog image");
+        ret = -EINVAL;
+        goto fail;
+    }

     qdict = qemu_opts_to_qdict(opts, NULL);
     qdict_put_str(qdict, "driver", "sheepdog");
@@ -2234,6 +2242,7 @@ fail:
     qapi_free_BlockdevCreateOptions(create_options);
     qobject_unref(qdict);
     g_free(redundancy);
+    g_free(backing_fmt);
     return ret;
 }

@@ -3191,6 +3200,11 @@ static QemuOptsList sd_create_opts = {
             .type = QEMU_OPT_STRING,
             .help = "File name of a base image"
         },
+        {
+            .name = BLOCK_OPT_BACKING_FMT,
+            .type = QEMU_OPT_STRING,
+            .help = "Must be 'sheepdog' if present",
+        },
         {
             .name = BLOCK_OPT_PREALLOC,
             .type = QEMU_OPT_STRING,
-- 
2.26.0.rc2

[PATCH v5 2/7] vmdk: Add trivial backing_fmt support

[Eric Blake]

vmdk already requires that if backing_file is present, that it be
another vmdk image (see vmdk_co_do_create).  Meanwhile, we want to
move towards always being explicit about the backing format for other
drivers where it matters.  So for convenience, make qemu-img create -F
vmdk work, while rejecting all other explicit formats (note that this
is only for QemuOpts usage; there is no change to the QAPI to allow a
format through -blockdev).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/vmdk.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/block/vmdk.c b/block/vmdk.c
index 218d9c980059..7ecaf0ee9be3 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -2613,6 +2613,14 @@ static int coroutine_fn vmdk_co_create_opts(BlockDriver *drv,
     bool zeroed_grain;
     bool compat6;
     VMDKCreateOptsData data;
+    char *backing_fmt = NULL;
+
+    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
+    if (backing_fmt && strcmp(backing_fmt, "vmdk") != 0) {
+        error_setg(errp, "backing_file must be a vmdk image");
+        ret = -EINVAL;
+        goto exit;
+    }

     if (filename_decompose(filename, path, prefix, postfix, PATH_MAX, errp)) {
         ret = -EINVAL;
@@ -2671,6 +2679,7 @@ static int coroutine_fn vmdk_co_create_opts(BlockDriver *drv,
                             vmdk_co_create_opts_cb, &data, errp);

 exit:
+    g_free(backing_fmt);
     g_free(adapter_type);
     g_free(backing_file);
     g_free(hw_version);
@@ -3007,6 +3016,11 @@ static QemuOptsList vmdk_create_opts = {
             .type = QEMU_OPT_STRING,
             .help = "File name of a base image"
         },
+        {
+            .name = BLOCK_OPT_BACKING_FMT,
+            .type = QEMU_OPT_STRING,
+            .help = "Must be 'vmdk' if present",
+        },
         {
             .name = BLOCK_OPT_COMPAT6,
             .type = QEMU_OPT_BOOL,
-- 
2.26.0.rc2

[PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

[Eric Blake]

qcow has no space in the metadata to store a backing format, and there
are existing qcow images backed both by raw or by other formats
(usually qcow) images, reliant on probing to tell the difference.
While we don't recommend the creation of new qcow images (as qcow2 is
hands-down better), we can at least insist that if the user does
request a specific format without using -u, then it must be non-raw
(as a raw backing file that gets inadvertently edited into some other
format can form a security hole); if the user does not request a
specific format or lies when using -u, then the status quo of probing
for the backing format remains intact (although an upcoming patch will
warn when omitting a format request).  Thus, when this series is
complete, the only way to use a backing file for qcow without
triggering a warning is when using -F if the backing file is non-raw
to begin with.  Note that this is only for QemuOpts usage; there is no
change to the QAPI to allow a format through -blockdev.

Add a new iotest 290 just for qcow, to demonstrate the new warning.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/qcow.c               | 16 ++++++++-
 tests/qemu-iotests/290     | 72 ++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/290.out | 42 ++++++++++++++++++++++
 tests/qemu-iotests/group   |  1 +
 4 files changed, 130 insertions(+), 1 deletion(-)
 create mode 100755 tests/qemu-iotests/290
 create mode 100644 tests/qemu-iotests/290.out

diff --git a/block/qcow.c b/block/qcow.c
index 8973e4e565a1..bbe48b7db4d7 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -940,11 +940,12 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,
 {
     BlockdevCreateOptions *create_options = NULL;
     BlockDriverState *bs = NULL;
-    QDict *qdict;
+    QDict *qdict = NULL;
     Visitor *v;
     const char *val;
     Error *local_err = NULL;
     int ret;
+    char *backing_fmt;

     static const QDictRenames opt_renames[] = {
         { BLOCK_OPT_BACKING_FILE,       "backing-file" },
@@ -953,6 +954,13 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,
     };

     /* Parse options and convert legacy syntax */
+    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
+    if (backing_fmt && !strcmp(backing_fmt, "raw")) {
+        error_setg(errp, "qcow cannot store backing format; an explicit "
+                   "backing format of raw is unsafe");
+        ret = -EINVAL;
+        goto fail;
+    }
     qdict = qemu_opts_to_qdict_filtered(opts, NULL, &qcow_create_opts, true);

     val = qdict_get_try_str(qdict, BLOCK_OPT_ENCRYPT);
@@ -1018,6 +1026,7 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,

     ret = 0;
 fail:
+    g_free(backing_fmt);
     qobject_unref(qdict);
     bdrv_unref(bs);
     qapi_free_BlockdevCreateOptions(create_options);
@@ -1152,6 +1161,11 @@ static QemuOptsList qcow_create_opts = {
             .type = QEMU_OPT_STRING,
             .help = "File name of a base image"
         },
+        {
+            .name = BLOCK_OPT_BACKING_FMT,
+            .type = QEMU_OPT_STRING,
+            .help = "Format of the backing image (caution: raw backing is unsafe)",
+        },
         {
             .name = BLOCK_OPT_ENCRYPT,
             .type = QEMU_OPT_BOOL,
diff --git a/tests/qemu-iotests/290 b/tests/qemu-iotests/290
new file mode 100755
index 000000000000..8482de58cb4b
--- /dev/null
+++ b/tests/qemu-iotests/290
@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+#
+# Test qcow backing file warnings
+#
+# Copyright (C) 2020 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+seq=`basename $0`
+echo "QA output created by $seq"
+
+status=1	# failure is the default!
+
+_cleanup()
+{
+	_cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+_supported_fmt qcow
+_supported_proto file
+_supported_os Linux
+
+size=128M
+
+echo
+echo "== qcow backed by qcow =="
+
+TEST_IMG="$TEST_IMG.base" _make_test_img $size
+_make_test_img -b "$TEST_IMG.base"
+_img_info
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT
+_img_info
+
+echo
+echo "== mismatched command line detection =="
+
+_make_test_img -b "$TEST_IMG.base" -F vmdk
+# Use of -u bypasses the backing format sanity check
+_make_test_img -u -b "$TEST_IMG.base" -F vmdk $size
+_img_info
+
+echo
+echo "== qcow backed by raw =="
+
+rm "$TEST_IMG.base"
+truncate --size=$size "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base"
+_img_info
+_make_test_img -b "$TEST_IMG.base" -F raw
+_img_info
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0
diff --git a/tests/qemu-iotests/290.out b/tests/qemu-iotests/290.out
new file mode 100644
index 000000000000..5b4ae3196d04
--- /dev/null
+++ b/tests/qemu-iotests/290.out
@@ -0,0 +1,42 @@
+QA output created by 290
+
+== qcow backed by qcow ==
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 128 MiB (134217728 bytes)
+cluster_size: 512
+backing file: TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 128 MiB (134217728 bytes)
+cluster_size: 512
+backing file: TEST_DIR/t.IMGFMT.base
+
+== mismatched command line detection ==
+qemu-img: TEST_DIR/t.IMGFMT: invalid VMDK image descriptor
+Could not open backing image to determine size.
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=vmdk
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 128 MiB (134217728 bytes)
+cluster_size: 512
+backing file: TEST_DIR/t.IMGFMT.base
+
+== qcow backed by raw ==
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 128 MiB (134217728 bytes)
+cluster_size: 512
+backing file: TEST_DIR/t.IMGFMT.base
+qemu-img: TEST_DIR/t.IMGFMT: IMGFMT cannot store backing format; an explicit backing format of raw is unsafe
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=raw
+image: TEST_DIR/t.IMGFMT
+file format: IMGFMT
+virtual size: 128 MiB (134217728 bytes)
+cluster_size: 512
+backing file: TEST_DIR/t.IMGFMT.base
+*** done
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index 79c6dfc85d1b..c4dc86a9cf73 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -296,3 +296,4 @@
 286 rw quick
 288 quick
 289 rw quick
+290 backing quick
-- 
2.26.0.rc2

Re: [PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

[Kevin Wolf]

Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
> qcow has no space in the metadata to store a backing format, and there
> are existing qcow images backed both by raw or by other formats
> (usually qcow) images, reliant on probing to tell the difference.
> While we don't recommend the creation of new qcow images (as qcow2 is
> hands-down better), we can at least insist that if the user does
> request a specific format without using -u, then it must be non-raw
> (as a raw backing file that gets inadvertently edited into some other
> format can form a security hole); if the user does not request a
> specific format or lies when using -u, then the status quo of probing
> for the backing format remains intact (although an upcoming patch will
> warn when omitting a format request).  Thus, when this series is
> complete, the only way to use a backing file for qcow without
> triggering a warning is when using -F if the backing file is non-raw
> to begin with.  Note that this is only for QemuOpts usage; there is no
> change to the QAPI to allow a format through -blockdev.
> 
> Add a new iotest 290 just for qcow, to demonstrate the new warning.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

Somehow this feels backwards. Not specifying the backing file format at
all isn't any safer than explicitly specifying raw.

If there is a difference at all, I would say that explicitly specifying
raw means that the user is aware what they are doing. So we would have
more reason to warn against raw images if the backing format isn't
specified at all because then the user might not be aware that they are
using a backing file that probes as raw.

>  block/qcow.c               | 16 ++++++++-
>  tests/qemu-iotests/290     | 72 ++++++++++++++++++++++++++++++++++++++
>  tests/qemu-iotests/290.out | 42 ++++++++++++++++++++++
>  tests/qemu-iotests/group   |  1 +
>  4 files changed, 130 insertions(+), 1 deletion(-)
>  create mode 100755 tests/qemu-iotests/290
>  create mode 100644 tests/qemu-iotests/290.out
> 
> diff --git a/block/qcow.c b/block/qcow.c
> index 8973e4e565a1..bbe48b7db4d7 100644
> --- a/block/qcow.c
> +++ b/block/qcow.c
> @@ -940,11 +940,12 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,
>  {
>      BlockdevCreateOptions *create_options = NULL;
>      BlockDriverState *bs = NULL;
> -    QDict *qdict;
> +    QDict *qdict = NULL;
>      Visitor *v;
>      const char *val;
>      Error *local_err = NULL;
>      int ret;
> +    char *backing_fmt;
> 
>      static const QDictRenames opt_renames[] = {
>          { BLOCK_OPT_BACKING_FILE,       "backing-file" },
> @@ -953,6 +954,13 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,
>      };
> 
>      /* Parse options and convert legacy syntax */
> +    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
> +    if (backing_fmt && !strcmp(backing_fmt, "raw")) {
> +        error_setg(errp, "qcow cannot store backing format; an explicit "
> +                   "backing format of raw is unsafe");

Does this message tell that an implicit backing format of raw is safe?

> +        ret = -EINVAL;
> +        goto fail;
> +    }

The commit message promises a warning. This is not a warning, but a hard
error.

>      qdict = qemu_opts_to_qdict_filtered(opts, NULL, &qcow_create_opts, true);
> 
>      val = qdict_get_try_str(qdict, BLOCK_OPT_ENCRYPT);
> @@ -1018,6 +1026,7 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,
> 
>      ret = 0;
>  fail:
> +    g_free(backing_fmt);
>      qobject_unref(qdict);
>      bdrv_unref(bs);
>      qapi_free_BlockdevCreateOptions(create_options);
> @@ -1152,6 +1161,11 @@ static QemuOptsList qcow_create_opts = {
>              .type = QEMU_OPT_STRING,
>              .help = "File name of a base image"
>          },
> +        {
> +            .name = BLOCK_OPT_BACKING_FMT,
> +            .type = QEMU_OPT_STRING,
> +            .help = "Format of the backing image (caution: raw backing is unsafe)",
> +        },
>          {
>              .name = BLOCK_OPT_ENCRYPT,
>              .type = QEMU_OPT_BOOL,
> diff --git a/tests/qemu-iotests/290 b/tests/qemu-iotests/290
> new file mode 100755
> index 000000000000..8482de58cb4b
> --- /dev/null
> +++ b/tests/qemu-iotests/290
> @@ -0,0 +1,72 @@
> +#!/usr/bin/env bash
> +#
> +# Test qcow backing file warnings
> +#
> +# Copyright (C) 2020 Red Hat, Inc.
> +#
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 2 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +#
> +
> +seq=`basename $0`
> +echo "QA output created by $seq"
> +
> +status=1	# failure is the default!
> +
> +_cleanup()
> +{
> +	_cleanup_test_img
> +}
> +trap "_cleanup; exit \$status" 0 1 2 3 15
> +
> +# get standard environment, filters and checks
> +. ./common.rc
> +. ./common.filter
> +
> +_supported_fmt qcow
> +_supported_proto file
> +_supported_os Linux
> +
> +size=128M
> +
> +echo
> +echo "== qcow backed by qcow =="
> +
> +TEST_IMG="$TEST_IMG.base" _make_test_img $size
> +_make_test_img -b "$TEST_IMG.base"
> +_img_info
> +_make_test_img -b "$TEST_IMG.base" -F $IMGFMT
> +_img_info
> +
> +echo
> +echo "== mismatched command line detection =="
> +
> +_make_test_img -b "$TEST_IMG.base" -F vmdk
> +# Use of -u bypasses the backing format sanity check
> +_make_test_img -u -b "$TEST_IMG.base" -F vmdk $size
> +_img_info
> +
> +echo
> +echo "== qcow backed by raw =="
> +
> +rm "$TEST_IMG.base"
> +truncate --size=$size "$TEST_IMG.base"
> +_make_test_img -b "$TEST_IMG.base"
> +_img_info
> +_make_test_img -b "$TEST_IMG.base" -F raw
> +_img_info

This test doesn't tell the difference between a warning and an error. In
both cases, the image would look the same: Either because it was
successfully created or because the old version is still there.

Kevin

Re: [PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

[Eric Blake]

On 5/5/20 2:35 AM, Kevin Wolf wrote:
> Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
>> qcow has no space in the metadata to store a backing format, and there
>> are existing qcow images backed both by raw or by other formats
>> (usually qcow) images, reliant on probing to tell the difference.
>> While we don't recommend the creation of new qcow images (as qcow2 is
>> hands-down better), we can at least insist that if the user does
>> request a specific format without using -u, then it must be non-raw
>> (as a raw backing file that gets inadvertently edited into some other
>> format can form a security hole); if the user does not request a
>> specific format or lies when using -u, then the status quo of probing
>> for the backing format remains intact (although an upcoming patch will
>> warn when omitting a format request).  Thus, when this series is
>> complete, the only way to use a backing file for qcow without
>> triggering a warning is when using -F if the backing file is non-raw
>> to begin with.  Note that this is only for QemuOpts usage; there is no
>> change to the QAPI to allow a format through -blockdev.
>>
>> Add a new iotest 290 just for qcow, to demonstrate the new warning.
>>
>> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> Somehow this feels backwards. Not specifying the backing file format at
> all isn't any safer than explicitly specifying raw.
> 
> If there is a difference at all, I would say that explicitly specifying
> raw means that the user is aware what they are doing. So we would have
> more reason to warn against raw images if the backing format isn't
> specified at all because then the user might not be aware that they are
> using a backing file that probes as raw.

Prior to this patch, -F does not work with qcow.  And even with this 
patch, we still cannot store the explicit value of -F in the qcow file. 
Anything that does not use -F must continue to work for now (although it 
may now warn, and in fact must warn if we deprecate it), while anything 
explicit is free to fail (since it failed already), but could also be 
made to work (if letting it work is nicer than making it fail, and where 
"work" may still include a warning, although it's pointless to have 
something brand new that works but is deprecated out of the box).  So 
the following is my summary of the two options we can choose between:

Option 1, qcow backed by raw is more common than qcow backed by other, 
so we want:
raw <- qcow, no -F: work without warning (but if backing file is edited, 
a future probe seeing non-raw would break image)
raw <- qcow, with -F: work without warning (but if backing file is 
edited, a future probe seeing non-raw would break image)
other <- qcow, no -F: works but issues a warning (but backing file will 
always probe correctly)
other <- qcow, with -F: fails (we cannot honor the user's explicit 
request, because we would still have to probe)

Option 2, qcow backed by other is more common than qcow backed by raw, 
so we want:
raw <- qcow, no -F: works but issues a warning (using a raw backing file 
without explicit buy-in is risky)
raw <- qcow, with -F: works but issues a warning (explicit buy-in will 
still require subsequent probing, and a backing file could change which 
would break image)
other <- qcow, no -F: works without warning
other <- qcow, with -F: works without warning (later probing will still 
see non-raw)

It looks like you are leaning more towards option 1, while my patch 
leaned more towards option 2.  Anyone else want to chime in with an 
opinion on which is safer vs. easier?


>> @@ -953,6 +954,13 @@ static int coroutine_fn qcow_co_create_opts(BlockDriver *drv,
>>       };
>>
>>       /* Parse options and convert legacy syntax */
>> +    backing_fmt = qemu_opt_get_del(opts, BLOCK_OPT_BACKING_FMT);
>> +    if (backing_fmt && !strcmp(backing_fmt, "raw")) {
>> +        error_setg(errp, "qcow cannot store backing format; an explicit "
>> +                   "backing format of raw is unsafe");
> 
> Does this message tell that an implicit backing format of raw is safe?

If we go with option 2, are we trying to deprecate ALL use of raw as a 
backing file to qcow, regardless of whether the user was explicit about 
it?  If we go with option 1, then we are instead deprecating any use of 
non-raw as a backing file to qcow.

At the end of the day, we are trying to discourage users from creating 
new qcow files in the first place; qcow2 is much better.  We still have 
to read existing qcow images with backing files, but maybe we want:

Option 3:
completely deprecate qcow images with backing files, as there is no safe 
way to do things favoring either raw (option 1) or non-raw (option 2), 
and therefore accept -F solely for convenience with the rest of the 
series, but always issue a warning regardless of whether -F was present.

> 
>> +        ret = -EINVAL;
>> +        goto fail;
>> +    }
> 
> The commit message promises a warning. This is not a warning, but a hard
> error.

Once we decide which behavior we want, I'll make sure the commit message 
matches the behavior.  Remember, for implicit images, we are starting a 
deprecation clock; for explicit images, it previously failed, so 
continuing to fail is still viable.


>> +echo
>> +echo "== qcow backed by raw =="
>> +
>> +rm "$TEST_IMG.base"
>> +truncate --size=$size "$TEST_IMG.base"
>> +_make_test_img -b "$TEST_IMG.base"
>> +_img_info
>> +_make_test_img -b "$TEST_IMG.base" -F raw
>> +_img_info
> 
> This test doesn't tell the difference between a warning and an error. In
> both cases, the image would look the same: Either because it was
> successfully created or because the old version is still there.

Indeed, I can make the test more robust.  But more importantly, we need 
consensus on _what_ we want behavior to be, and write the test along 
those lines.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

[Eric Blake]

On 5/5/20 10:30 AM, Eric Blake wrote:
> On 5/5/20 2:35 AM, Kevin Wolf wrote:
>> Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
>>> qcow has no space in the metadata to store a backing format, and there
>>> are existing qcow images backed both by raw or by other formats
>>> (usually qcow) images, reliant on probing to tell the difference.
>>> While we don't recommend the creation of new qcow images (as qcow2 is
>>> hands-down better), we can at least insist that if the user does
>>> request a specific format without using -u, then it must be non-raw
>>> (as a raw backing file that gets inadvertently edited into some other
>>> format can form a security hole); if the user does not request a
>>> specific format or lies when using -u, then the status quo of probing
>>> for the backing format remains intact (although an upcoming patch will
>>> warn when omitting a format request).  Thus, when this series is
>>> complete, the only way to use a backing file for qcow without
>>> triggering a warning is when using -F if the backing file is non-raw
>>> to begin with.  Note that this is only for QemuOpts usage; there is no
>>> change to the QAPI to allow a format through -blockdev.
>>>
>>> Add a new iotest 290 just for qcow, to demonstrate the new warning.
>>>
>>> Signed-off-by: Eric Blake <eblake@redhat.com>
>>
>> Somehow this feels backwards. Not specifying the backing file format at
>> all isn't any safer than explicitly specifying raw.
>>
>> If there is a difference at all, I would say that explicitly specifying
>> raw means that the user is aware what they are doing. So we would have
>> more reason to warn against raw images if the backing format isn't
>> specified at all because then the user might not be aware that they are
>> using a backing file that probes as raw.
> 
> Prior to this patch, -F does not work with qcow.  And even with this 
> patch, we still cannot store the explicit value of -F in the qcow file. 
> Anything that does not use -F must continue to work for now (although it 
> may now warn, and in fact must warn if we deprecate it), while anything 
> explicit is free to fail (since it failed already), but could also be 
> made to work (if letting it work is nicer than making it fail, and where 
> "work" may still include a warning, although it's pointless to have 
> something brand new that works but is deprecated out of the box).  So 
> the following is my summary of the two options we can choose between:
> 
> Option 1, qcow backed by raw is more common than qcow backed by other, 
> so we want:
> raw <- qcow, no -F: work without warning (but if backing file is edited, 
> a future probe seeing non-raw would break image)
> raw <- qcow, with -F: work without warning (but if backing file is 
> edited, a future probe seeing non-raw would break image)
> other <- qcow, no -F: works but issues a warning (but backing file will 
> always probe correctly)
> other <- qcow, with -F: fails (we cannot honor the user's explicit 
> request, because we would still have to probe)
> 
> Option 2, qcow backed by other is more common than qcow backed by raw, 
> so we want:
> raw <- qcow, no -F: works but issues a warning (using a raw backing file 
> without explicit buy-in is risky)
> raw <- qcow, with -F: works but issues a warning (explicit buy-in will 
> still require subsequent probing, and a backing file could change which 
> would break image)
> other <- qcow, no -F: works without warning
> other <- qcow, with -F: works without warning (later probing will still 
> see non-raw)
> 
> It looks like you are leaning more towards option 1, while my patch 
> leaned more towards option 2.  Anyone else want to chime in with an 
> opinion on which is safer vs. easier?

> Option 3:
> completely deprecate qcow images with backing files, as there is no safe 
> way to do things favoring either raw (option 1) or non-raw (option 2), 
> and therefore accept -F solely for convenience with the rest of the 
> series, but always issue a warning regardless of whether -F was present.


Hearing no other opinion in the meantime, I've come up with option 4:

raw <- qcow, no -F: works but issues a warning to use -F (the user 
should be explicit that they know they are using raw)
raw <- qcow, with -F raw: a probe is attempted, if it returns anything 
other than raw, then fail (since we can't store the backing type, and 
the user's explicit type didn't match reality); otherwise works without 
warning (users tend to treat backing files as read-only, so even though 
editing a backing file could make it appear non-raw, that's less likely 
to happen)
other <- qcow, no -F: works without warning (we'll probe in future 
opens, but the probe will see the same file type and not corrupt user data)
other <- qcow, with -F: a probe is attempted and must match, but 
otherwise works without warning (we'll still have to probe in future 
opens, but it's no worse than before)



-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

[Kevin Wolf]

Am 22.06.2020 um 23:58 hat Eric Blake geschrieben:
> On 5/5/20 10:30 AM, Eric Blake wrote:
> > On 5/5/20 2:35 AM, Kevin Wolf wrote:
> > > Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
> > > > qcow has no space in the metadata to store a backing format, and there
> > > > are existing qcow images backed both by raw or by other formats
> > > > (usually qcow) images, reliant on probing to tell the difference.
> > > > While we don't recommend the creation of new qcow images (as qcow2 is
> > > > hands-down better), we can at least insist that if the user does
> > > > request a specific format without using -u, then it must be non-raw
> > > > (as a raw backing file that gets inadvertently edited into some other
> > > > format can form a security hole); if the user does not request a
> > > > specific format or lies when using -u, then the status quo of probing
> > > > for the backing format remains intact (although an upcoming patch will
> > > > warn when omitting a format request).  Thus, when this series is
> > > > complete, the only way to use a backing file for qcow without
> > > > triggering a warning is when using -F if the backing file is non-raw
> > > > to begin with.  Note that this is only for QemuOpts usage; there is no
> > > > change to the QAPI to allow a format through -blockdev.
> > > > 
> > > > Add a new iotest 290 just for qcow, to demonstrate the new warning.
> > > > 
> > > > Signed-off-by: Eric Blake <eblake@redhat.com>
> > > 
> > > Somehow this feels backwards. Not specifying the backing file format at
> > > all isn't any safer than explicitly specifying raw.
> > > 
> > > If there is a difference at all, I would say that explicitly specifying
> > > raw means that the user is aware what they are doing. So we would have
> > > more reason to warn against raw images if the backing format isn't
> > > specified at all because then the user might not be aware that they are
> > > using a backing file that probes as raw.
> > 
> > Prior to this patch, -F does not work with qcow.  And even with this
> > patch, we still cannot store the explicit value of -F in the qcow file.
> > Anything that does not use -F must continue to work for now (although it
> > may now warn, and in fact must warn if we deprecate it), while anything
> > explicit is free to fail (since it failed already), but could also be
> > made to work (if letting it work is nicer than making it fail, and where
> > "work" may still include a warning, although it's pointless to have
> > something brand new that works but is deprecated out of the box).  So
> > the following is my summary of the two options we can choose between:
> > 
> > Option 1, qcow backed by raw is more common than qcow backed by other,
> > so we want:
> > raw <- qcow, no -F: work without warning (but if backing file is edited,
> > a future probe seeing non-raw would break image)
> > raw <- qcow, with -F: work without warning (but if backing file is
> > edited, a future probe seeing non-raw would break image)
> > other <- qcow, no -F: works but issues a warning (but backing file will
> > always probe correctly)
> > other <- qcow, with -F: fails (we cannot honor the user's explicit
> > request, because we would still have to probe)
> > 
> > Option 2, qcow backed by other is more common than qcow backed by raw,
> > so we want:
> > raw <- qcow, no -F: works but issues a warning (using a raw backing file
> > without explicit buy-in is risky)
> > raw <- qcow, with -F: works but issues a warning (explicit buy-in will
> > still require subsequent probing, and a backing file could change which
> > would break image)
> > other <- qcow, no -F: works without warning
> > other <- qcow, with -F: works without warning (later probing will still
> > see non-raw)
> > 
> > It looks like you are leaning more towards option 1, while my patch
> > leaned more towards option 2.  Anyone else want to chime in with an
> > opinion on which is safer vs. easier?
> 
> > Option 3:
> > completely deprecate qcow images with backing files, as there is no safe
> > way to do things favoring either raw (option 1) or non-raw (option 2),
> > and therefore accept -F solely for convenience with the rest of the
> > series, but always issue a warning regardless of whether -F was present.
> 
> 
> Hearing no other opinion in the meantime, I've come up with option 4:
> 
> raw <- qcow, no -F: works but issues a warning to use -F (the user should be
> explicit that they know they are using raw)
> raw <- qcow, with -F raw: a probe is attempted, if it returns anything other
> than raw, then fail (since we can't store the backing type, and the user's
> explicit type didn't match reality); otherwise works without warning (users
> tend to treat backing files as read-only, so even though editing a backing
> file could make it appear non-raw, that's less likely to happen)

Actually, even for a backing file, I think bs->probed should be set, so
the raw driver would return an I/O error if you write the magic of an
image format to the first sector. We should just add a test case to
verify this behaviour for backing files (e.g. in the context of a commit
job).

Of course, if you edit the backing file outside of QEMU, that's your
problem.

> other <- qcow, no -F: works without warning (we'll probe in future opens,
> but the probe will see the same file type and not corrupt user data)
> other <- qcow, with -F: a probe is attempted and must match, but otherwise
> works without warning (we'll still have to probe in future opens, but it's
> no worse than before)

This plan makes sense to me.

Kevin

[PATCH v5 4/7] qcow2: Deprecate use of qemu-img amend to change backing file

[Eric Blake]

The use of 'qemu-img amend' to change qcow2 backing files is not
tested very well.  In particular, our implementation has a bug where
if a new backing file is provided without a format, then the prior
format is blindly reused, even if this results in data corruption, but
this is not caught by iotests.

There are also situations where amending other options needs access to
the original backing file (for example, on a downgrade to a v2 image,
knowing whether a v3 zero cluster must be allocated or may be left
unallocated depends on knowing whether the backing file already reads
as zero), but the command line does not have a nice way to tell us
both the backing file to use for opening the image as well as the
backing file to install after the operation is complete.

Even if we do allow changing the backing file, it is redundant with
the existing ability to change backing files via 'qemu-img rebase -u'.
It is time to deprecate this support (leaving the existing behavior
intact, even if it is buggy), and at a point in the future, require
the use of only 'qemu-img rebase' for adjusting backing chain
relations, saving 'qemu-img amend' for changes unrelated to the
backing chain.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/system/deprecated.rst | 12 ++++++++++++
 docs/tools/qemu-img.rst    |  4 ++++
 block/qcow2.c              |  5 +++++
 tests/qemu-iotests/061.out |  1 +
 tests/qemu-iotests/082.out |  2 ++
 5 files changed, 24 insertions(+)

diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index c633fe2beffd..aac6be58917a 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -417,6 +417,18 @@ The above, converted to the current supported format::
 Related binaries
 ----------------

+qemu-img amend to adjust backing file (since 5.0.0)
+'''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The use of ``qemu-img amend`` to modify the name or format of a qcow2
+backing image is deprecated; this functionality was never fully
+documented or tested, and interferes with other amend operations that
+need access to the original backing image (such as deciding whether a
+v3 zero cluster may be left unallocated when converting to a v2
+image).  Rather, any changes to the backing chain should be performed
+with ``qemu-img rebase -u`` either before or after the remaining
+changes being performed by amend, as appropriate.
+
 ``qemu-img convert -n -o`` (since 4.2.0)
 ''''''''''''''''''''''''''''''''''''''''

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index 0080f83a76c9..83d57586be96 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -254,6 +254,10 @@ Command description:
   Amends the image format specific *OPTIONS* for the image file
   *FILENAME*. Not all file formats support this operation.

+  The set of options that can be amended are dependent on the image
+  format, but note that amending the backing chain relationship should
+  instead be performed with ``qemu-img rebase``.
+
 .. option:: bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL] [-i AIO] [-n] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME

   Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
diff --git a/block/qcow2.c b/block/qcow2.c
index 2bb536b01494..e4b772726010 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -5303,6 +5303,11 @@ static int qcow2_amend_options(BlockDriverState *bs, QemuOpts *opts,
     }

     if (backing_file || backing_format) {
+        if (g_strcmp0(backing_file, s->image_backing_file) ||
+            g_strcmp0(backing_format, s->image_backing_format)) {
+            warn_report("Deprecated use of amend to alter the backing file; "
+                        "use qemu-img rebase instead");
+        }
         ret = qcow2_change_backing_file(bs,
                     backing_file ?: s->image_backing_file,
                     backing_format ?: s->image_backing_format);
diff --git a/tests/qemu-iotests/061.out b/tests/qemu-iotests/061.out
index 413cc4e0f4ab..048248c121cb 100644
--- a/tests/qemu-iotests/061.out
+++ b/tests/qemu-iotests/061.out
@@ -342,6 +342,7 @@ wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+qemu-img: warning: Deprecated use of amend to alter the backing file; use qemu-img rebase instead
 read 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.
diff --git a/tests/qemu-iotests/082.out b/tests/qemu-iotests/082.out
index 9d4ed4dc9d61..94ea990a2754 100644
--- a/tests/qemu-iotests/082.out
+++ b/tests/qemu-iotests/082.out
@@ -806,10 +806,12 @@ Creation options for 'qcow2':
 Note that not all of these options may be amendable.

 Testing: amend -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2
+qemu-img: warning: Deprecated use of amend to alter the backing file; use qemu-img rebase instead

 Testing: rebase -u -b  -f qcow2 TEST_DIR/t.qcow2

 Testing: amend -f qcow2 -o backing_file=TEST_DIR/t.qcow2,,? TEST_DIR/t.qcow2
+qemu-img: warning: Deprecated use of amend to alter the backing file; use qemu-img rebase instead

 Testing: rebase -u -b  -f qcow2 TEST_DIR/t.qcow2

-- 
2.26.0.rc2

Re: [PATCH v5 4/7] qcow2: Deprecate use of qemu-img amend to change backing file

[Kevin Wolf]

Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
> The use of 'qemu-img amend' to change qcow2 backing files is not
> tested very well.  In particular, our implementation has a bug where
> if a new backing file is provided without a format, then the prior
> format is blindly reused, even if this results in data corruption, but
> this is not caught by iotests.
> 
> There are also situations where amending other options needs access to
> the original backing file (for example, on a downgrade to a v2 image,
> knowing whether a v3 zero cluster must be allocated or may be left
> unallocated depends on knowing whether the backing file already reads
> as zero), but the command line does not have a nice way to tell us
> both the backing file to use for opening the image as well as the
> backing file to install after the operation is complete.
> 
> Even if we do allow changing the backing file, it is redundant with
> the existing ability to change backing files via 'qemu-img rebase -u'.
> It is time to deprecate this support (leaving the existing behavior
> intact, even if it is buggy), and at a point in the future, require
> the use of only 'qemu-img rebase' for adjusting backing chain
> relations, saving 'qemu-img amend' for changes unrelated to the
> backing chain.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

I think the original idea was that in BlockDriver, special interfaces
like the ones used by qemu-img rebase could eventually go away if we
have a single consistent interface to change image options, which is
amend. So ideally, bdrv_change_backing_file() should become a wrapper
around amend.

I'm not even sure if the behaviour you mention should be considered a
bug because amend is a low-level interface. But even if that's the case,
it would be a matter of simply adding a check.

Kevin

[PATCH v5 5/7] iotests: Specify explicit backing format where sensible

[Eric Blake]

There are many existing qcow2 images that specify a backing file but
no format.  This has been the source of CVEs in the past, but has
become more prominent of a problem now that libvirt has switched to
-blockdev.  With older -drive, at least the probing was always done by
qemu (so the only risk of a changed format between successive boots of
a guest was if qemu was upgraded and probed differently).  But with
newer -blockdev, libvirt must specify a format; if libvirt guesses raw
where the image was formatted, this results in data corruption visible
to the guest; conversely, if libvirt guesses qcow2 where qemu was
using raw, this can result in potential security holes, so modern
libvirt instead refuses to use images without explicit backing format.

The change in libvirt to reject images without explicit backing format
has pointed out that a number of tools have been far too reliant on
probing in the past.  It's time to set a better example in our own
iotests of properly setting this parameter.

iotest calls to create, rebase, and convert are all impacted to some
degree.  It's a bit annoying that we are inconsistent on command line
- while all of those accept -o backing_file=...,backing_fmt=..., the
shortcuts are different: create and rebase have -b and -F, while
convert has -B but no -F.  (amend has no shortcuts, but the previous
patch just deprecated the use of amend to change backing chains).

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 tests/qemu-iotests/017        |  2 +-
 tests/qemu-iotests/017.out    |  2 +-
 tests/qemu-iotests/018        |  2 +-
 tests/qemu-iotests/018.out    |  2 +-
 tests/qemu-iotests/019        |  5 ++--
 tests/qemu-iotests/019.out    |  2 +-
 tests/qemu-iotests/020        |  4 +--
 tests/qemu-iotests/020.out    |  4 +--
 tests/qemu-iotests/024        |  8 +++---
 tests/qemu-iotests/024.out    |  5 ++--
 tests/qemu-iotests/028        |  4 +--
 tests/qemu-iotests/028.out    |  2 +-
 tests/qemu-iotests/030        | 26 +++++++++++++------
 tests/qemu-iotests/034        |  2 +-
 tests/qemu-iotests/034.out    |  2 +-
 tests/qemu-iotests/037        |  2 +-
 tests/qemu-iotests/037.out    |  2 +-
 tests/qemu-iotests/038        |  2 +-
 tests/qemu-iotests/038.out    |  2 +-
 tests/qemu-iotests/039        |  3 ++-
 tests/qemu-iotests/039.out    |  2 +-
 tests/qemu-iotests/040        | 47 +++++++++++++++++++++++++----------
 tests/qemu-iotests/041        | 37 ++++++++++++++++++---------
 tests/qemu-iotests/042        |  4 +--
 tests/qemu-iotests/043        | 18 +++++++-------
 tests/qemu-iotests/043.out    | 16 +++++++-----
 tests/qemu-iotests/046        |  2 +-
 tests/qemu-iotests/046.out    |  2 +-
 tests/qemu-iotests/050        |  4 +--
 tests/qemu-iotests/050.out    |  2 +-
 tests/qemu-iotests/051        |  2 +-
 tests/qemu-iotests/051.out    |  2 +-
 tests/qemu-iotests/051.pc.out |  2 +-
 tests/qemu-iotests/056        |  3 ++-
 tests/qemu-iotests/060        |  2 +-
 tests/qemu-iotests/060.out    |  2 +-
 tests/qemu-iotests/061        | 10 ++++----
 tests/qemu-iotests/061.out    | 10 ++++----
 tests/qemu-iotests/069        |  2 +-
 tests/qemu-iotests/069.out    |  2 +-
 tests/qemu-iotests/073        |  2 +-
 tests/qemu-iotests/073.out    |  2 +-
 tests/qemu-iotests/082        | 10 +++++---
 tests/qemu-iotests/082.out    | 12 ++++-----
 tests/qemu-iotests/085        |  4 +--
 tests/qemu-iotests/085.out    |  6 ++---
 tests/qemu-iotests/089        |  2 +-
 tests/qemu-iotests/089.out    |  2 +-
 tests/qemu-iotests/095        |  4 +--
 tests/qemu-iotests/095.out    |  4 +--
 tests/qemu-iotests/097        |  4 +--
 tests/qemu-iotests/097.out    | 16 ++++++------
 tests/qemu-iotests/098        |  2 +-
 tests/qemu-iotests/098.out    |  8 +++---
 tests/qemu-iotests/110        |  4 +--
 tests/qemu-iotests/110.out    |  4 +--
 tests/qemu-iotests/122        | 27 ++++++++++++--------
 tests/qemu-iotests/122.out    |  8 +++---
 tests/qemu-iotests/126        |  4 +--
 tests/qemu-iotests/126.out    |  4 +--
 tests/qemu-iotests/127        |  4 +--
 tests/qemu-iotests/127.out    |  4 +--
 tests/qemu-iotests/129        |  3 ++-
 tests/qemu-iotests/133        |  2 +-
 tests/qemu-iotests/133.out    |  2 +-
 tests/qemu-iotests/139        |  2 +-
 tests/qemu-iotests/141        |  4 +--
 tests/qemu-iotests/141.out    |  4 +--
 tests/qemu-iotests/142        |  2 +-
 tests/qemu-iotests/142.out    |  2 +-
 tests/qemu-iotests/153        | 14 +++++------
 tests/qemu-iotests/153.out    | 35 ++++++++++++++------------
 tests/qemu-iotests/154        | 42 +++++++++++++++----------------
 tests/qemu-iotests/154.out    | 42 +++++++++++++++----------------
 tests/qemu-iotests/155        | 12 ++++++---
 tests/qemu-iotests/156        |  9 ++++---
 tests/qemu-iotests/156.out    |  6 ++---
 tests/qemu-iotests/158        |  2 +-
 tests/qemu-iotests/158.out    |  2 +-
 tests/qemu-iotests/161        |  8 +++---
 tests/qemu-iotests/161.out    |  8 +++---
 tests/qemu-iotests/176        |  4 +--
 tests/qemu-iotests/176.out    | 32 ++++++++++++------------
 tests/qemu-iotests/177        |  2 +-
 tests/qemu-iotests/177.out    |  2 +-
 tests/qemu-iotests/179        |  2 +-
 tests/qemu-iotests/179.out    |  2 +-
 tests/qemu-iotests/189        |  2 +-
 tests/qemu-iotests/189.out    |  2 +-
 tests/qemu-iotests/191        | 12 ++++-----
 tests/qemu-iotests/191.out    | 12 ++++-----
 tests/qemu-iotests/195        |  6 ++---
 tests/qemu-iotests/195.out    |  6 ++---
 tests/qemu-iotests/198        |  2 +-
 tests/qemu-iotests/198.out    |  3 ++-
 tests/qemu-iotests/204        |  2 +-
 tests/qemu-iotests/204.out    |  2 +-
 tests/qemu-iotests/216        |  2 +-
 tests/qemu-iotests/224        |  4 +--
 tests/qemu-iotests/225        |  2 +-
 tests/qemu-iotests/225.out    |  2 +-
 tests/qemu-iotests/228        |  5 ++--
 tests/qemu-iotests/245        |  3 ++-
 tests/qemu-iotests/249        |  4 +--
 tests/qemu-iotests/249.out    |  4 +--
 tests/qemu-iotests/252        |  2 +-
 tests/qemu-iotests/257        |  3 ++-
 tests/qemu-iotests/267        |  4 +--
 tests/qemu-iotests/267.out    |  6 ++---
 tests/qemu-iotests/270        |  2 +-
 tests/qemu-iotests/270.out    |  2 +-
 tests/qemu-iotests/273        |  4 +--
 tests/qemu-iotests/273.out    |  4 +--
 tests/qemu-iotests/279        |  4 +--
 tests/qemu-iotests/279.out    |  4 +--
 115 files changed, 410 insertions(+), 336 deletions(-)

diff --git a/tests/qemu-iotests/017 b/tests/qemu-iotests/017
index 0a4b854e6520..585512bb296b 100755
--- a/tests/qemu-iotests/017
+++ b/tests/qemu-iotests/017
@@ -66,7 +66,7 @@ echo "Creating test image with backing file"
 echo

 TEST_IMG=$TEST_IMG_SAVE
-_make_test_img -b "$TEST_IMG.base" 6G
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G

 echo "Filling test image"
 echo
diff --git a/tests/qemu-iotests/017.out b/tests/qemu-iotests/017.out
index 8fc9241942e8..f439d3ece3a7 100644
--- a/tests/qemu-iotests/017.out
+++ b/tests/qemu-iotests/017.out
@@ -269,7 +269,7 @@ wrote 65536/65536 bytes at offset 4295032832
 No errors were found on the image.
 Creating test image with backing file

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Filling test image

 === IO: pattern 1
diff --git a/tests/qemu-iotests/018 b/tests/qemu-iotests/018
index c69ce0920920..191b461a4d29 100755
--- a/tests/qemu-iotests/018
+++ b/tests/qemu-iotests/018
@@ -66,7 +66,7 @@ echo "Creating test image with backing file"
 echo

 TEST_IMG="$TEST_IMG_SAVE.orig"
-_make_test_img -b "$TEST_IMG_SAVE.base" 6G
+_make_test_img -b "$TEST_IMG_SAVE.base" -F $IMGFMT 6G

 echo "Filling test image"
 echo
diff --git a/tests/qemu-iotests/018.out b/tests/qemu-iotests/018.out
index 5df966727f98..0ab664ca4b4c 100644
--- a/tests/qemu-iotests/018.out
+++ b/tests/qemu-iotests/018.out
@@ -269,7 +269,7 @@ wrote 65536/65536 bytes at offset 4295032832
 No errors were found on the image.
 Creating test image with backing file

-Formatting 'TEST_DIR/t.IMGFMT.orig', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT.orig', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Filling test image

 === IO: pattern 1
diff --git a/tests/qemu-iotests/019 b/tests/qemu-iotests/019
index 813a84acac47..d3c11256dc0f 100755
--- a/tests/qemu-iotests/019
+++ b/tests/qemu-iotests/019
@@ -74,7 +74,7 @@ echo "Creating test image with backing file"
 echo

 TEST_IMG="$TEST_IMG_SAVE.orig"
-_make_test_img -b "$TEST_IMG_SAVE.base" 6G
+_make_test_img -b "$TEST_IMG_SAVE.base" -F $IMGFMT 6G

 echo "Filling test image"
 echo
@@ -98,7 +98,8 @@ for backing_option in "-B " "-o backing_file="; do
     echo
     echo Testing conversion with $backing_option"$TEST_IMG.base" | _filter_testdir | _filter_imgfmt
     echo
-    $QEMU_IMG convert -f $IMGFMT -O $IMGFMT $backing_option"$TEST_IMG.base" "$TEST_IMG.orig" "$TEST_IMG"
+    $QEMU_IMG convert -f $IMGFMT -O $IMGFMT $backing_option"$TEST_IMG.base" \
+        -o backing_fmt=$IMGFMT "$TEST_IMG.orig" "$TEST_IMG"

     echo "Checking if backing clusters are allocated when they shouldn't"
     echo
diff --git a/tests/qemu-iotests/019.out b/tests/qemu-iotests/019.out
index 17a7c036b989..0fa73bd69ab8 100644
--- a/tests/qemu-iotests/019.out
+++ b/tests/qemu-iotests/019.out
@@ -269,7 +269,7 @@ wrote 65536/65536 bytes at offset 4296015872
 No errors were found on the image.
 Creating test image with backing file

-Formatting 'TEST_DIR/t.IMGFMT.orig', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT.orig', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Filling test image

 === IO: pattern 43
diff --git a/tests/qemu-iotests/020 b/tests/qemu-iotests/020
index 20f8f185d071..f90f133f1262 100755
--- a/tests/qemu-iotests/020
+++ b/tests/qemu-iotests/020
@@ -70,7 +70,7 @@ echo "Creating test image with backing file"
 echo

 TEST_IMG="$TEST_IMG_SAVE"
-_make_test_img -b "$TEST_IMG.base" 6G
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G

 echo "Filling test image"
 echo
@@ -126,7 +126,7 @@ _make_test_img -b "json:{'driver': '$IMGFMT',
                              'image': {
                                  'driver': 'file',
                                  'filename': '$TEST_IMG.base'
-                             }}}"
+                             }}}" -F $IMGFMT

 # Just write anything so committing will not be a no-op
 $QEMU_IO -c 'writev 0 64k' "$TEST_IMG" | _filter_qemu_io
diff --git a/tests/qemu-iotests/020.out b/tests/qemu-iotests/020.out
index 4b722b2dd022..8c0bf238b297 100644
--- a/tests/qemu-iotests/020.out
+++ b/tests/qemu-iotests/020.out
@@ -269,7 +269,7 @@ wrote 65536/65536 bytes at offset 4295032832
 No errors were found on the image.
 Creating test image with backing file

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Filling test image

 === IO: pattern 1
@@ -1090,7 +1090,7 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=json:{'driv
                              'image': {
                                  'driver': 'file',,
                                  'filename': 'TEST_DIR/t.IMGFMT.base'
-                             }}}
+                             }}} backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qemu-img: Block job failed: No space left on device
diff --git a/tests/qemu-iotests/024 b/tests/qemu-iotests/024
index e2e766241e1a..3c5d57310e94 100755
--- a/tests/qemu-iotests/024
+++ b/tests/qemu-iotests/024
@@ -83,7 +83,7 @@ TEST_IMG="$TEST_IMG_SAVE"
 echo "Creating COW image"
 echo

-_make_test_img -b "$TEST_IMG.base_old" 1G
+_make_test_img -b "$TEST_IMG.base_old" -F $IMGFMT 1G
 io_pattern writev 0 $((4 * CLUSTER_SIZE)) 0 1 0x33
 io_pattern writev $((8 * CLUSTER_SIZE)) $((4 * CLUSTER_SIZE)) 0 1 0x33

@@ -109,7 +109,7 @@ io_pattern readv $((15 * CLUSTER_SIZE)) $CLUSTER_SIZE 0 1 0x00
 echo
 echo Rebase and test again
 echo
-$QEMU_IMG rebase -b "$TEST_IMG.base_new" "$TEST_IMG"
+$QEMU_IMG rebase -b "$TEST_IMG.base_new" -F $IMGFMT "$TEST_IMG"
 io_pattern readv $((0 * CLUSTER_SIZE)) $CLUSTER_SIZE 0 1 0x33
 io_pattern readv $((1 * CLUSTER_SIZE)) $CLUSTER_SIZE 0 1 0x33
 io_pattern readv $((2 * CLUSTER_SIZE)) $CLUSTER_SIZE 0 1 0x33
@@ -159,7 +159,7 @@ OVERLAY="$TEST_DIR/$OVERLAY_WREL"

 TEST_IMG=$BASE_OLD _make_test_img 1M
 TEST_IMG=$BASE_NEW _make_test_img 1M
-TEST_IMG=$OVERLAY _make_test_img -b "$BASE_OLD_OREL" 1M
+TEST_IMG=$OVERLAY _make_test_img -b "$BASE_OLD_OREL" -F $IMGFMT 1M

 echo

@@ -176,7 +176,7 @@ $QEMU_IO "$BASE_NEW" \
 echo

 pushd "$TEST_DIR" >/dev/null
-$QEMU_IMG rebase -f "$IMGFMT" -b "$BASE_NEW_OREL" "$OVERLAY_WREL"
+$QEMU_IMG rebase -f "$IMGFMT" -b "$BASE_NEW_OREL" -F $IMGFMT "$OVERLAY_WREL"
 popd >/dev/null

 # Verify the backing path is correct
diff --git a/tests/qemu-iotests/024.out b/tests/qemu-iotests/024.out
index 024dc786b3c4..f4fd77fd5b4d 100644
--- a/tests/qemu-iotests/024.out
+++ b/tests/qemu-iotests/024.out
@@ -33,7 +33,7 @@ wrote 131072/131072 bytes at offset 786432
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 Creating COW image

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 backing_file=TEST_DIR/t.IMGFMT.base_old
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 backing_file=TEST_DIR/t.IMGFMT.base_old backing_fmt=IMGFMT
 === IO: pattern 0x33
 wrote 262144/262144 bytes at offset 0
 256 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -146,7 +146,7 @@ read 65536/65536 bytes at offset 983040

 Formatting 'TEST_DIR/subdir/t.IMGFMT.base_old', fmt=IMGFMT size=1048576
 Formatting 'TEST_DIR/subdir/t.IMGFMT.base_new', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/subdir/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=t.IMGFMT.base_old
+Formatting 'TEST_DIR/subdir/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=t.IMGFMT.base_old backing_fmt=IMGFMT

 wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
@@ -158,6 +158,7 @@ wrote 65536/65536 bytes at offset 196608
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

 backing file: t.IMGFMT.base_new (actual path: TEST_DIR/subdir/t.IMGFMT.base_new)
+backing file format: IMGFMT

 read 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
diff --git a/tests/qemu-iotests/028 b/tests/qemu-iotests/028
index 797dae535042..5d043cef9289 100755
--- a/tests/qemu-iotests/028
+++ b/tests/qemu-iotests/028
@@ -76,7 +76,7 @@ echo "Creating test image with backing file"
 echo

 TEST_IMG="$TEST_IMG_SAVE"
-_make_test_img -b "$TEST_IMG.base" $image_size
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $image_size

 echo "Filling test image"
 echo
@@ -101,7 +101,7 @@ io_zero readv $(( offset + 32 * 1024 )) 512 1024 32
 _check_test_img

 # Rebase it on top of its base image
-$QEMU_IMG rebase -b "$TEST_IMG.base" "$TEST_IMG"
+$QEMU_IMG rebase -b "$TEST_IMG.base" -F $IMGFMT "$TEST_IMG"

 echo
 echo block-backup
diff --git a/tests/qemu-iotests/028.out b/tests/qemu-iotests/028.out
index 37aed8443628..12f82c6a6c39 100644
--- a/tests/qemu-iotests/028.out
+++ b/tests/qemu-iotests/028.out
@@ -70,7 +70,7 @@ wrote 512/512 bytes at offset 3221225984
 No errors were found on the image.
 Creating test image with backing file

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294968832 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294968832 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Filling test image

 === IO: pattern 196
diff --git a/tests/qemu-iotests/030 b/tests/qemu-iotests/030
index aa911d266a13..322e31e2cd93 100755
--- a/tests/qemu-iotests/030
+++ b/tests/qemu-iotests/030
@@ -32,8 +32,12 @@ class TestSingleDrive(iotests.QMPTestCase):

     def setUp(self):
         iotests.create_image(backing_img, TestSingleDrive.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, mid_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img,
+                 '-F', 'raw', mid_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % mid_img,
+                 '-F', iotests.imgfmt, test_img)
         qemu_io('-f', 'raw', '-c', 'write -P 0x1 0 512', backing_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 524288 512', mid_img)
         self.vm = iotests.VM().add_drive("blkdebug::" + test_img,
@@ -199,7 +203,8 @@ class TestParallelOps(iotests.QMPTestCase):
         iotests.create_image(self.imgs[0], self.image_len)
         for i in range(1, self.num_imgs):
             qemu_img('create', '-f', iotests.imgfmt,
-                     '-o', 'backing_file=%s' % self.imgs[i-1], self.imgs[i])
+                     '-o', 'backing_file=%s' % self.imgs[i-1],
+                     '-F', 'raw' if i == 1 else iotests.imgfmt, self.imgs[i])

         # Put data into the images we are copying data from
         odd_img_indexes = [x for x in reversed(range(self.num_imgs)) if x % 2 == 1]
@@ -544,7 +549,8 @@ class TestQuorum(iotests.QMPTestCase):
             qemu_io('-f', iotests.imgfmt,
                     '-c', 'write -P 0x55 0 1024', backing_img)
             qemu_img('create', '-f', iotests.imgfmt,
-                     '-o', 'backing_file=%s' % backing_img, child_img)
+                     '-o', 'backing_file=%s' % backing_img,
+                     '-F', iotests.imgfmt, child_img)
             opts.append("children.%d.file.filename=%s" % (i, child_img))
             opts.append("children.%d.node-name=node%d" % (i, i))

@@ -585,7 +591,9 @@ class TestSmallerBackingFile(iotests.QMPTestCase):

     def setUp(self):
         iotests.create_image(backing_img, self.backing_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img, str(self.image_len))
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img,
+                 '-F', 'raw', test_img, str(self.image_len))
         self.vm = iotests.VM().add_drive(test_img)
         self.vm.launch()

@@ -848,7 +856,9 @@ class TestStreamStop(iotests.QMPTestCase):
     def setUp(self):
         qemu_img('create', backing_img, str(TestStreamStop.image_len))
         qemu_io('-f', 'raw', '-c', 'write -P 0x1 0 32M', backing_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img,
+                 '-F', 'raw', test_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 32M 32M', test_img)
         self.vm = iotests.VM().add_drive("blkdebug::" + test_img)
         self.vm.launch()
@@ -879,7 +889,9 @@ class TestSetSpeed(iotests.QMPTestCase):
     def setUp(self):
         qemu_img('create', backing_img, str(TestSetSpeed.image_len))
         qemu_io('-f', 'raw', '-c', 'write -P 0x1 0 32M', backing_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img,
+                 '-F', 'raw', test_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 32M 32M', test_img)
         self.vm = iotests.VM().add_drive('blkdebug::' + test_img)
         self.vm.launch()
diff --git a/tests/qemu-iotests/034 b/tests/qemu-iotests/034
index da4cea1571e4..ac2d687c71da 100755
--- a/tests/qemu-iotests/034
+++ b/tests/qemu-iotests/034
@@ -58,7 +58,7 @@ $QEMU_IO -c "write -P 0x55 0 1M" "$TEST_IMG" | _filter_qemu_io

 TEST_IMG="$TEST_IMG_SAVE"

-_make_test_img -b "$TEST_IMG.base" 6G
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G

 echo
 echo "== zero write with backing file =="
diff --git a/tests/qemu-iotests/034.out b/tests/qemu-iotests/034.out
index 0764ead8b95d..478205ad2542 100644
--- a/tests/qemu-iotests/034.out
+++ b/tests/qemu-iotests/034.out
@@ -4,7 +4,7 @@ QA output created by 034
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
 wrote 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == zero write with backing file ==
 wrote 196608/196608 bytes at offset 65536
diff --git a/tests/qemu-iotests/037 b/tests/qemu-iotests/037
index e6517acbd4cd..e1187ac24a0b 100755
--- a/tests/qemu-iotests/037
+++ b/tests/qemu-iotests/037
@@ -74,7 +74,7 @@ backing_io 0 256 write | $QEMU_IO "$TEST_IMG" | _filter_qemu_io

 TEST_IMG="$TEST_IMG_SAVE"

-_make_test_img -b "$TEST_IMG.base" 6G
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G

 echo
 echo "== COW in a single cluster =="
diff --git a/tests/qemu-iotests/037.out b/tests/qemu-iotests/037.out
index cd6710c90160..30ef989b64d6 100644
--- a/tests/qemu-iotests/037.out
+++ b/tests/qemu-iotests/037.out
@@ -514,7 +514,7 @@ wrote 512/512 bytes at offset 130048
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 512/512 bytes at offset 130560
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == COW in a single cluster ==
 wrote 2048/2048 bytes at offset 0
diff --git a/tests/qemu-iotests/038 b/tests/qemu-iotests/038
index 707e2d72e92d..a253231f5baf 100755
--- a/tests/qemu-iotests/038
+++ b/tests/qemu-iotests/038
@@ -71,7 +71,7 @@ backing_io 0 256 write | $QEMU_IO "$TEST_IMG" | _filter_qemu_io

 TEST_IMG="$TEST_IMG_SAVE"

-_make_test_img -b "$TEST_IMG.base" 6G
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G

 echo
 echo "== Some concurrent requests touching the same cluster =="
diff --git a/tests/qemu-iotests/038.out b/tests/qemu-iotests/038.out
index 0bdfb19faa63..fe2108593ae6 100644
--- a/tests/qemu-iotests/038.out
+++ b/tests/qemu-iotests/038.out
@@ -514,7 +514,7 @@ wrote 65536/65536 bytes at offset 16646144
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 16711680
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == Some concurrent requests touching the same cluster ==
 wrote 65536/65536 bytes at offset XXX
diff --git a/tests/qemu-iotests/039 b/tests/qemu-iotests/039
index ddce48ab4752..42f65031383a 100755
--- a/tests/qemu-iotests/039
+++ b/tests/qemu-iotests/039
@@ -133,7 +133,8 @@ echo "== Committing to a backing file with lazy_refcounts=on =="

 TEST_IMG="$TEST_IMG".base _make_test_img -o "compat=1.1,lazy_refcounts=on" $size

-_make_test_img -o "compat=1.1,lazy_refcounts=on,backing_file=$TEST_IMG.base" $size
+_make_test_img -o "compat=1.1,lazy_refcounts=on,backing_file=$TEST_IMG.base" \
+    -F $IMGFMT $size

 $QEMU_IO -c "write 0 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG commit "$TEST_IMG"
diff --git a/tests/qemu-iotests/039.out b/tests/qemu-iotests/039.out
index bdafa3ace360..e52484d4be1b 100644
--- a/tests/qemu-iotests/039.out
+++ b/tests/qemu-iotests/039.out
@@ -66,7 +66,7 @@ No errors were found on the image.

 == Committing to a backing file with lazy_refcounts=on ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 0
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 Image committed.
diff --git a/tests/qemu-iotests/040 b/tests/qemu-iotests/040
index 32c82b4ec68a..f58f50d023e6 100755
--- a/tests/qemu-iotests/040
+++ b/tests/qemu-iotests/040
@@ -80,8 +80,11 @@ class TestSingleDrive(ImageCommitTestCase):

     def setUp(self):
         iotests.create_image(backing_img, self.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, mid_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', mid_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % mid_img,
+                 '-F', iotests.imgfmt, test_img)
         qemu_io('-f', 'raw', '-c', 'write -P 0xab 0 524288', backing_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xef 524288 524288', mid_img)
         self.vm = iotests.VM().add_drive(test_img, "node-name=top,backing.node-name=mid,backing.backing.node-name=base", interface="none")
@@ -305,10 +308,16 @@ class TestRelativePaths(ImageCommitTestCase):
             if exception.errno != errno.EEXIST:
                 raise
         iotests.create_image(self.backing_img_abs, TestRelativePaths.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % self.backing_img_abs, self.mid_img_abs)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % self.mid_img_abs, self.test_img)
-        qemu_img('rebase', '-u', '-b', self.backing_img, self.mid_img_abs)
-        qemu_img('rebase', '-u', '-b', self.mid_img, self.test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % self.backing_img_abs,
+                 '-F', 'raw', self.mid_img_abs)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % self.mid_img_abs,
+                 '-F', iotests.imgfmt, self.test_img)
+        qemu_img('rebase', '-u', '-b', self.backing_img,
+                 '-F', 'raw', self.mid_img_abs)
+        qemu_img('rebase', '-u', '-b', self.mid_img,
+                 '-F', iotests.imgfmt, self.test_img)
         qemu_io('-f', 'raw', '-c', 'write -P 0xab 0 524288', self.backing_img_abs)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xef 524288 524288', self.mid_img_abs)
         self.vm = iotests.VM().add_drive(self.test_img)
@@ -371,8 +380,11 @@ class TestSetSpeed(ImageCommitTestCase):

     def setUp(self):
         qemu_img('create', backing_img, str(TestSetSpeed.image_len))
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, mid_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', mid_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % mid_img,
+                 '-F', iotests.imgfmt, test_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x1 0 512', test_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xef 524288 524288', mid_img)
         self.vm = iotests.VM().add_drive('blkdebug::' + test_img)
@@ -410,9 +422,14 @@ class TestReopenOverlay(ImageCommitTestCase):

     def setUp(self):
         iotests.create_image(self.img0, self.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % self.img0, self.img1)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % self.img1, self.img2)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % self.img2, self.img3)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % self.img0, '-F', 'raw', self.img1)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % self.img1,
+                 '-F', iotests.imgfmt, self.img2)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % self.img2,
+                 '-F', iotests.imgfmt, self.img3)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xab 0 128K', self.img1)
         self.vm = iotests.VM().add_drive(self.img3)
         self.vm.launch()
@@ -435,8 +452,12 @@ class TestErrorHandling(iotests.QMPTestCase):

     def setUp(self):
         iotests.create_image(backing_img, self.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, mid_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % mid_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img,
+                 '-F', 'raw', mid_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % mid_img,
+                 '-F', iotests.imgfmt, test_img)

         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x11 0 512k', mid_img)
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0x22 0 512k', test_img)
diff --git a/tests/qemu-iotests/041 b/tests/qemu-iotests/041
index 5d67bf14bfe8..547d75cc491a 100755
--- a/tests/qemu-iotests/041
+++ b/tests/qemu-iotests/041
@@ -44,7 +44,8 @@ class TestSingleDrive(iotests.QMPTestCase):

     def setUp(self):
         iotests.create_image(backing_img, self.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', test_img)
         self.vm = iotests.VM().add_drive(test_img, "node-name=top,backing.node-name=base")
         if iotests.qemu_default_machine == 'pc':
             self.vm.add_drive(None, 'media=cdrom', 'ide')
@@ -157,7 +158,8 @@ class TestSingleDrive(iotests.QMPTestCase):
         self.assert_no_active_block_jobs()

         qemu_img('create', '-f', iotests.imgfmt, '-o', 'cluster_size=%d,backing_file=%s'
-                        % (self.image_len, backing_img), target_img)
+                        % (self.image_len, backing_img),
+                 '-F', 'raw', target_img)
         result = self.vm.qmp(self.qmp_cmd, device='drive0', sync='full',
                              mode='existing', target=self.qmp_target)
         self.assert_qmp(result, 'return', {})
@@ -227,7 +229,8 @@ class TestSingleBlockdev(TestSingleDrive):

     def setUp(self):
         TestSingleDrive.setUp(self)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, target_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', target_img)
         args = {'driver': iotests.imgfmt,
                 'node-name': self.qmp_target,
                 'file': { 'filename': target_img, 'driver': 'file' } }
@@ -265,7 +268,8 @@ class TestMirrorNoBacking(iotests.QMPTestCase):

     def setUp(self):
         iotests.create_image(backing_img, TestMirrorNoBacking.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', test_img)
         self.vm = iotests.VM().add_drive(test_img)
         self.vm.launch()

@@ -282,7 +286,8 @@ class TestMirrorNoBacking(iotests.QMPTestCase):
     def test_complete(self):
         self.assert_no_active_block_jobs()

-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, target_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', target_img)
         result = self.vm.qmp('drive-mirror', device='drive0', sync='full',
                              mode='existing', target=target_img)
         self.assert_qmp(result, 'return', {})
@@ -297,7 +302,8 @@ class TestMirrorNoBacking(iotests.QMPTestCase):
     def test_cancel(self):
         self.assert_no_active_block_jobs()

-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, target_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', target_img)
         result = self.vm.qmp('drive-mirror', device='drive0', sync='full',
                              mode='existing', target=target_img)
         self.assert_qmp(result, 'return', {})
@@ -316,7 +322,8 @@ class TestMirrorNoBacking(iotests.QMPTestCase):
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'size=%d'
                         %(TestMirrorNoBacking.image_len), target_backing_img)
         qemu_img('create', '-f', iotests.imgfmt, '-o', 'cluster_size=%d,backing_file=%s'
-                        % (TestMirrorNoBacking.image_len, target_backing_img), target_img)
+                        % (TestMirrorNoBacking.image_len, target_backing_img),
+                 '-F', iotests.imgfmt, target_img)

         result = self.vm.qmp('drive-mirror', device='drive0', sync='full',
                              mode='existing', target=target_img)
@@ -335,7 +342,8 @@ class TestMirrorResized(iotests.QMPTestCase):

     def setUp(self):
         iotests.create_image(backing_img, TestMirrorResized.backing_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', test_img)
         qemu_img('resize', test_img, '2M')
         self.vm = iotests.VM().add_drive(test_img)
         self.vm.launch()
@@ -481,7 +489,9 @@ new_state = "1"
         # Test COW into the target image.  The first half of the
         # cluster at MIRROR_GRANULARITY has to be copied from
         # backing_img, even though sync='top'.
-        qemu_img('create', '-f', iotests.imgfmt, '-ocluster_size=131072,backing_file=%s' %(backing_img), target_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-ocluster_size=131072,backing_file=%s' %(backing_img),
+                 '-F', 'raw', target_img)
         result = self.vm.qmp('drive-mirror', device='drive0', sync='top',
                              on_source_error='ignore',
                              mode='existing', target=target_img)
@@ -501,7 +511,8 @@ new_state = "1"
         self.vm.shutdown()

         # Detach blkdebug to compare images successfully
-        qemu_img('rebase', '-f', iotests.imgfmt, '-u', '-b', backing_img, test_img)
+        qemu_img('rebase', '-f', iotests.imgfmt, '-u', '-b', backing_img,
+                 '-F', 'raw', test_img)
         self.assertTrue(iotests.compare_images(test_img, target_img),
                         'target image does not match source after mirroring')

@@ -573,7 +584,8 @@ new_state = "1"
         self.blkdebug_file = target_img + ".blkdebug"
         iotests.create_image(backing_img, TestWriteErrors.image_len)
         self.create_blkdebug_file(self.blkdebug_file, "write_aio", 5)
-        qemu_img('create', '-f', iotests.imgfmt, '-obacking_file=%s' %(backing_img), test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-obacking_file=%s' %(backing_img), '-F', 'raw', test_img)
         self.vm = iotests.VM().add_drive(test_img)
         self.target_img = 'blkdebug:%s:%s' % (self.blkdebug_file, target_img)
         qemu_img('create', '-f', iotests.imgfmt, '-osize=%d' %(TestWriteErrors.image_len), target_img)
@@ -668,7 +680,8 @@ class TestSetSpeed(iotests.QMPTestCase):

     def setUp(self):
         qemu_img('create', backing_img, str(TestSetSpeed.image_len))
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', test_img)
         self.vm = iotests.VM().add_drive(test_img)
         self.vm.launch()

diff --git a/tests/qemu-iotests/042 b/tests/qemu-iotests/042
index 0e85b356b76a..e8f23a174ce7 100755
--- a/tests/qemu-iotests/042
+++ b/tests/qemu-iotests/042
@@ -64,8 +64,8 @@ _check_test_img
 echo
 echo "== Rebasing the image =="

-$QEMU_IMG rebase -u -b "$TEST_IMG.orig" "$TEST_IMG"
-$QEMU_IMG rebase -b "$TEST_IMG.orig" "$TEST_IMG"
+$QEMU_IMG rebase -u -b "$TEST_IMG.orig" -F $IMGFMT "$TEST_IMG"
+$QEMU_IMG rebase -b "$TEST_IMG.orig" -F $IMGFMT "$TEST_IMG"
 _check_test_img

 # success, all done
diff --git a/tests/qemu-iotests/043 b/tests/qemu-iotests/043
index b102e49208b8..e4022b7044de 100755
--- a/tests/qemu-iotests/043
+++ b/tests/qemu-iotests/043
@@ -46,7 +46,7 @@ _supported_proto file

 size=128M
 _make_test_img $size
-$QEMU_IMG rebase -u -b "$TEST_IMG" "$TEST_IMG"
+$QEMU_IMG rebase -u -b "$TEST_IMG" -F $IMGFMT "$TEST_IMG"

 echo
 echo "== backing file references self =="
@@ -54,8 +54,8 @@ _img_info --backing-chain

 _make_test_img $size
 mv "$TEST_IMG" "$TEST_IMG.base"
-_make_test_img -b "$TEST_IMG.base" $size
-$QEMU_IMG rebase -u -b "$TEST_IMG" "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $size
+$QEMU_IMG rebase -u -b "$TEST_IMG" -F $IMGFMT "$TEST_IMG.base"

 echo
 echo "== parent references self =="
@@ -63,12 +63,12 @@ _img_info --backing-chain

 _make_test_img $size
 mv "$TEST_IMG" "$TEST_IMG.1.base"
-_make_test_img -b "$TEST_IMG.1.base" $size
+_make_test_img -b "$TEST_IMG.1.base" -F $IMGFMT $size
 mv "$TEST_IMG" "$TEST_IMG.2.base"
-_make_test_img -b "$TEST_IMG.2.base" $size
+_make_test_img -b "$TEST_IMG.2.base" -F $IMGFMT $size
 mv "$TEST_IMG" "$TEST_IMG.3.base"
-_make_test_img -b "$TEST_IMG.3.base" $size
-$QEMU_IMG rebase -u -b "$TEST_IMG.2.base" "$TEST_IMG.1.base"
+_make_test_img -b "$TEST_IMG.3.base" -F $IMGFMT $size
+$QEMU_IMG rebase -u -b "$TEST_IMG.2.base" -F $IMGFMT "$TEST_IMG.1.base"

 echo
 echo "== ancestor references another ancestor =="
@@ -76,9 +76,9 @@ _img_info --backing-chain

 _make_test_img $size
 mv "$TEST_IMG" "$TEST_IMG.1.base"
-_make_test_img -b "$TEST_IMG.1.base" $size
+_make_test_img -b "$TEST_IMG.1.base" -F $IMGFMT $size
 mv "$TEST_IMG" "$TEST_IMG.2.base"
-_make_test_img -b "$TEST_IMG.2.base" $size
+_make_test_img -b "$TEST_IMG.2.base" -F $IMGFMT $size

 echo
 echo "== finite chain of length 3 (human) =="
diff --git a/tests/qemu-iotests/043.out b/tests/qemu-iotests/043.out
index 0280f999ce12..7251e26d4ff9 100644
--- a/tests/qemu-iotests/043.out
+++ b/tests/qemu-iotests/043.out
@@ -4,20 +4,20 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
 == backing file references self ==
 qemu-img: Backing file 'TEST_DIR/t.IMGFMT' creates an infinite loop.
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == parent references self ==
 qemu-img: Backing file 'TEST_DIR/t.IMGFMT' creates an infinite loop.
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.1.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.2.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.3.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.1.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.2.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.3.base backing_fmt=IMGFMT

 == ancestor references another ancestor ==
 qemu-img: Backing file 'TEST_DIR/t.IMGFMT.2.base' creates an infinite loop.
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.1.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.2.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.1.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.2.base backing_fmt=IMGFMT

 == finite chain of length 3 (human) ==
 image: TEST_DIR/t.IMGFMT
@@ -25,12 +25,14 @@ file format: IMGFMT
 virtual size: 128 MiB (134217728 bytes)
 cluster_size: 65536
 backing file: TEST_DIR/t.IMGFMT.2.base
+backing file format: IMGFMT

 image: TEST_DIR/t.IMGFMT.2.base
 file format: IMGFMT
 virtual size: 128 MiB (134217728 bytes)
 cluster_size: 65536
 backing file: TEST_DIR/t.IMGFMT.1.base
+backing file format: IMGFMT

 image: TEST_DIR/t.IMGFMT.1.base
 file format: IMGFMT
@@ -40,6 +42,7 @@ cluster_size: 65536
 == finite chain of length 3 (json) ==
 [
     {
+        "backing-filename-format": "IMGFMT",
         "virtual-size": 134217728,
         "filename": "TEST_DIR/t.IMGFMT",
         "cluster-size": 65536,
@@ -49,6 +52,7 @@ cluster_size: 65536
         "dirty-flag": false
     },
     {
+        "backing-filename-format": "IMGFMT",
         "virtual-size": 134217728,
         "filename": "TEST_DIR/t.IMGFMT.2.base",
         "cluster-size": 65536,
diff --git a/tests/qemu-iotests/046 b/tests/qemu-iotests/046
index a066eec60598..7384136dc016 100755
--- a/tests/qemu-iotests/046
+++ b/tests/qemu-iotests/046
@@ -69,7 +69,7 @@ backing_io 0 32 write | $QEMU_IO "$TEST_IMG" | _filter_qemu_io

 mv "$TEST_IMG" "$TEST_IMG.base"

-_make_test_img -b "$TEST_IMG.base" 6G
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 6G

 echo
 echo "== Some concurrent requests touching the same cluster =="
diff --git a/tests/qemu-iotests/046.out b/tests/qemu-iotests/046.out
index ca2c7404a914..1a9dfe200f6f 100644
--- a/tests/qemu-iotests/046.out
+++ b/tests/qemu-iotests/046.out
@@ -66,7 +66,7 @@ wrote 65536/65536 bytes at offset 1966080
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 2031616
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=6442450944 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == Some concurrent requests touching the same cluster ==
 blkdebug: Suspended request 'A'
diff --git a/tests/qemu-iotests/050 b/tests/qemu-iotests/050
index cdc53565410d..4b0a390c437c 100755
--- a/tests/qemu-iotests/050
+++ b/tests/qemu-iotests/050
@@ -58,13 +58,13 @@ $QEMU_IO -c "write -P 0x5a 0 1048576" "$TEST_IMG" | _filter_qemu_io

 TEST_IMG="$TEST_IMG_SAVE"

-_make_test_img -b "$TEST_IMG.old" $size
+_make_test_img -b "$TEST_IMG.old" -F $IMGFMT $size
 $QEMU_IO -c "write -z 0 1048576" "$TEST_IMG" | _filter_qemu_io

 echo
 echo "== Rebasing the image =="

-$QEMU_IMG rebase -b "$TEST_IMG.new" "$TEST_IMG"
+$QEMU_IMG rebase -b "$TEST_IMG.new" -F $IMGFMT "$TEST_IMG"
 $QEMU_IO -c "read -P 0x00 0 1048576" "$TEST_IMG" | _filter_qemu_io

 # success, all done
diff --git a/tests/qemu-iotests/050.out b/tests/qemu-iotests/050.out
index 3602d580dc05..ab3daeddca3e 100644
--- a/tests/qemu-iotests/050.out
+++ b/tests/qemu-iotests/050.out
@@ -7,7 +7,7 @@ wrote 1048576/1048576 bytes at offset 0
 Formatting 'TEST_DIR/t.IMGFMT.new', fmt=IMGFMT size=10485760
 wrote 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=10485760 backing_file=TEST_DIR/t.IMGFMT.old
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=10485760 backing_file=TEST_DIR/t.IMGFMT.old backing_fmt=IMGFMT
 wrote 1048576/1048576 bytes at offset 0
 1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

diff --git a/tests/qemu-iotests/051 b/tests/qemu-iotests/051
index 034d3a325077..bee26075b207 100755
--- a/tests/qemu-iotests/051
+++ b/tests/qemu-iotests/051
@@ -70,7 +70,7 @@ device_id="drive0"
 _make_test_img $size
 cp "$TEST_IMG" "$TEST_IMG.orig"
 mv "$TEST_IMG" "$TEST_IMG.base"
-_make_test_img -b "$TEST_IMG.base" $size
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $size

 echo
 echo === Unknown option ===
diff --git a/tests/qemu-iotests/051.out b/tests/qemu-iotests/051.out
index 554c5ca90a02..de4771bcb36d 100644
--- a/tests/qemu-iotests/051.out
+++ b/tests/qemu-iotests/051.out
@@ -1,6 +1,6 @@
 QA output created by 051
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 === Unknown option ===

diff --git a/tests/qemu-iotests/051.pc.out b/tests/qemu-iotests/051.pc.out
index 0ea80d35f0e2..0a261753faac 100644
--- a/tests/qemu-iotests/051.pc.out
+++ b/tests/qemu-iotests/051.pc.out
@@ -1,6 +1,6 @@
 QA output created by 051
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 === Unknown option ===

diff --git a/tests/qemu-iotests/056 b/tests/qemu-iotests/056
index f73fc74457df..052456aa003f 100755
--- a/tests/qemu-iotests/056
+++ b/tests/qemu-iotests/056
@@ -54,7 +54,8 @@ class TestSyncModesNoneAndTop(iotests.QMPTestCase):

     def setUp(self):
         create_image(backing_img, TestSyncModesNoneAndTop.image_len)
-        qemu_img('create', '-f', iotests.imgfmt, '-o', 'backing_file=%s' % backing_img, test_img)
+        qemu_img('create', '-f', iotests.imgfmt,
+                 '-o', 'backing_file=%s' % backing_img, '-F', 'raw', test_img)
         qemu_io('-c', 'write -P0x41 0 512', test_img)
         qemu_io('-c', 'write -P0xd5 1M 32k', test_img)
         qemu_io('-c', 'write -P0xdc 32M 124k', test_img)
diff --git a/tests/qemu-iotests/060 b/tests/qemu-iotests/060
index 043f12904aab..2f0894e255ea 100755
--- a/tests/qemu-iotests/060
+++ b/tests/qemu-iotests/060
@@ -163,7 +163,7 @@ $QEMU_IO -c 'write 0k 64k' "$BACKING_IMG" | _filter_qemu_io
 # compat=0.10 is required in order to make the following discard actually
 # unallocate the sector rather than make it a zero sector - we want COW, after
 # all.
-_make_test_img -o 'compat=0.10' -b "$BACKING_IMG" 1G
+_make_test_img -o 'compat=0.10' -b "$BACKING_IMG" -F $IMGFMT 1G
 # Write two clusters, the second one enforces creation of an L2 table after
 # the first data cluster.
 $QEMU_IO -c 'write 0k 64k' -c 'write 512M 64k' "$TEST_IMG" | _filter_qemu_io
diff --git a/tests/qemu-iotests/060.out b/tests/qemu-iotests/060.out
index d27692a33c0d..6b0a3b360d41 100644
--- a/tests/qemu-iotests/060.out
+++ b/tests/qemu-iotests/060.out
@@ -100,7 +100,7 @@ read 512/512 bytes at offset 0
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1073741824
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1073741824 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 536870912
diff --git a/tests/qemu-iotests/061 b/tests/qemu-iotests/061
index ce285d308408..4c8c776a91c6 100755
--- a/tests/qemu-iotests/061
+++ b/tests/qemu-iotests/061
@@ -189,7 +189,7 @@ echo "=== Testing zero expansion on backed image ==="
 echo
 TEST_IMG="$TEST_IMG.base" _make_test_img -o "compat=1.1" 64M
 $QEMU_IO -c "write -P 0x2a 0 128k" "$TEST_IMG.base" | _filter_qemu_io
-_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" 64M
+_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" -F $IMGFMT 64M
 $QEMU_IO -c "read -P 0x2a 0 128k" -c "write -z 0 64k" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG amend -o "compat=0.10" "$TEST_IMG"
 _check_test_img
@@ -200,7 +200,7 @@ echo "=== Testing zero expansion on backed inactive clusters ==="
 echo
 TEST_IMG="$TEST_IMG.base" _make_test_img -o "compat=1.1" 64M
 $QEMU_IO -c "write -P 0x2a 0 128k" "$TEST_IMG.base" | _filter_qemu_io
-_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" 64M
+_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" -F $IMGFMT 64M
 $QEMU_IO -c "write -z 0 64k" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG snapshot -c foo "$TEST_IMG"
 $QEMU_IO -c "write -P 0x42 0 128k" "$TEST_IMG" | _filter_qemu_io
@@ -216,7 +216,7 @@ echo "=== Testing zero expansion on backed image with shared L2 table ==="
 echo
 TEST_IMG="$TEST_IMG.base" _make_test_img -o "compat=1.1" 64M
 $QEMU_IO -c "write -P 0x2a 0 128k" "$TEST_IMG.base" | _filter_qemu_io
-_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" 64M
+_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" -F $IMGFMT 64M
 $QEMU_IO -c "write -z 0 128k" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG snapshot -c foo "$TEST_IMG"
 $QEMU_IMG amend -o "compat=0.10" "$TEST_IMG"
@@ -239,7 +239,7 @@ echo
 echo "=== Testing progress report without snapshot ==="
 echo
 TEST_IMG="$TEST_IMG.base" _make_test_img -o "compat=1.1" 4G
-_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" 4G
+_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" -F $IMGFMT 4G
 $QEMU_IO -c "write -z 0  64k" \
          -c "write -z 1G 64k" \
          -c "write -z 2G 64k" \
@@ -251,7 +251,7 @@ echo
 echo "=== Testing progress report with snapshot ==="
 echo
 TEST_IMG="$TEST_IMG.base" _make_test_img -o "compat=1.1" 4G
-_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" 4G
+_make_test_img -o "compat=1.1" -b "$TEST_IMG.base" -F $IMGFMT 4G
 $QEMU_IO -c "write -z 0  64k" \
          -c "write -z 1G 64k" \
          -c "write -z 2G 64k" \
diff --git a/tests/qemu-iotests/061.out b/tests/qemu-iotests/061.out
index 048248c121cb..0f005d2e9be4 100644
--- a/tests/qemu-iotests/061.out
+++ b/tests/qemu-iotests/061.out
@@ -396,7 +396,7 @@ read 131072/131072 bytes at offset 0
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 read 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 0
@@ -412,7 +412,7 @@ read 65536/65536 bytes at offset 65536
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 0
@@ -431,7 +431,7 @@ read 65536/65536 bytes at offset 65536
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 131072/131072 bytes at offset 0
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.
@@ -455,7 +455,7 @@ read 67108864/67108864 bytes at offset 0
 === Testing progress report without snapshot ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=4294967296
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 1073741824
@@ -470,7 +470,7 @@ No errors were found on the image.
 === Testing progress report with snapshot ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=4294967296
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 65536/65536 bytes at offset 1073741824
diff --git a/tests/qemu-iotests/069 b/tests/qemu-iotests/069
index b997b127f025..a4da83b2d9da 100755
--- a/tests/qemu-iotests/069
+++ b/tests/qemu-iotests/069
@@ -46,7 +46,7 @@ echo
 echo "=== Creating an image with a backing file and deleting that file ==="
 echo
 TEST_IMG="$TEST_IMG.base" _make_test_img $IMG_SIZE
-_make_test_img -b "$TEST_IMG.base" $IMG_SIZE
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $IMG_SIZE
 _rm_test_img "$TEST_IMG.base"
 # Just open the image and close it right again (this should print an error message)
 $QEMU_IO -c quit "$TEST_IMG" 2>&1 | _filter_testdir | _filter_imgfmt
diff --git a/tests/qemu-iotests/069.out b/tests/qemu-iotests/069.out
index c78e8c2b7204..126b4d2d519b 100644
--- a/tests/qemu-iotests/069.out
+++ b/tests/qemu-iotests/069.out
@@ -3,6 +3,6 @@ QA output created by 069
 === Creating an image with a backing file and deleting that file ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=131072
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=131072 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 qemu-io: can't open device TEST_DIR/t.IMGFMT: Could not open backing file: Could not open 'TEST_DIR/t.IMGFMT.base': No such file or directory
 *** done
diff --git a/tests/qemu-iotests/073 b/tests/qemu-iotests/073
index 23a1bdf89056..1dce47870914 100755
--- a/tests/qemu-iotests/073
+++ b/tests/qemu-iotests/073
@@ -52,7 +52,7 @@ echo "== creating backing file =="

 TEST_IMG="$TEST_IMG.base" _make_test_img $size

-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT
 $QEMU_IO -c "write -P 0xa5 0 $size" "$TEST_IMG.base" | _filter_qemu_io

 echo
diff --git a/tests/qemu-iotests/073.out b/tests/qemu-iotests/073.out
index de5452492c70..7a718b525e68 100644
--- a/tests/qemu-iotests/073.out
+++ b/tests/qemu-iotests/073.out
@@ -2,7 +2,7 @@ QA output created by 073

 == creating backing file ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 134217728/134217728 bytes at offset 0
 128 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

diff --git a/tests/qemu-iotests/082 b/tests/qemu-iotests/082
index 3286c2c6dbb7..52957fd86c2f 100755
--- a/tests/qemu-iotests/082
+++ b/tests/qemu-iotests/082
@@ -83,8 +83,10 @@ run_qemu_img create -f $IMGFMT -o cluster_size=4k -o help "$TEST_IMG" $size
 run_qemu_img create -f $IMGFMT -o cluster_size=4k -o \? "$TEST_IMG" $size

 # Looks like a help option, but is part of the backing file name
-run_qemu_img create -f $IMGFMT -u -o backing_file="$TEST_IMG",,help "$TEST_IMG" $size
-run_qemu_img create -f $IMGFMT -u -o backing_file="$TEST_IMG",,\? "$TEST_IMG" $size
+run_qemu_img create -f $IMGFMT -u -o backing_file="$TEST_IMG",,help \
+    -F $IMGFMT "$TEST_IMG" $size
+run_qemu_img create -f $IMGFMT -u -o backing_file="$TEST_IMG",,\? \
+    -F $IMGFMT "$TEST_IMG" $size

 # Try to trick qemu-img into creating escaped commas
 run_qemu_img create -f $IMGFMT -o backing_file="$TEST_IMG", -o help "$TEST_IMG" $size
@@ -140,8 +142,8 @@ run_qemu_img convert -O $IMGFMT -o cluster_size=4k -o help "$TEST_IMG" "$TEST_IM
 run_qemu_img convert -O $IMGFMT -o cluster_size=4k -o \? "$TEST_IMG" "$TEST_IMG".base

 # Looks like a help option, but is part of the backing file name
-run_qemu_img convert -O $IMGFMT -o backing_file="$TEST_IMG",,help "$TEST_IMG" "$TEST_IMG".base
-run_qemu_img convert -O $IMGFMT -o backing_file="$TEST_IMG",,\? "$TEST_IMG" "$TEST_IMG".base
+run_qemu_img convert -O $IMGFMT -o backing_fmt=$IMGFMT,backing_file="$TEST_IMG",,help "$TEST_IMG" "$TEST_IMG".base
+run_qemu_img convert -O $IMGFMT -o backing_fmt=$IMGFMT,backing_file="$TEST_IMG",,\? "$TEST_IMG" "$TEST_IMG".base

 # Try to trick qemu-img into creating escaped commas
 run_qemu_img convert -O $IMGFMT -o backing_file="$TEST_IMG", -o help "$TEST_IMG" "$TEST_IMG".base
diff --git a/tests/qemu-iotests/082.out b/tests/qemu-iotests/082.out
index 94ea990a2754..813e9b1a949f 100644
--- a/tests/qemu-iotests/082.out
+++ b/tests/qemu-iotests/082.out
@@ -226,11 +226,11 @@ Supported options:
   refcount_bits=<num>    - Width of a reference count entry in bits
   size=<size>            - Virtual disk size

-Testing: create -f qcow2 -u -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2 128M
-Formatting 'TEST_DIR/t.qcow2', fmt=qcow2 size=134217728 backing_file=TEST_DIR/t.qcow2,,help cluster_size=65536 lazy_refcounts=off refcount_bits=16
+Testing: create -f qcow2 -u -o backing_file=TEST_DIR/t.qcow2,,help -F qcow2 TEST_DIR/t.qcow2 128M
+Formatting 'TEST_DIR/t.qcow2', fmt=qcow2 size=134217728 backing_file=TEST_DIR/t.qcow2,,help backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16

-Testing: create -f qcow2 -u -o backing_file=TEST_DIR/t.qcow2,,? TEST_DIR/t.qcow2 128M
-Formatting 'TEST_DIR/t.qcow2', fmt=qcow2 size=134217728 backing_file=TEST_DIR/t.qcow2,,? cluster_size=65536 lazy_refcounts=off refcount_bits=16
+Testing: create -f qcow2 -u -o backing_file=TEST_DIR/t.qcow2,,? -F qcow2 TEST_DIR/t.qcow2 128M
+Formatting 'TEST_DIR/t.qcow2', fmt=qcow2 size=134217728 backing_file=TEST_DIR/t.qcow2,,? backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16

 Testing: create -f qcow2 -o backing_file=TEST_DIR/t.qcow2, -o help TEST_DIR/t.qcow2 128M
 qemu-img: Invalid option list: backing_file=TEST_DIR/t.qcow2,
@@ -506,10 +506,10 @@ Supported options:
   refcount_bits=<num>    - Width of a reference count entry in bits
   size=<size>            - Virtual disk size

-Testing: convert -O qcow2 -o backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
+Testing: convert -O qcow2 -o backing_fmt=qcow2,backing_file=TEST_DIR/t.qcow2,,help TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 qemu-img: Could not open 'TEST_DIR/t.qcow2.base': Could not open backing file: Could not open 'TEST_DIR/t.qcow2,help': No such file or directory

-Testing: convert -O qcow2 -o backing_file=TEST_DIR/t.qcow2,,? TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
+Testing: convert -O qcow2 -o backing_fmt=qcow2,backing_file=TEST_DIR/t.qcow2,,? TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
 qemu-img: Could not open 'TEST_DIR/t.qcow2.base': Could not open backing file: Could not open 'TEST_DIR/t.qcow2,?': No such file or directory

 Testing: convert -O qcow2 -o backing_file=TEST_DIR/t.qcow2, -o help TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.base
diff --git a/tests/qemu-iotests/085 b/tests/qemu-iotests/085
index dd3c993a2d57..e99eb44581e8 100755
--- a/tests/qemu-iotests/085
+++ b/tests/qemu-iotests/085
@@ -106,7 +106,7 @@ add_snapshot_image()
 {
     base_image="${TEST_DIR}/$((${1}-1))-${snapshot_virt0}"
     snapshot_file="${TEST_DIR}/${1}-${snapshot_virt0}"
-    TEST_IMG=$snapshot_file _make_test_img -u -b "${base_image}" "$size"
+    TEST_IMG=$snapshot_file _make_test_img -u -b "${base_image}" -F $IMGFMT "$size"
     do_blockdev_add "$1" "'backing': null, " "${snapshot_file}"
 }

@@ -225,7 +225,7 @@ echo
 SNAPSHOTS=$((${SNAPSHOTS}+1))

 TEST_IMG="$TEST_IMG.base" _make_test_img "$size"
-_make_test_img -b "${TEST_IMG}.base" "$size"
+_make_test_img -b "${TEST_IMG}.base" -F $IMGFMT "$size"
 do_blockdev_add ${SNAPSHOTS} "" "${TEST_IMG}"
 blockdev_snapshot ${SNAPSHOTS} error

diff --git a/tests/qemu-iotests/085.out b/tests/qemu-iotests/085.out
index fd11aae67822..b84b20bd5741 100644
--- a/tests/qemu-iotests/085.out
+++ b/tests/qemu-iotests/085.out
@@ -68,12 +68,12 @@ Formatting 'TEST_DIR/10-snapshot-v1.qcow2', fmt=qcow2 size=134217728 backing_fil

 === Create a couple of snapshots using blockdev-snapshot ===

-Formatting 'TEST_DIR/11-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/10-snapshot-v0.IMGFMT
+Formatting 'TEST_DIR/11-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/10-snapshot-v0.IMGFMT backing_fmt=IMGFMT
 { 'execute': 'blockdev-add', 'arguments': { 'driver': 'IMGFMT', 'node-name': 'snap_11', 'backing': null, 'file': { 'driver': 'file', 'filename': 'TEST_DIR/11-snapshot-v0.IMGFMT', 'node-name': 'file_11' } } }
 {"return": {}}
 { 'execute': 'blockdev-snapshot', 'arguments': { 'node': 'virtio0', 'overlay':'snap_11' } }
 {"return": {}}
-Formatting 'TEST_DIR/12-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/11-snapshot-v0.IMGFMT
+Formatting 'TEST_DIR/12-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/11-snapshot-v0.IMGFMT backing_fmt=IMGFMT
 { 'execute': 'blockdev-add', 'arguments': { 'driver': 'IMGFMT', 'node-name': 'snap_12', 'backing': null, 'file': { 'driver': 'file', 'filename': 'TEST_DIR/12-snapshot-v0.IMGFMT', 'node-name': 'file_12' } } }
 {"return": {}}
 { 'execute': 'blockdev-snapshot', 'arguments': { 'node': 'virtio0', 'overlay':'snap_12' } }
@@ -101,7 +101,7 @@ Formatting 'TEST_DIR/12-snapshot-v0.IMGFMT', fmt=IMGFMT size=134217728 backing_f
 === Invalid command - snapshot node has a backing image ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 { 'execute': 'blockdev-add', 'arguments': { 'driver': 'IMGFMT', 'node-name': 'snap_13', 'file': { 'driver': 'file', 'filename': 'TEST_DIR/t.IMGFMT', 'node-name': 'file_13' } } }
 {"return": {}}
 { 'execute': 'blockdev-snapshot', 'arguments': { 'node': 'virtio0', 'overlay':'snap_13' } }
diff --git a/tests/qemu-iotests/089 b/tests/qemu-iotests/089
index 059ad75e28d6..66c5415abe8c 100755
--- a/tests/qemu-iotests/089
+++ b/tests/qemu-iotests/089
@@ -84,7 +84,7 @@ echo
 echo "=== Testing correct handling of 'backing':null ==="
 echo

-_make_test_img -b "$TEST_IMG.base" $IMG_SIZE
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $IMG_SIZE

 # This should read 42
 $QEMU_IO -c 'read -P 42 0 512' "$TEST_IMG" | _filter_qemu_io
diff --git a/tests/qemu-iotests/089.out b/tests/qemu-iotests/089.out
index 20c8ce8f0ef4..15682c28862a 100644
--- a/tests/qemu-iotests/089.out
+++ b/tests/qemu-iotests/089.out
@@ -21,7 +21,7 @@ read 512/512 bytes at offset 0

 === Testing correct handling of 'backing':null ===

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 read 512/512 bytes at offset 0
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 512/512 bytes at offset 0
diff --git a/tests/qemu-iotests/095 b/tests/qemu-iotests/095
index 155ae86aa780..77a5f0f3f597 100755
--- a/tests/qemu-iotests/095
+++ b/tests/qemu-iotests/095
@@ -51,9 +51,9 @@ size_larger=100M

 TEST_IMG="$TEST_IMG.base" _make_test_img $size_smaller

-TEST_IMG="$TEST_IMG.snp1" _make_test_img -b "$TEST_IMG.base" $size_larger
+TEST_IMG="$TEST_IMG.snp1" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT $size_larger

-_make_test_img -b "${TEST_IMG}.snp1" $size_larger
+_make_test_img -b "${TEST_IMG}.snp1" -F $IMGFMT $size_larger

 echo
 echo "=== Base image info before commit and resize ==="
diff --git a/tests/qemu-iotests/095.out b/tests/qemu-iotests/095.out
index 614e5ccbbb38..e66ced58f855 100644
--- a/tests/qemu-iotests/095.out
+++ b/tests/qemu-iotests/095.out
@@ -1,7 +1,7 @@
 QA output created by 095
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=5242880
-Formatting 'TEST_DIR/t.IMGFMT.snp1', fmt=IMGFMT size=104857600 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=104857600 backing_file=TEST_DIR/t.IMGFMT.snp1
+Formatting 'TEST_DIR/t.IMGFMT.snp1', fmt=IMGFMT size=104857600 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=104857600 backing_file=TEST_DIR/t.IMGFMT.snp1 backing_fmt=IMGFMT

 === Base image info before commit and resize ===
 image: TEST_DIR/t.IMGFMT.base
diff --git a/tests/qemu-iotests/097 b/tests/qemu-iotests/097
index 690f3d3ce1ed..d910a8b107ea 100755
--- a/tests/qemu-iotests/097
+++ b/tests/qemu-iotests/097
@@ -67,8 +67,8 @@ echo

 len=$((2100 * 1024 * 1024 + 512)) # larger than 2G, and not cluster aligned
 TEST_IMG="$TEST_IMG.base" _make_test_img $len
-TEST_IMG="$TEST_IMG.itmd" _make_test_img -b "$TEST_IMG.base" $len
-_make_test_img -b "$TEST_IMG.itmd" $len
+TEST_IMG="$TEST_IMG.itmd" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT $len
+_make_test_img -b "$TEST_IMG.itmd" -F $IMGFMT $len

 $QEMU_IO -c "write -P 1 0x7ffd0000 192k" "$TEST_IMG.base" | _filter_qemu_io
 $QEMU_IO -c "write -P 2 0x7ffe0000 128k" "$TEST_IMG.itmd" | _filter_qemu_io
diff --git a/tests/qemu-iotests/097.out b/tests/qemu-iotests/097.out
index f6705a1cc74a..3038ddab3100 100644
--- a/tests/qemu-iotests/097.out
+++ b/tests/qemu-iotests/097.out
@@ -3,8 +3,8 @@ QA output created by 097
 === Test pass 0 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -40,8 +40,8 @@ Offset          Length          File
 === Test pass 1 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -78,8 +78,8 @@ Offset          Length          File
 === Test pass 2 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -116,8 +116,8 @@ Offset          Length          File
 === Test pass 3 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
diff --git a/tests/qemu-iotests/098 b/tests/qemu-iotests/098
index 1e29d96b3d7e..f2ccdd790980 100755
--- a/tests/qemu-iotests/098
+++ b/tests/qemu-iotests/098
@@ -52,7 +52,7 @@ echo "=== $event ==="
 echo

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
-_make_test_img -b "$TEST_IMG.base" 64M
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT 64M

 # Some data that can be leaked when emptying the top image
 $QEMU_IO -c 'write 0 64k' "$TEST_IMG" | _filter_qemu_io
diff --git a/tests/qemu-iotests/098.out b/tests/qemu-iotests/098.out
index 7634d0e8b02b..14d3018ecce7 100644
--- a/tests/qemu-iotests/098.out
+++ b/tests/qemu-iotests/098.out
@@ -3,7 +3,7 @@ QA output created by 098
 === l1_update ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
@@ -12,7 +12,7 @@ No errors were found on the image.
 === empty_image_prepare ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
@@ -25,7 +25,7 @@ No errors were found on the image.
 === reftable_update ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
@@ -39,7 +39,7 @@ No errors were found on the image.
 === refblock_alloc ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 0
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 qemu-img: Could not empty blkdebug:TEST_DIR/blkdebug.conf:TEST_DIR/t.IMGFMT: Input/output error
diff --git a/tests/qemu-iotests/110 b/tests/qemu-iotests/110
index 139c02c2cf6b..f1813d0dfb50 100755
--- a/tests/qemu-iotests/110
+++ b/tests/qemu-iotests/110
@@ -50,7 +50,7 @@ echo '=== Reconstructable filename ==='
 echo

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
-_make_test_img -b "$TEST_IMG_REL.base" 64M
+_make_test_img -b "$TEST_IMG_REL.base" -F $IMGFMT 64M
 # qemu should be able to reconstruct the filename, so relative backing names
 # should work
 # (We have to filter the backing file format because vmdk always
@@ -91,7 +91,7 @@ echo '=== Backing name is always relative to the backed image ==='
 echo

 # omit the image size; it should work anyway
-_make_test_img -b "$TEST_IMG_REL.base"
+_make_test_img -b "$TEST_IMG_REL.base" -F $IMGFMT

 echo
 echo '=== Nodes without a common directory ==='
diff --git a/tests/qemu-iotests/110.out b/tests/qemu-iotests/110.out
index f835553a9930..cf8f6c025d60 100644
--- a/tests/qemu-iotests/110.out
+++ b/tests/qemu-iotests/110.out
@@ -3,7 +3,7 @@ QA output created by 110
 === Reconstructable filename ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=t.IMGFMT.base backing_fmt=IMGFMT
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
 virtual size: 64 MiB (67108864 bytes)
@@ -18,7 +18,7 @@ backing file: t.IMGFMT.base (actual path: TEST_DIR/t.IMGFMT.base)

 === Backing name is always relative to the backed image ===

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=t.IMGFMT.base backing_fmt=IMGFMT

 === Nodes without a common directory ===

diff --git a/tests/qemu-iotests/122 b/tests/qemu-iotests/122
index f7a3ae684a7c..61a0d7aeec2a 100755
--- a/tests/qemu-iotests/122
+++ b/tests/qemu-iotests/122
@@ -52,9 +52,10 @@ echo
 echo "=== Check allocation status regression with -B ==="
 echo

-_make_test_img -b "$TEST_IMG".base
+_make_test_img -b "$TEST_IMG".base -F $IMGFMT
 $QEMU_IO -c "write -P 0x22 0 3M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base "$TEST_IMG" "$TEST_IMG".orig
+$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base \
+    -o backing_fmt=$IMGFMT "$TEST_IMG" "$TEST_IMG".orig
 $QEMU_IMG map "$TEST_IMG".orig | _filter_qemu_img_map


@@ -62,18 +63,22 @@ echo
 echo "=== Check that zero clusters are kept in overlay ==="
 echo

-_make_test_img -b "$TEST_IMG".base
+_make_test_img -b "$TEST_IMG".base -F $IMGFMT

 $QEMU_IO -c "write -P 0 0 3M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base "$TEST_IMG" "$TEST_IMG".orig
+$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base -o backing_fmt=$IMGFMT \
+    "$TEST_IMG" "$TEST_IMG".orig
 $QEMU_IO -c "read -P 0 0 3M" "$TEST_IMG".orig 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IMG convert -O $IMGFMT -c -B "$TEST_IMG".base "$TEST_IMG" "$TEST_IMG".orig
+$QEMU_IMG convert -O $IMGFMT -c -B "$TEST_IMG".base -o backing_fmt=$IMGFMT \
+    "$TEST_IMG" "$TEST_IMG".orig
 $QEMU_IO -c "read -P 0 0 3M" "$TEST_IMG".orig 2>&1 | _filter_qemu_io | _filter_testdir

 $QEMU_IO -c "write -z 0 3M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base "$TEST_IMG" "$TEST_IMG".orig
+$QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base -o backing_fmt=$IMGFMT \
+    "$TEST_IMG" "$TEST_IMG".orig
 $QEMU_IO -c "read -P 0 0 3M" "$TEST_IMG".orig 2>&1 | _filter_qemu_io | _filter_testdir
-$QEMU_IMG convert -O $IMGFMT -c -B "$TEST_IMG".base "$TEST_IMG" "$TEST_IMG".orig
+$QEMU_IMG convert -O $IMGFMT -c -B "$TEST_IMG".base -o backing_fmt=$IMGFMT \
+    "$TEST_IMG" "$TEST_IMG".orig
 $QEMU_IO -c "read -P 0 0 3M" "$TEST_IMG".orig 2>&1 | _filter_qemu_io | _filter_testdir


@@ -94,11 +99,11 @@ TEST_IMG="$TEST_IMG".base _make_test_img 256M
 # We do not want such a zero write, however, because we are past the
 # end of the backing file on the target as well, so we do not need to
 # write anything there.
-_make_test_img -b "$TEST_IMG".base 768M
+_make_test_img -b "$TEST_IMG".base 768M -F $IMGFMT

 # Use compat=0.10 as the output so there is no zero cluster support
 $QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base -o compat=0.10 \
-    "$TEST_IMG" "$TEST_IMG".orig
+    -o backing_fmt=$IMGFMT "$TEST_IMG" "$TEST_IMG".orig
 # See that nothing has been allocated past 64M
 $QEMU_IMG map "$TEST_IMG".orig | _filter_qemu_img_map

@@ -110,7 +115,7 @@ $QEMU_IO -c 'write -P 0x11 255M 1M' "$TEST_IMG".base 2>&1 | _filter_qemu_io
 $QEMU_IO -c 'write -P 0x22 600M 1M' "$TEST_IMG" 2>&1 | _filter_qemu_io

 $QEMU_IMG convert -O $IMGFMT -B "$TEST_IMG".base -o compat=0.10 \
-    "$TEST_IMG" "$TEST_IMG".orig
+    -o backing_fmt=$IMGFMT "$TEST_IMG" "$TEST_IMG".orig

 $QEMU_IMG map "$TEST_IMG".orig | _filter_qemu_img_map
 $QEMU_IO -c 'read -P 0x11 255M 1M' \
@@ -199,7 +204,7 @@ $QEMU_IMG map --output=json "$TEST_IMG".orig | _filter_qemu_img_map
 TEST_IMG="$TEST_IMG".base _make_test_img 64M
 $QEMU_IO -c "write -P 0x11 0 32M" "$TEST_IMG".base 2>&1 | _filter_qemu_io | _filter_testdir

-_make_test_img -b "$TEST_IMG".base 64M
+_make_test_img -b "$TEST_IMG".base 64M -F $IMGFMT
 $QEMU_IO -c "write -P 0x22 0 3M" "$TEST_IMG" 2>&1 | _filter_qemu_io | _filter_testdir

 echo
diff --git a/tests/qemu-iotests/122.out b/tests/qemu-iotests/122.out
index 1a35951a80a8..513c34ef3b9d 100644
--- a/tests/qemu-iotests/122.out
+++ b/tests/qemu-iotests/122.out
@@ -5,7 +5,7 @@ wrote 67108864/67108864 bytes at offset 0

 === Check allocation status regression with -B ===

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 3145728/3145728 bytes at offset 0
 3 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 Offset          Length          File
@@ -14,7 +14,7 @@ Offset          Length          File

 === Check that zero clusters are kept in overlay ===

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 3145728/3145728 bytes at offset 0
 3 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 read 3145728/3145728 bytes at offset 0
@@ -31,7 +31,7 @@ read 3145728/3145728 bytes at offset 0
 === Converting to an overlay larger than its backing file ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=268435456
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=805306368 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=805306368 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Offset          Length          File

 wrote 1048576/1048576 bytes at offset 267386880
@@ -141,7 +141,7 @@ read 63963136/63963136 bytes at offset 3145728
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 wrote 33554432/33554432 bytes at offset 0
 32 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 3145728/3145728 bytes at offset 0
 3 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

diff --git a/tests/qemu-iotests/126 b/tests/qemu-iotests/126
index b7fce1e59dd3..dd5a2112278d 100755
--- a/tests/qemu-iotests/126
+++ b/tests/qemu-iotests/126
@@ -61,7 +61,7 @@ BASE_IMG="$TEST_DIR/image:base.$IMGFMT"
 TOP_IMG="$TEST_DIR/image:top.$IMGFMT"

 TEST_IMG=$BASE_IMG _make_test_img 64M
-TEST_IMG=$TOP_IMG _make_test_img -b ./image:base.$IMGFMT
+TEST_IMG=$TOP_IMG _make_test_img -b ./image:base.$IMGFMT -F $IMGFMT

 # (1) The default cluster size depends on the image format
 # (2) vmdk only supports vmdk backing files, so it always reports the
@@ -82,7 +82,7 @@ BASE_IMG="base.$IMGFMT"
 TOP_IMG="file:image:top.$IMGFMT"

 TEST_IMG=$BASE_IMG _make_test_img 64M
-TEST_IMG=$TOP_IMG _make_test_img -b "$BASE_IMG"
+TEST_IMG=$TOP_IMG _make_test_img -b "$BASE_IMG" -F $IMGFMT

 TEST_IMG=$TOP_IMG _img_info | grep -ve 'cluster_size' -e 'backing file format'

diff --git a/tests/qemu-iotests/126.out b/tests/qemu-iotests/126.out
index e3c4d619162d..7d6634685eb5 100644
--- a/tests/qemu-iotests/126.out
+++ b/tests/qemu-iotests/126.out
@@ -8,14 +8,14 @@ Formatting 'file:TEST_DIR/a:b.IMGFMT', fmt=IMGFMT size=67108864
 === Testing relative backing filename resolution ===

 Formatting 'TEST_DIR/image:base.IMGFMT', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/image:top.IMGFMT', fmt=IMGFMT size=67108864 backing_file=./image:base.IMGFMT
+Formatting 'TEST_DIR/image:top.IMGFMT', fmt=IMGFMT size=67108864 backing_file=./image:base.IMGFMT backing_fmt=IMGFMT
 image: TEST_DIR/image:top.IMGFMT
 file format: IMGFMT
 virtual size: 64 MiB (67108864 bytes)
 backing file: ./image:base.IMGFMT (actual path: TEST_DIR/./image:base.IMGFMT)

 Formatting 'base.IMGFMT', fmt=IMGFMT size=67108864
-Formatting 'file:image:top.IMGFMT', fmt=IMGFMT size=67108864 backing_file=base.IMGFMT
+Formatting 'file:image:top.IMGFMT', fmt=IMGFMT size=67108864 backing_file=base.IMGFMT backing_fmt=IMGFMT
 image: ./image:top.IMGFMT
 file format: IMGFMT
 virtual size: 64 MiB (67108864 bytes)
diff --git a/tests/qemu-iotests/127 b/tests/qemu-iotests/127
index a4fc8660381b..e2ac2f55364d 100755
--- a/tests/qemu-iotests/127
+++ b/tests/qemu-iotests/127
@@ -48,8 +48,8 @@ _require_devices virtio-scsi scsi-hd
 IMG_SIZE=64K

 _make_test_img $IMG_SIZE
-TEST_IMG="$TEST_IMG.overlay0" _make_test_img -b "$TEST_IMG" $IMG_SIZE
-TEST_IMG="$TEST_IMG.overlay1" _make_test_img -b "$TEST_IMG" $IMG_SIZE
+TEST_IMG="$TEST_IMG.overlay0" _make_test_img -b "$TEST_IMG" -F $IMGFMT $IMG_SIZE
+TEST_IMG="$TEST_IMG.overlay1" _make_test_img -b "$TEST_IMG" -F $IMGFMT $IMG_SIZE

 # So that we actually have something to mirror and the job does not return
 # immediately (which may be bad because then we cannot know whether the
diff --git a/tests/qemu-iotests/127.out b/tests/qemu-iotests/127.out
index 623d52bbd150..efd6cb327fe5 100644
--- a/tests/qemu-iotests/127.out
+++ b/tests/qemu-iotests/127.out
@@ -1,7 +1,7 @@
 QA output created by 127
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=65536
-Formatting 'TEST_DIR/t.IMGFMT.overlay0', fmt=IMGFMT size=65536 backing_file=TEST_DIR/t.IMGFMT
-Formatting 'TEST_DIR/t.IMGFMT.overlay1', fmt=IMGFMT size=65536 backing_file=TEST_DIR/t.IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.overlay0', fmt=IMGFMT size=65536 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.overlay1', fmt=IMGFMT size=65536 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
 wrote 42/42 bytes at offset 0
 42 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 { 'execute': 'qmp_capabilities' }
diff --git a/tests/qemu-iotests/129 b/tests/qemu-iotests/129
index b0da4a5541ab..0e13244d850e 100755
--- a/tests/qemu-iotests/129
+++ b/tests/qemu-iotests/129
@@ -29,7 +29,8 @@ class TestStopWithBlockJob(iotests.QMPTestCase):

     def setUp(self):
         iotests.qemu_img('create', '-f', iotests.imgfmt, self.base_img, "1G")
-        iotests.qemu_img('create', '-f', iotests.imgfmt, self.test_img, "-b", self.base_img)
+        iotests.qemu_img('create', '-f', iotests.imgfmt, self.test_img,
+                         "-b", self.base_img, '-F', iotests.imgfmt)
         iotests.qemu_io('-f', iotests.imgfmt, '-c', 'write -P0x5d 1M 128M', self.test_img)
         self.vm = iotests.VM().add_drive(self.test_img)
         self.vm.launch()
diff --git a/tests/qemu-iotests/133 b/tests/qemu-iotests/133
index 6f7cacc0910b..4070fd9457b2 100755
--- a/tests/qemu-iotests/133
+++ b/tests/qemu-iotests/133
@@ -40,7 +40,7 @@ _supported_fmt qcow2
 _supported_proto file

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 echo
 echo "=== Check that node-name can't be changed ==="
diff --git a/tests/qemu-iotests/133.out b/tests/qemu-iotests/133.out
index 1491ef45a984..d70c2e8041ce 100644
--- a/tests/qemu-iotests/133.out
+++ b/tests/qemu-iotests/133.out
@@ -1,6 +1,6 @@
 QA output created by 133
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 === Check that node-name can't be changed ===

diff --git a/tests/qemu-iotests/139 b/tests/qemu-iotests/139
index 7120d3142b77..1452fd24b335 100755
--- a/tests/qemu-iotests/139
+++ b/tests/qemu-iotests/139
@@ -71,7 +71,7 @@ class TestBlockdevDel(iotests.QMPTestCase):
     def addBlockDriverStateOverlay(self, node):
         self.checkBlockDriverState(node, False)
         iotests.qemu_img('create', '-u', '-f', iotests.imgfmt,
-                         '-b', base_img, new_img, '1M')
+                         '-b', base_img, '-F', iotests.imgfmt, new_img, '1M')
         opts = {'driver': iotests.imgfmt,
                 'node-name': node,
                 'backing': None,
diff --git a/tests/qemu-iotests/141 b/tests/qemu-iotests/141
index 5192d256e314..21aa0b42d813 100755
--- a/tests/qemu-iotests/141
+++ b/tests/qemu-iotests/141
@@ -89,8 +89,8 @@ test_blockjob()


 TEST_IMG="$TEST_DIR/b.$IMGFMT" _make_test_img 1M
-TEST_IMG="$TEST_DIR/m.$IMGFMT" _make_test_img -b "$TEST_DIR/b.$IMGFMT" 1M
-_make_test_img -b "$TEST_DIR/m.$IMGFMT" 1M
+TEST_IMG="$TEST_DIR/m.$IMGFMT" _make_test_img -b "$TEST_DIR/b.$IMGFMT" -F $IMGFMT 1M
+_make_test_img -b "$TEST_DIR/m.$IMGFMT" 1M -F $IMGFMT

 _launch_qemu -nodefaults

diff --git a/tests/qemu-iotests/141.out b/tests/qemu-iotests/141.out
index 263b680bdfb3..08e0aecd65f4 100644
--- a/tests/qemu-iotests/141.out
+++ b/tests/qemu-iotests/141.out
@@ -1,7 +1,7 @@
 QA output created by 141
 Formatting 'TEST_DIR/b.IMGFMT', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/m.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/b.IMGFMT
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/m.IMGFMT
+Formatting 'TEST_DIR/m.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/b.IMGFMT backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/m.IMGFMT backing_fmt=IMGFMT
 {'execute': 'qmp_capabilities'}
 {"return": {}}

diff --git a/tests/qemu-iotests/142 b/tests/qemu-iotests/142
index daefcbaa5878..69fd10ef5103 100755
--- a/tests/qemu-iotests/142
+++ b/tests/qemu-iotests/142
@@ -67,7 +67,7 @@ size=128M

 TEST_IMG="$TEST_IMG.base" _make_test_img $size
 TEST_IMG="$TEST_IMG.snap" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base" $size
+_make_test_img -b "$TEST_IMG.base" $size -F $IMGFMT

 echo
 echo === Simple test for all cache modes ===
diff --git a/tests/qemu-iotests/142.out b/tests/qemu-iotests/142.out
index 3667e38def8a..a92b948edd95 100644
--- a/tests/qemu-iotests/142.out
+++ b/tests/qemu-iotests/142.out
@@ -1,7 +1,7 @@
 QA output created by 142
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
 Formatting 'TEST_DIR/t.IMGFMT.snap', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 === Simple test for all cache modes ===

diff --git a/tests/qemu-iotests/153 b/tests/qemu-iotests/153
index 2b1311176801..4d41f25e2ced 100755
--- a/tests/qemu-iotests/153
+++ b/tests/qemu-iotests/153
@@ -94,7 +94,7 @@ for opts1 in "" "read-only=on" "read-only=on,force-share=on"; do

     echo
     echo "== Creating test image =="
-    _make_test_img -b "${TEST_IMG}.base"
+    _make_test_img -b "${TEST_IMG}.base" -F $IMGFMT

     echo
     echo "== Launching QEMU, opts: '$opts1' =="
@@ -125,7 +125,7 @@ for opts1 in "" "read-only=on" "read-only=on,force-share=on"; do
         _run_cmd $QEMU_IMG amend -o "" $L "${TEST_IMG}"
         _run_cmd $QEMU_IMG commit      $L "${TEST_IMG}"
         _run_cmd $QEMU_IMG resize      $L "${TEST_IMG}" $size
-        _run_cmd $QEMU_IMG rebase      $L "${TEST_IMG}" -b "${TEST_IMG}.base"
+        _run_cmd $QEMU_IMG rebase      $L "${TEST_IMG}" -b "${TEST_IMG}.base" -F $IMGFMT
         _run_cmd $QEMU_IMG snapshot -l $L "${TEST_IMG}"
         _run_cmd $QEMU_IMG convert     $L "${TEST_IMG}" "${TEST_IMG}.convert"
         _run_cmd $QEMU_IMG dd          $L if="${TEST_IMG}" of="${TEST_IMG}.convert" bs=512 count=1
@@ -135,7 +135,7 @@ for opts1 in "" "read-only=on" "read-only=on,force-share=on"; do
         # qemu-img create does not support -U
         if [ -z "$L" ]; then
             _run_cmd $QEMU_IMG create -f $IMGFMT "${TEST_IMG}" \
-                                      -b ${TEST_IMG}.base
+                                      -b ${TEST_IMG}.base -F $IMGFMT
             # Read the file format.  It used to be the case that
             # file-posix simply truncated the file, but the qcow2
             # driver then failed to format it because it was unable
@@ -168,9 +168,9 @@ done
 echo
 echo "== Creating ${TEST_IMG}.[abc] ==" | _filter_testdir
 (
-    $QEMU_IMG create -f qcow2 "${TEST_IMG}.a" -b "${TEST_IMG}"
-    $QEMU_IMG create -f qcow2 "${TEST_IMG}.b" -b "${TEST_IMG}"
-    $QEMU_IMG create -f qcow2 "${TEST_IMG}.c" -b "${TEST_IMG}.b"
+    $QEMU_IMG create -f qcow2 "${TEST_IMG}.a" -b "${TEST_IMG}" -F $IMGFMT
+    $QEMU_IMG create -f qcow2 "${TEST_IMG}.b" -b "${TEST_IMG}" -F $IMGFMT
+    $QEMU_IMG create -f qcow2 "${TEST_IMG}.c" -b "${TEST_IMG}.b" -F $IMGFMT
 ) | _filter_img_create

 echo
@@ -219,7 +219,7 @@ _send_qemu_cmd $QEMU_HANDLE \
 _run_cmd $QEMU_IO "${TEST_IMG}" -c 'write 0 512'

 echo "Creating overlay with qemu-img when the guest is running should be allowed"
-_run_cmd $QEMU_IMG create -f $IMGFMT -b "${TEST_IMG}" "${TEST_IMG}.overlay"
+_run_cmd $QEMU_IMG create -f $IMGFMT -b "${TEST_IMG}" -F $IMGFMT "${TEST_IMG}.overlay"

 echo "== Closing an image should unlock it =="
 _send_qemu_cmd $QEMU_HANDLE \
diff --git a/tests/qemu-iotests/153.out b/tests/qemu-iotests/153.out
index f7464dd8d345..24fcf78d63b2 100644
--- a/tests/qemu-iotests/153.out
+++ b/tests/qemu-iotests/153.out
@@ -6,7 +6,7 @@ QEMU_PROG: -drive if=none,file=null-co://,readonly=off,force-share=on: force-sha
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=33554432

 == Creating test image ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == Launching QEMU, opts: '' ==

@@ -68,7 +68,7 @@ _qemu_img_wrapper resize TEST_DIR/t.qcow2 32M
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

-_qemu_img_wrapper rebase TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper rebase TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

@@ -92,10 +92,11 @@ _qemu_img_wrapper bench -w -c 1 TEST_DIR/t.qcow2
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

-_qemu_img_wrapper create -f qcow2 TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper create -f qcow2 TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 qemu-img: TEST_DIR/t.qcow2: Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?
 file format: IMGFMT
+backing file format: IMGFMT

 == Running utility commands -U ==

@@ -130,7 +131,7 @@ _qemu_img_wrapper resize -U TEST_DIR/t.qcow2 32M
 qemu-img: unrecognized option '-U'
 Try 'qemu-img --help' for more information

-_qemu_img_wrapper rebase -U TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper rebase -U TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

@@ -152,7 +153,7 @@ Round done
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=33554432

 == Creating test image ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == Launching QEMU, opts: 'read-only=on' ==

@@ -199,7 +200,7 @@ _qemu_img_wrapper resize TEST_DIR/t.qcow2 32M
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

-_qemu_img_wrapper rebase TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper rebase TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

@@ -215,10 +216,11 @@ _qemu_img_wrapper bench -w -c 1 TEST_DIR/t.qcow2
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

-_qemu_img_wrapper create -f qcow2 TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper create -f qcow2 TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 qemu-img: TEST_DIR/t.qcow2: Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?
 file format: IMGFMT
+backing file format: IMGFMT

 == Running utility commands -U ==

@@ -253,7 +255,7 @@ _qemu_img_wrapper resize -U TEST_DIR/t.qcow2 32M
 qemu-img: unrecognized option '-U'
 Try 'qemu-img --help' for more information

-_qemu_img_wrapper rebase -U TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper rebase -U TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 qemu-img: Could not open 'TEST_DIR/t.qcow2': Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?

@@ -275,7 +277,7 @@ Round done
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=33554432

 == Creating test image ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 == Launching QEMU, opts: 'read-only=on,force-share=on' ==

@@ -309,7 +311,7 @@ _qemu_img_wrapper commit TEST_DIR/t.qcow2

 _qemu_img_wrapper resize TEST_DIR/t.qcow2 32M

-_qemu_img_wrapper rebase TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper rebase TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2

 _qemu_img_wrapper snapshot -l TEST_DIR/t.qcow2

@@ -321,8 +323,9 @@ _qemu_img_wrapper bench -c 1 TEST_DIR/t.qcow2

 _qemu_img_wrapper bench -w -c 1 TEST_DIR/t.qcow2

-_qemu_img_wrapper create -f qcow2 TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper create -f qcow2 TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2
 file format: IMGFMT
+backing file format: IMGFMT

 == Running utility commands -U ==

@@ -357,7 +360,7 @@ _qemu_img_wrapper resize -U TEST_DIR/t.qcow2 32M
 qemu-img: unrecognized option '-U'
 Try 'qemu-img --help' for more information

-_qemu_img_wrapper rebase -U TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base
+_qemu_img_wrapper rebase -U TEST_DIR/t.qcow2 -b TEST_DIR/t.qcow2.base -F qcow2

 _qemu_img_wrapper snapshot -l -U TEST_DIR/t.qcow2

@@ -398,9 +401,9 @@ Is another process using the image [TEST_DIR/t.qcow2]?
 == Two devices with the same image (read-only=on,force-share=on - read-only=on,force-share=on) ==

 == Creating TEST_DIR/t.qcow2.[abc] ==
-Formatting 'TEST_DIR/t.IMGFMT.a', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT
-Formatting 'TEST_DIR/t.IMGFMT.b', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT
-Formatting 'TEST_DIR/t.IMGFMT.c', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.b
+Formatting 'TEST_DIR/t.IMGFMT.a', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.b', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.c', fmt=IMGFMT size=33554432 backing_file=TEST_DIR/t.IMGFMT.b backing_fmt=IMGFMT

 == Two devices sharing the same file in backing chain ==

@@ -430,7 +433,7 @@ qemu-io: can't open device TEST_DIR/t.qcow2: Failed to get "write" lock
 Is another process using the image [TEST_DIR/t.qcow2]?
 Creating overlay with qemu-img when the guest is running should be allowed

-_qemu_img_wrapper create -f qcow2 -b TEST_DIR/t.qcow2 TEST_DIR/t.qcow2.overlay
+_qemu_img_wrapper create -f qcow2 -b TEST_DIR/t.qcow2 -F qcow2 TEST_DIR/t.qcow2.overlay
 == Closing an image should unlock it ==
 { 'execute': 'human-monitor-command', 'arguments': { 'command-line': 'drive_del d0' } }
 {"return": ""}
diff --git a/tests/qemu-iotests/154 b/tests/qemu-iotests/154
index d68f66b9e058..7f1c0d9bd981 100755
--- a/tests/qemu-iotests/154
+++ b/tests/qemu-iotests/154
@@ -50,7 +50,7 @@ echo
 echo == backing file contains zeros ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Make sure that the whole cluster is allocated even for partial write_zeroes
 # when the backing file contains zeros
@@ -74,7 +74,7 @@ echo
 echo == backing file contains non-zero data before write_zeroes ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Single cluster; non-zero data at the cluster start
 # ... | XX -- 00 -- | ...
@@ -97,7 +97,7 @@ echo
 echo == backing file contains non-zero data after write_zeroes ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Single cluster; non-zero data directly after request
 # ... | -- 00 XX -- | ...
@@ -120,7 +120,7 @@ echo
 echo == write_zeroes covers non-zero data ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # non-zero data at front of request
 # Backing file: -- XX -- --
@@ -160,7 +160,7 @@ echo
 echo == spanning two clusters, non-zero before request ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Two clusters; non-zero data before request:
 # 1. At cluster start:          32k: XX -- -- 00 | 00 -- -- --
@@ -190,7 +190,7 @@ echo
 echo == spanning two clusters, non-zero after request ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Two clusters; non-zero data after request:
 # 1. Directly after request:    32k: -- -- -- 00 | 00 XX -- --
@@ -220,7 +220,7 @@ echo
 echo == spanning two clusters, partially overwriting backing file ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Backing file: -- -- XX XX | XX XX -- --
 # Active layer: -- -- XX 00 | 00 XX -- --
@@ -239,7 +239,7 @@ echo
 echo == spanning multiple clusters, non-zero in first cluster ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Backing file: 64k: XX XX -- -- | -- -- -- -- | -- -- -- --
 # Active layer: 64k: XX XX 00 00 | 00 00 00 00 | 00 -- -- --
@@ -255,7 +255,7 @@ echo
 echo == spanning multiple clusters, non-zero in intermediate cluster ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Backing file: 64k: -- -- -- -- | -- XX XX -- | -- -- -- --
 # Active layer: 64k: -- -- 00 00 | 00 00 00 00 | 00 -- -- --
@@ -270,7 +270,7 @@ echo
 echo == spanning multiple clusters, non-zero in final cluster ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Backing file: 64k: -- -- -- -- | -- -- -- -- | -- -- XX XX
 # Active layer: 64k: -- -- 00 00 | 00 00 00 00 | 00 -- XX XX
@@ -286,7 +286,7 @@ echo
 echo == spanning multiple clusters, partially overwriting backing file ==

 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Backing file: 64k: -- XX XX XX | XX XX XX XX | XX XX XX --
 # Active layer: 64k: -- XX 00 00 | 00 00 00 00 | 00 XX XX --
@@ -338,7 +338,7 @@ CLUSTER_SIZE=2048 TEST_IMG="$TEST_IMG.base" _make_test_img $((size + 1024))
 # Write at the front: sector-wise, the request is:
 # backing: 128m... | -- --
 # active:  128m... | 00 -- -- --
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $size 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -346,7 +346,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Write at the back: sector-wise, the request is:
 # backing: 128m... | -- --
 # active:  128m... | -- -- -- 00
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $((size + 1536)) 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -354,7 +354,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Write at middle: sector-wise, the request is:
 # backing: 128m... | -- --
 # active:  128m... | -- 00 00 --
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $((size + 512)) 1024" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -362,7 +362,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Write entire cluster: sector-wise, the request is:
 # backing: 128m... | -- --
 # active:  128m... | 00 00 00 00
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -373,7 +373,7 @@ $QEMU_IO -c "write -z $size 512" "$TEST_IMG.base" | _filter_qemu_io
 # Write at the front: sector-wise, the request is:
 # backing: 128m... | 00 00
 # active:  128m... | 00 -- -- --
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $size 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -381,7 +381,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Write at the back: sector-wise, the request is:
 # backing: 128m... | 00 00
 # active:  128m... | -- -- -- 00
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $((size + 1536)) 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -389,7 +389,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Write at middle: sector-wise, the request is:
 # backing: 128m... | 00 00
 # active:  128m... | -- 00 00 --
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $((size + 512)) 1024" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -397,7 +397,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Write entire cluster: sector-wise, the request is:
 # backing: 128m... | 00 00
 # active:  128m... | 00 00 00 00
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -z $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "alloc $size 2048" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
@@ -427,7 +427,7 @@ echo == unaligned image tail cluster, allocation required ==
 # Backing file: 128m... | XX --
 # Active layer: 128m... | -- -- 00 --
 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $((size + 1024))
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -P 1 $((size)) 512" "$TEST_IMG.base" | _filter_qemu_io
 $QEMU_IO -c "write -z $((size + 1024)) 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "read -P 1 $((size)) 512" "$TEST_IMG" | _filter_qemu_io
@@ -438,7 +438,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map
 # Backing file: 128m: ... | -- XX
 # Active layer: 128m: ... | 00 -- -- 00
 CLUSTER_SIZE=512 TEST_IMG="$TEST_IMG.base" _make_test_img $((size + 1024))
-_make_test_img -b "$TEST_IMG.base" $((size + 2048))
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $((size + 2048))
 $QEMU_IO -c "write -P 1 $((size + 512)) 512" "$TEST_IMG.base" | _filter_qemu_io
 $QEMU_IO -c "write -z $((size)) 512" "$TEST_IMG" | _filter_qemu_io
 $QEMU_IO -c "read -P 0 $((size)) 512" "$TEST_IMG" | _filter_qemu_io
diff --git a/tests/qemu-iotests/154.out b/tests/qemu-iotests/154.out
index fa3673317fd8..4863e2483864 100644
--- a/tests/qemu-iotests/154.out
+++ b/tests/qemu-iotests/154.out
@@ -2,7 +2,7 @@ QA output created by 154

 == backing file contains zeros ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 0
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2048/2048 bytes at offset 10240
@@ -22,7 +22,7 @@ wrote 2048/2048 bytes at offset 27648

 == backing file contains non-zero data before write_zeroes ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 32768
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 1024/1024 bytes at offset 34816
@@ -49,7 +49,7 @@ read 2048/2048 bytes at offset 67584

 == backing file contains non-zero data after write_zeroes ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 34816
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 1024/1024 bytes at offset 33792
@@ -76,7 +76,7 @@ read 3072/3072 bytes at offset 40960

 == write_zeroes covers non-zero data ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 5120
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2048/2048 bytes at offset 5120
@@ -113,7 +113,7 @@ read 4096/4096 bytes at offset 28672

 == spanning two clusters, non-zero before request ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 32768
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2048/2048 bytes at offset 35840
@@ -155,7 +155,7 @@ read 5120/5120 bytes at offset 68608

 == spanning two clusters, non-zero after request ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 37888
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2048/2048 bytes at offset 35840
@@ -197,7 +197,7 @@ read 1024/1024 bytes at offset 72704

 == spanning two clusters, partially overwriting backing file ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 4096/4096 bytes at offset 2048
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2048/2048 bytes at offset 3072
@@ -217,7 +217,7 @@ read 2048/2048 bytes at offset 6144

 == spanning multiple clusters, non-zero in first cluster ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 65536
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 7168/7168 bytes at offset 67584
@@ -233,7 +233,7 @@ read 10240/10240 bytes at offset 67584

 == spanning multiple clusters, non-zero in intermediate cluster ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 70656
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 7168/7168 bytes at offset 67584
@@ -246,7 +246,7 @@ read 12288/12288 bytes at offset 65536

 == spanning multiple clusters, non-zero in final cluster ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 75776
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 7168/7168 bytes at offset 67584
@@ -262,7 +262,7 @@ read 2048/2048 bytes at offset 75776

 == spanning multiple clusters, partially overwriting backing file ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 10240/10240 bytes at offset 66560
 10 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 7168/7168 bytes at offset 67584
@@ -305,25 +305,25 @@ wrote 2048/2048 bytes at offset 134217728
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134219776, "depth": 0, "zero": true, "data": false}]
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134219264
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 134218240
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 134217728
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
@@ -331,25 +331,25 @@ wrote 2048/2048 bytes at offset 134217728
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134219264
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 1024/1024 bytes at offset 134218240
 1 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": true, "data": false}]
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2048/2048 bytes at offset 134217728
 2 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 2048/2048 bytes allocated at offset 128 MiB
@@ -377,7 +377,7 @@ read 1024/1024 bytes at offset 134217728

 == unaligned image tail cluster, allocation required ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134217728
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 512/512 bytes at offset 134218752
@@ -389,7 +389,7 @@ read 1536/1536 bytes at offset 134218240
 [{ "start": 0, "length": 134217728, "depth": 1, "zero": true, "data": false},
 { "start": 134217728, "length": 2048, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134218752
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134219776 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 512/512 bytes at offset 134218240
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 512/512 bytes at offset 134217728
diff --git a/tests/qemu-iotests/155 b/tests/qemu-iotests/155
index 571bce9de460..ffa229f183eb 100755
--- a/tests/qemu-iotests/155
+++ b/tests/qemu-iotests/155
@@ -60,9 +60,12 @@ class BaseClass(iotests.QMPTestCase):

     def setUp(self):
         qemu_img('create', '-f', iotests.imgfmt, back0_img, '1440K')
-        qemu_img('create', '-f', iotests.imgfmt, '-b', back0_img, back1_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-b', back1_img, back2_img)
-        qemu_img('create', '-f', iotests.imgfmt, '-b', back2_img, source_img)
+        qemu_img('create', '-f', iotests.imgfmt, '-b', back0_img,
+                 '-F', iotests.imgfmt, back1_img)
+        qemu_img('create', '-f', iotests.imgfmt, '-b', back1_img,
+                 '-F', iotests.imgfmt, back2_img)
+        qemu_img('create', '-f', iotests.imgfmt, '-b', back2_img,
+                 '-F', iotests.imgfmt, source_img)

         self.vm = iotests.VM()
         # Add the BDS via blockdev-add so it stays around after the mirror block
@@ -89,7 +92,8 @@ class BaseClass(iotests.QMPTestCase):
         if self.existing:
             if self.target_backing:
                 qemu_img('create', '-f', iotests.imgfmt,
-                         '-b', self.target_backing, target_img, '1440K')
+                         '-b', self.target_backing, '-F', 'raw',
+                         target_img, '1440K')
             else:
                 qemu_img('create', '-f', iotests.imgfmt, target_img, '1440K')

diff --git a/tests/qemu-iotests/156 b/tests/qemu-iotests/156
index 5559df63a53e..7c69a6c3fa32 100755
--- a/tests/qemu-iotests/156
+++ b/tests/qemu-iotests/156
@@ -56,7 +56,7 @@ _unsupported_imgopts data_file

 # Create source disk
 TEST_IMG="$TEST_IMG.backing" _make_test_img 1M
-_make_test_img -b "$TEST_IMG.backing" 1M
+_make_test_img -b "$TEST_IMG.backing" -F $IMGFMT 1M

 $QEMU_IO -c 'write -P 1 0 256k' "$TEST_IMG.backing" | _filter_qemu_io
 $QEMU_IO -c 'write -P 2 64k 192k' "$TEST_IMG" | _filter_qemu_io
@@ -68,7 +68,7 @@ _send_qemu_cmd $QEMU_HANDLE \
     'return'

 # Create snapshot
-TEST_IMG="$TEST_IMG.overlay" _make_test_img -u -b "$TEST_IMG" 1M
+TEST_IMG="$TEST_IMG.overlay" _make_test_img -u -b "$TEST_IMG" -F $IMGFMT 1M
 _send_qemu_cmd $QEMU_HANDLE \
     "{ 'execute': 'blockdev-snapshot-sync',
        'arguments': { 'device': 'source',
@@ -85,7 +85,8 @@ _send_qemu_cmd $QEMU_HANDLE \
     'return'

 # Create target image
-TEST_IMG="$TEST_IMG.target.overlay" _make_test_img -u -b "$TEST_IMG.target" 1M
+TEST_IMG="$TEST_IMG.target.overlay" _make_test_img -u -b "$TEST_IMG.target" \
+    -F $IMGFMT 1M

 # Mirror snapshot
 _send_qemu_cmd $QEMU_HANDLE \
@@ -111,7 +112,7 @@ _send_qemu_cmd $QEMU_HANDLE \
 # Copy source backing chain to the target before completing the job
 cp "$TEST_IMG.backing" "$TEST_IMG.target.backing"
 cp "$TEST_IMG" "$TEST_IMG.target"
-$QEMU_IMG rebase -u -b "$TEST_IMG.target.backing" "$TEST_IMG.target"
+$QEMU_IMG rebase -u -b "$TEST_IMG.target.backing" -F $IMGFMT "$TEST_IMG.target"

 # Complete block job
 _send_qemu_cmd $QEMU_HANDLE \
diff --git a/tests/qemu-iotests/156.out b/tests/qemu-iotests/156.out
index 084be5ad7151..cce167b63fe3 100644
--- a/tests/qemu-iotests/156.out
+++ b/tests/qemu-iotests/156.out
@@ -1,20 +1,20 @@
 QA output created by 156
 Formatting 'TEST_DIR/t.IMGFMT.backing', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.backing
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.backing backing_fmt=IMGFMT
 wrote 262144/262144 bytes at offset 0
 256 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 196608/196608 bytes at offset 65536
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 { 'execute': 'qmp_capabilities' }
 {"return": {}}
-Formatting 'TEST_DIR/t.IMGFMT.overlay', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.overlay', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT backing_fmt=IMGFMT
 { 'execute': 'blockdev-snapshot-sync', 'arguments': { 'device': 'source', 'snapshot-file': 'TEST_DIR/t.IMGFMT.overlay', 'format': 'IMGFMT', 'mode': 'existing' } }
 {"return": {}}
 { 'execute': 'human-monitor-command', 'arguments': { 'command-line': 'qemu-io source "write -P 3 128k 128k"' } }
 wrote 131072/131072 bytes at offset 131072
 128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 {"return": ""}
-Formatting 'TEST_DIR/t.IMGFMT.target.overlay', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.target
+Formatting 'TEST_DIR/t.IMGFMT.target.overlay', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.target backing_fmt=IMGFMT
 { 'execute': 'drive-mirror', 'arguments': { 'device': 'source', 'target': 'TEST_DIR/t.IMGFMT.target.overlay', 'mode': 'existing', 'sync': 'top' } }
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "created", "id": "source"}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "JOB_STATUS_CHANGE", "data": {"status": "running", "id": "source"}}
diff --git a/tests/qemu-iotests/158 b/tests/qemu-iotests/158
index ba4db6116ac3..3175968e2b6a 100755
--- a/tests/qemu-iotests/158
+++ b/tests/qemu-iotests/158
@@ -64,7 +64,7 @@ echo "== verify pattern =="
 $QEMU_IO --object $SECRET -c "read -P 0xa 0 $size" --image-opts $IMGSPECBASE | _filter_qemu_io | _filter_testdir

 echo "== create overlay =="
-_make_test_img -u --object $SECRET -o "encryption=on,encrypt.key-secret=sec0" -b "$TEST_IMG_BASE" $size
+_make_test_img -u --object $SECRET -o "encryption=on,encrypt.key-secret=sec0" -b "$TEST_IMG_BASE" -F $IMGFMT $size

 echo
 echo "== writing part of a cluster =="
diff --git a/tests/qemu-iotests/158.out b/tests/qemu-iotests/158.out
index 6def216e5574..d999f51ec0c0 100644
--- a/tests/qemu-iotests/158.out
+++ b/tests/qemu-iotests/158.out
@@ -10,7 +10,7 @@ wrote 134217728/134217728 bytes at offset 0
 read 134217728/134217728 bytes at offset 0
 128 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 == create overlay ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base encryption=on encrypt.key-secret=sec0
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT encryption=on encrypt.key-secret=sec0

 == writing part of a cluster ==
 wrote 1024/1024 bytes at offset 0
diff --git a/tests/qemu-iotests/161 b/tests/qemu-iotests/161
index f572a19af28c..e270976d8783 100755
--- a/tests/qemu-iotests/161
+++ b/tests/qemu-iotests/161
@@ -49,8 +49,8 @@ IMG_SIZE=1M

 # Create the images
 TEST_IMG="$TEST_IMG.base" _make_test_img $IMG_SIZE | _filter_imgfmt
-TEST_IMG="$TEST_IMG.int" _make_test_img -b "$TEST_IMG.base" | _filter_imgfmt
-_make_test_img -b "$TEST_IMG.int" | _filter_imgfmt
+TEST_IMG="$TEST_IMG.int" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT | _filter_imgfmt
+_make_test_img -b "$TEST_IMG.int" -F $IMGFMT -F $IMGFMT | _filter_imgfmt

 # First test: reopen $TEST.IMG changing the detect-zeroes option on
 # its backing file ($TEST_IMG.int).
@@ -106,8 +106,8 @@ echo "*** Commit and then change an option on the backing file"
 echo
 # Create the images again
 TEST_IMG="$TEST_IMG.base" _make_test_img $IMG_SIZE | _filter_imgfmt
-TEST_IMG="$TEST_IMG.int" _make_test_img -b "$TEST_IMG.base" | _filter_imgfmt
-_make_test_img -b "$TEST_IMG.int" | _filter_imgfmt
+TEST_IMG="$TEST_IMG.int" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT| _filter_imgfmt
+_make_test_img -b "$TEST_IMG.int" -F $IMGFMT | _filter_imgfmt

 _launch_qemu -drive if=none,file="${TEST_IMG}"
 _send_qemu_cmd $QEMU_HANDLE \
diff --git a/tests/qemu-iotests/161.out b/tests/qemu-iotests/161.out
index aef9741ed31c..3d8d89a9da3f 100644
--- a/tests/qemu-iotests/161.out
+++ b/tests/qemu-iotests/161.out
@@ -1,7 +1,7 @@
 QA output created by 161
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/t.IMGFMT.int', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.int
+Formatting 'TEST_DIR/t.IMGFMT.int', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.int backing_fmt=IMGFMT

 *** Change an option on the backing file

@@ -29,8 +29,8 @@ Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.
 *** Commit and then change an option on the backing file

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/t.IMGFMT.int', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.int
+Formatting 'TEST_DIR/t.IMGFMT.int', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.int backing_fmt=IMGFMT
 { 'execute': 'qmp_capabilities' }
 {"return": {}}
 { 'execute': 'block-commit', 'arguments': { 'device': 'none0', 'top': 'TEST_DIR/t.IMGFMT.int' } }
diff --git a/tests/qemu-iotests/176 b/tests/qemu-iotests/176
index 117c8b695490..2565ff12eef8 100755
--- a/tests/qemu-iotests/176
+++ b/tests/qemu-iotests/176
@@ -84,8 +84,8 @@ echo

 len=$((2100 * 1024 * 1024 + 512)) # larger than 2G, and not cluster aligned
 TEST_IMG="$TEST_IMG.base" _make_test_img $len
-TEST_IMG="$TEST_IMG.itmd" _make_test_img -b "$TEST_IMG.base" $len
-_make_test_img -b "$TEST_IMG.itmd" $len
+TEST_IMG="$TEST_IMG.itmd" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT $len
+_make_test_img -b "$TEST_IMG.itmd" -F $IMGFMT $len
 # Update the top image to use a feature that is incompatible with fast path
 case $reason in
     snapshot) $QEMU_IMG snapshot -c snap "$TEST_IMG" ;;
diff --git a/tests/qemu-iotests/176.out b/tests/qemu-iotests/176.out
index cc33def96b25..9d09b6045202 100644
--- a/tests/qemu-iotests/176.out
+++ b/tests/qemu-iotests/176.out
@@ -3,8 +3,8 @@ QA output created by 176
 === Test pass snapshot.0 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -43,8 +43,8 @@ ID        TAG
 === Test pass snapshot.1 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -84,8 +84,8 @@ ID        TAG
 === Test pass snapshot.2 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -125,8 +125,8 @@ ID        TAG
 === Test pass snapshot.3 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 wrote 196608/196608 bytes at offset 2147287040
 192 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 131072/131072 bytes at offset 2147352576
@@ -163,8 +163,8 @@ ID        TAG
 === Test pass bitmap.0 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 QMP_VERSION
 {"return": {}}
 {"return": {}}
@@ -212,8 +212,8 @@ QMP_VERSION
 === Test pass bitmap.1 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 QMP_VERSION
 {"return": {}}
 {"return": {}}
@@ -262,8 +262,8 @@ QMP_VERSION
 === Test pass bitmap.2 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 QMP_VERSION
 {"return": {}}
 {"return": {}}
@@ -312,8 +312,8 @@ QMP_VERSION
 === Test pass bitmap.3 ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=2202010112
-Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd
+Formatting 'TEST_DIR/t.IMGFMT.itmd', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=2202010112 backing_file=TEST_DIR/t.IMGFMT.itmd backing_fmt=IMGFMT
 QMP_VERSION
 {"return": {}}
 {"return": {}}
diff --git a/tests/qemu-iotests/177 b/tests/qemu-iotests/177
index 752d29f8ad50..c201a9d6c64a 100755
--- a/tests/qemu-iotests/177
+++ b/tests/qemu-iotests/177
@@ -51,7 +51,7 @@ echo "== setting up files =="

 TEST_IMG="$TEST_IMG.base" _make_test_img $size
 $QEMU_IO -c "write -P 11 0 $size" "$TEST_IMG.base" | _filter_qemu_io
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT
 $QEMU_IO -c "write -P 22 0 $size" "$TEST_IMG" | _filter_qemu_io

 # Limited to 64k max-transfer
diff --git a/tests/qemu-iotests/177.out b/tests/qemu-iotests/177.out
index e887542678ba..ba1e1e212e47 100644
--- a/tests/qemu-iotests/177.out
+++ b/tests/qemu-iotests/177.out
@@ -4,7 +4,7 @@ QA output created by 177
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
 wrote 134217728/134217728 bytes at offset 0
 128 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 134217728/134217728 bytes at offset 0
 128 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

diff --git a/tests/qemu-iotests/179 b/tests/qemu-iotests/179
index 9372dc30ef77..11a20cb1bfa8 100755
--- a/tests/qemu-iotests/179
+++ b/tests/qemu-iotests/179
@@ -49,7 +49,7 @@ echo '=== Testing write zeroes with unmap ==='
 echo

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 # Offsets chosen at or near 2M boundaries so test works at all cluster sizes
 # 8k and larger (smaller clusters fail due to non-contiguous allocations)
diff --git a/tests/qemu-iotests/179.out b/tests/qemu-iotests/179.out
index 80722b22891d..1f7680002ca6 100644
--- a/tests/qemu-iotests/179.out
+++ b/tests/qemu-iotests/179.out
@@ -3,7 +3,7 @@ QA output created by 179
 === Testing write zeroes with unmap ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 2097152/2097152 bytes at offset 2097152
 2 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 wrote 2097152/2097152 bytes at offset 6291456
diff --git a/tests/qemu-iotests/189 b/tests/qemu-iotests/189
index c9ce9d3bede3..99a900c998c2 100755
--- a/tests/qemu-iotests/189
+++ b/tests/qemu-iotests/189
@@ -65,7 +65,7 @@ echo "== verify pattern =="
 $QEMU_IO --object $SECRET0 -c "read -P 0xa 0 $size" --image-opts $IMGSPECBASE | _filter_qemu_io | _filter_testdir

 echo "== create overlay =="
-_make_test_img --object $SECRET1 -o "encrypt.format=luks,encrypt.key-secret=sec1,encrypt.iter-time=10" -u -b "$TEST_IMG_BASE" $size
+_make_test_img --object $SECRET1 -o "encrypt.format=luks,encrypt.key-secret=sec1,encrypt.iter-time=10" -u -b "$TEST_IMG_BASE" -F $IMGFMT $size

 echo
 echo "== writing part of a cluster =="
diff --git a/tests/qemu-iotests/189.out b/tests/qemu-iotests/189.out
index a0b7c9c24c24..035068764f73 100644
--- a/tests/qemu-iotests/189.out
+++ b/tests/qemu-iotests/189.out
@@ -10,7 +10,7 @@ wrote 16777216/16777216 bytes at offset 0
 read 16777216/16777216 bytes at offset 0
 16 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 == create overlay ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=16777216 backing_file=TEST_DIR/t.IMGFMT.base encrypt.format=luks encrypt.key-secret=sec1 encrypt.iter-time=10
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=16777216 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT encrypt.format=luks encrypt.key-secret=sec1 encrypt.iter-time=10

 == writing part of a cluster ==
 wrote 1024/1024 bytes at offset 0
diff --git a/tests/qemu-iotests/191 b/tests/qemu-iotests/191
index b05db6814157..d17462e1e42b 100755
--- a/tests/qemu-iotests/191
+++ b/tests/qemu-iotests/191
@@ -54,8 +54,8 @@ echo

 TEST_IMG="${TEST_IMG}.base" _make_test_img $size
 TEST_IMG="${TEST_IMG}.mid" _make_test_img -o "backing_fmt=$IMGFMT" -b "${TEST_IMG}.base"
-_make_test_img -b "${TEST_IMG}.mid"
-TEST_IMG="${TEST_IMG}.ovl2" _make_test_img -b "${TEST_IMG}.mid"
+_make_test_img -b "${TEST_IMG}.mid" -F $IMGFMT
+TEST_IMG="${TEST_IMG}.ovl2" _make_test_img -b "${TEST_IMG}.mid" -F $IMGFMT

 $QEMU_IO -c 'write -P 0x55 1M 64k' "${TEST_IMG}.mid" | _filter_qemu_io

@@ -102,10 +102,10 @@ echo === Preparing and starting VM with -drive ===
 echo

 TEST_IMG="${TEST_IMG}.base" _make_test_img $size
-TEST_IMG="${TEST_IMG}.mid" _make_test_img -b "${TEST_IMG}.base"
-_make_test_img -b "${TEST_IMG}.mid"
-TEST_IMG="${TEST_IMG}.ovl2" _make_test_img -b "${TEST_IMG}.mid"
-TEST_IMG="${TEST_IMG}.ovl3" _make_test_img -b "${TEST_IMG}.ovl2"
+TEST_IMG="${TEST_IMG}.mid" _make_test_img -b "${TEST_IMG}.base" -F $IMGFMT
+_make_test_img -b "${TEST_IMG}.mid" -F $IMGFMT
+TEST_IMG="${TEST_IMG}.ovl2" _make_test_img -b "${TEST_IMG}.mid" -F $IMGFMT
+TEST_IMG="${TEST_IMG}.ovl3" _make_test_img -b "${TEST_IMG}.ovl2" -F $IMGFMT

 $QEMU_IO -c 'write -P 0x55 1M 64k' "${TEST_IMG}.mid" | _filter_qemu_io

diff --git a/tests/qemu-iotests/191.out b/tests/qemu-iotests/191.out
index 34cec7b9ecf3..11aaf3b691fd 100644
--- a/tests/qemu-iotests/191.out
+++ b/tests/qemu-iotests/191.out
@@ -4,8 +4,8 @@ QA output created by 191

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
 Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
-Formatting 'TEST_DIR/t.IMGFMT.ovl2', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.ovl2', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 1048576
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 { 'execute': 'qmp_capabilities' }
@@ -413,10 +413,10 @@ backing file format: IMGFMT
 === Preparing and starting VM with -drive ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
-Formatting 'TEST_DIR/t.IMGFMT.ovl2', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
-Formatting 'TEST_DIR/t.IMGFMT.ovl3', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.ovl2
+Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.ovl2', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT.ovl3', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.ovl2 backing_fmt=IMGFMT
 wrote 65536/65536 bytes at offset 1048576
 64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 { 'execute': 'qmp_capabilities' }
diff --git a/tests/qemu-iotests/195 b/tests/qemu-iotests/195
index 48984b7ac1ed..2351d55fe1f1 100755
--- a/tests/qemu-iotests/195
+++ b/tests/qemu-iotests/195
@@ -56,8 +56,8 @@ run_qemu()

 size=64M
 TEST_IMG="$TEST_IMG.base" _make_test_img $size
-TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base"
-_make_test_img -b "$TEST_IMG.mid"
+TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT
+_make_test_img -b "$TEST_IMG.mid" -F $IMGFMT

 echo
 echo "Change backing file of mid (opened read-only)"
@@ -75,7 +75,7 @@ echo
 echo "Change backing file of top (opened writable)"
 echo

-TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base"
+TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 run_qemu -drive if=none,file="$TEST_IMG",node-name=top <<EOF
 {"execute":"qmp_capabilities"}
diff --git a/tests/qemu-iotests/195.out b/tests/qemu-iotests/195.out
index e6df0d6781ba..ec84df5012a3 100644
--- a/tests/qemu-iotests/195.out
+++ b/tests/qemu-iotests/195.out
@@ -1,7 +1,7 @@
 QA output created by 195
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
+Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT

 Change backing file of mid (opened read-only)

@@ -42,7 +42,7 @@ backing file format: IMGFMT

 Change backing file of top (opened writable)

-Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Testing: -drive if=none,file=TEST_DIR/t.IMGFMT,node-name=top
 {
     QMP_VERSION
diff --git a/tests/qemu-iotests/198 b/tests/qemu-iotests/198
index fb0d5a29d357..33853df1c8bd 100755
--- a/tests/qemu-iotests/198
+++ b/tests/qemu-iotests/198
@@ -62,7 +62,7 @@ echo "== writing whole image base =="
 $QEMU_IO --object $SECRET0 -c "write -P 0xa 0 $size" --image-opts $IMGSPECBASE | _filter_qemu_io | _filter_testdir

 echo "== create overlay =="
-_make_test_img --object $SECRET1 -o "encrypt.format=luks,encrypt.key-secret=sec1,encrypt.iter-time=10" -u -b "$TEST_IMG_BASE" $size
+_make_test_img --object $SECRET1 -o "encrypt.format=luks,encrypt.key-secret=sec1,encrypt.iter-time=10" -u -b "$TEST_IMG_BASE" -F $IMGFMT $size

 echo
 echo "== writing whole image layer =="
diff --git a/tests/qemu-iotests/198.out b/tests/qemu-iotests/198.out
index 831ce3a289d3..8bc39dd0d5ee 100644
--- a/tests/qemu-iotests/198.out
+++ b/tests/qemu-iotests/198.out
@@ -6,7 +6,7 @@ Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=16777216 encrypt.format=luk
 wrote 16777216/16777216 bytes at offset 0
 16 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 == create overlay ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=16777216 backing_file=TEST_DIR/t.IMGFMT.base encrypt.format=luks encrypt.key-secret=sec1 encrypt.iter-time=10
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=16777216 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT encrypt.format=luks encrypt.key-secret=sec1 encrypt.iter-time=10

 == writing whole image layer ==
 wrote 16777216/16777216 bytes at offset 0
@@ -78,6 +78,7 @@ image: json:{ /* filtered */ }
 file format: IMGFMT
 virtual size: 16 MiB (16777216 bytes)
 backing file: TEST_DIR/t.IMGFMT.base
+backing file format: IMGFMT
 Format specific information:
     encrypt:
         ivgen alg: plain64
diff --git a/tests/qemu-iotests/204 b/tests/qemu-iotests/204
index abb73dc381c9..6770fa9b9636 100755
--- a/tests/qemu-iotests/204
+++ b/tests/qemu-iotests/204
@@ -52,7 +52,7 @@ echo "== setting up files =="

 TEST_IMG="$TEST_IMG.base" _make_test_img $size
 $QEMU_IO -c "write -P 11 0 $size" "$TEST_IMG.base" | _filter_qemu_io
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT
 $QEMU_IO -c "write -P 22 0 110M" "$TEST_IMG" | _filter_qemu_io

 # Limited to 64k max-transfer
diff --git a/tests/qemu-iotests/204.out b/tests/qemu-iotests/204.out
index f3a10fbe9035..457f72df8f88 100644
--- a/tests/qemu-iotests/204.out
+++ b/tests/qemu-iotests/204.out
@@ -4,7 +4,7 @@ QA output created by 204
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
 wrote 134217728/134217728 bytes at offset 0
 128 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 wrote 115343360/115343360 bytes at offset 0
 110 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)

diff --git a/tests/qemu-iotests/216 b/tests/qemu-iotests/216
index 372f042d3ebf..088b318a3755 100755
--- a/tests/qemu-iotests/216
+++ b/tests/qemu-iotests/216
@@ -53,7 +53,7 @@ with iotests.FilePath('base.img') as base_img_path, \
     assert qemu_img('create', '-f', iotests.imgfmt, base_img_path, '64M') == 0
     assert qemu_io_silent(base_img_path, '-c', 'write -P 1 0M 1M') == 0
     assert qemu_img('create', '-f', iotests.imgfmt, '-b', base_img_path,
-                    top_img_path) == 0
+                    '-F', iotests.imgfmt, top_img_path) == 0
     assert qemu_io_silent(top_img_path,  '-c', 'write -P 2 1M 1M') == 0

     log('Done')
diff --git a/tests/qemu-iotests/224 b/tests/qemu-iotests/224
index e91fb26fd8eb..94e412a680df 100755
--- a/tests/qemu-iotests/224
+++ b/tests/qemu-iotests/224
@@ -49,9 +49,9 @@ for filter_node_name in False, True:
         assert qemu_img('create', '-f', iotests.imgfmt,
                         base_img_path, '64M') == 0
         assert qemu_img('create', '-f', iotests.imgfmt, '-b', base_img_path,
-                        mid_img_path) == 0
+                        '-F', iotests.imgfmt, mid_img_path) == 0
         assert qemu_img('create', '-f', iotests.imgfmt, '-b', mid_img_path,
-                        top_img_path) == 0
+                        '-F', iotests.imgfmt, top_img_path) == 0

         # Something to commit
         assert qemu_io_silent(mid_img_path, '-c', 'write -P 1 0 1M') == 0
diff --git a/tests/qemu-iotests/225 b/tests/qemu-iotests/225
index c9a334c7e998..0186ec815660 100755
--- a/tests/qemu-iotests/225
+++ b/tests/qemu-iotests/225
@@ -48,7 +48,7 @@ _unsupported_imgopts "subformat=monolithicFlat" \

 TEST_IMG="$TEST_IMG.base" _make_test_img 1M
 TEST_IMG="$TEST_IMG.not_base" _make_test_img 1M
-_make_test_img -b "$TEST_IMG.base"
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT

 make_opts()
 {
diff --git a/tests/qemu-iotests/225.out b/tests/qemu-iotests/225.out
index 4dc8ee282fad..0998ae094c2b 100644
--- a/tests/qemu-iotests/225.out
+++ b/tests/qemu-iotests/225.out
@@ -1,7 +1,7 @@
 QA output created by 225
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1048576
 Formatting 'TEST_DIR/t.IMGFMT.not_base', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT

 === Testing fitting VMDK backing image ===

diff --git a/tests/qemu-iotests/228 b/tests/qemu-iotests/228
index 64bc82ee2346..98e92f750249 100755
--- a/tests/qemu-iotests/228
+++ b/tests/qemu-iotests/228
@@ -54,7 +54,8 @@ with iotests.FilePath('base.img') as base_img_path, \
     assert qemu_img('create', '-f', iotests.imgfmt, base_img_path, '64M') == 0
     # Choose a funny way to describe the backing filename
     assert qemu_img('create', '-f', iotests.imgfmt, '-b',
-                    'file:' + base_img_path, top_img_path) == 0
+                    'file:' + base_img_path, '-F', iotests.imgfmt,
+                    top_img_path) == 0

     vm.launch()

@@ -169,7 +170,7 @@ with iotests.FilePath('base.img') as base_img_path, \
     # filename unless the backing file is opened implicitly with the
     # overlay)
     assert qemu_img('create', '-f', iotests.imgfmt, '-b', base_img_path,
-                    top_img_path) == 0
+                    '-F', iotests.imgfmt, top_img_path) == 0

     # You can only reliably override backing options by using a node
     # reference (or by specifying file.filename, but, well...)
diff --git a/tests/qemu-iotests/245 b/tests/qemu-iotests/245
index 1001275a4451..a3badee77043 100755
--- a/tests/qemu-iotests/245
+++ b/tests/qemu-iotests/245
@@ -44,7 +44,8 @@ class TestBlockdevReopen(iotests.QMPTestCase):

     def setUp(self):
         qemu_img('create', '-f', iotests.imgfmt, hd_path[0], '3M')
-        qemu_img('create', '-f', iotests.imgfmt, '-b', hd_path[0], hd_path[1])
+        qemu_img('create', '-f', iotests.imgfmt, '-b', hd_path[0],
+                 '-F', iotests.imgfmt, hd_path[1])
         qemu_img('create', '-f', iotests.imgfmt, hd_path[2], '3M')
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xa0  0 1M', hd_path[0])
         qemu_io('-f', iotests.imgfmt, '-c', 'write -P 0xa1 1M 1M', hd_path[1])
diff --git a/tests/qemu-iotests/249 b/tests/qemu-iotests/249
index 2b99c9789ed2..68f13ed32896 100755
--- a/tests/qemu-iotests/249
+++ b/tests/qemu-iotests/249
@@ -49,8 +49,8 @@ IMG_SIZE=1M

 # Create the images: base <- int <- active
 TEST_IMG="$TEST_IMG.base" _make_test_img $IMG_SIZE | _filter_imgfmt
-TEST_IMG="$TEST_IMG.int" _make_test_img -b "$TEST_IMG.base" | _filter_imgfmt
-_make_test_img -b "$TEST_IMG.int" | _filter_imgfmt
+TEST_IMG="$TEST_IMG.int" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT | _filter_imgfmt
+_make_test_img -b "$TEST_IMG.int" -F $IMGFMT | _filter_imgfmt

 # Launch QEMU with these two drives:
 # none0: base (read-only)
diff --git a/tests/qemu-iotests/249.out b/tests/qemu-iotests/249.out
index 51307ed0f795..85acda4635b3 100644
--- a/tests/qemu-iotests/249.out
+++ b/tests/qemu-iotests/249.out
@@ -1,7 +1,7 @@
 QA output created by 249
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=1048576
-Formatting 'TEST_DIR/t.IMGFMT.int', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.int
+Formatting 'TEST_DIR/t.IMGFMT.int', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=1048576 backing_file=TEST_DIR/t.IMGFMT.int backing_fmt=IMGFMT
 { 'execute': 'qmp_capabilities' }
 {"return": {}}

diff --git a/tests/qemu-iotests/252 b/tests/qemu-iotests/252
index 83280c1715dc..6662f4c9dec7 100755
--- a/tests/qemu-iotests/252
+++ b/tests/qemu-iotests/252
@@ -101,7 +101,7 @@ $QEMU_IMG map --output=json "$TEST_IMG" | _filter_qemu_img_map

 echo

-$QEMU_IMG rebase -b "$TEST_IMG.base_new" "$TEST_IMG"
+$QEMU_IMG rebase -b "$TEST_IMG.base_new" -F $IMGFMT "$TEST_IMG"

 # Verify the data is correct
 $QEMU_IO "$TEST_IMG" \
diff --git a/tests/qemu-iotests/257 b/tests/qemu-iotests/257
index 004a433b8be2..e1e60772195e 100755
--- a/tests/qemu-iotests/257
+++ b/tests/qemu-iotests/257
@@ -238,7 +238,8 @@ def compare_images(image, reference, baseimg=None, expected_match=True):
     """
     expected_ret = 0 if expected_match else 1
     if baseimg:
-        assert qemu_img("rebase", "-u", "-b", baseimg, image) == 0
+        assert qemu_img("rebase", "-u", "-b", baseimg, '-F', iotests.imgfmt,
+                        image) == 0
     ret = qemu_img("compare", image, reference)
     log('qemu_img compare "{:s}" "{:s}" ==> {:s}, {:s}'.format(
         image, reference,
diff --git a/tests/qemu-iotests/267 b/tests/qemu-iotests/267
index 3146273eef33..e44be49c771d 100755
--- a/tests/qemu-iotests/267
+++ b/tests/qemu-iotests/267
@@ -73,7 +73,7 @@ size=128M
 run_test()
 {
     if [ -n "$BACKING_FILE" ]; then
-        _make_test_img -b "$BACKING_FILE" $size
+        _make_test_img -b "$BACKING_FILE" -F $IMGFMT $size
     else
         _make_test_img $size
     fi
@@ -149,7 +149,7 @@ echo
 echo "=== -blockdev with NBD server on the backing file ==="
 echo

-_make_test_img -b "$TEST_IMG.base" $size
+_make_test_img -b "$TEST_IMG.base" -F $IMGFMT $size
 cat <<EOF |
 nbd_server_start unix:$SOCK_DIR/nbd
 nbd_server_add -w backing-fmt
diff --git a/tests/qemu-iotests/267.out b/tests/qemu-iotests/267.out
index 8dddb4baa443..d6d80c099f94 100644
--- a/tests/qemu-iotests/267.out
+++ b/tests/qemu-iotests/267.out
@@ -128,7 +128,7 @@ ID        TAG                 VM SIZE                DATE       VM CLOCK
 === -blockdev with a backing file ===

 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Testing: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT.base,node-name=backing-file -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=file -blockdev driver=IMGFMT,file=file,backing=backing-file,node-name=fmt
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) savevm snap0
@@ -139,7 +139,7 @@ ID        TAG                 VM SIZE                DATE       VM CLOCK
 (qemu) loadvm snap0
 (qemu) quit

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Testing: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT.base,node-name=backing-file -blockdev driver=IMGFMT,file=backing-file,node-name=backing-fmt -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=file -blockdev driver=IMGFMT,file=file,backing=backing-fmt,node-name=fmt
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) savevm snap0
@@ -158,7 +158,7 @@ Internal snapshots on backing file:

 === -blockdev with NBD server on the backing file ===

-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
 Testing: -blockdev driver=file,filename=TEST_DIR/t.IMGFMT.base,node-name=backing-file -blockdev driver=IMGFMT,file=backing-file,node-name=backing-fmt -blockdev driver=file,filename=TEST_DIR/t.IMGFMT,node-name=file -blockdev driver=IMGFMT,file=file,backing=backing-fmt,node-name=fmt
 QEMU X.Y.Z monitor - type 'help' for more information
 (qemu) nbd_server_start unix:SOCK_DIR/nbd
diff --git a/tests/qemu-iotests/270 b/tests/qemu-iotests/270
index b9a12b908c3e..00339c0f7833 100755
--- a/tests/qemu-iotests/270
+++ b/tests/qemu-iotests/270
@@ -53,7 +53,7 @@ $QEMU_IO -c 'write 0 512' "$TEST_IMG.base" | _filter_qemu_io
 # We need a large cluster size, see below for why (above the $QEMU_IO
 # invocation)
 _make_test_img -o cluster_size=2M,data_file="$TEST_IMG.orig" \
-    -b "$TEST_IMG.base" 4G
+    -b "$TEST_IMG.base" -F $IMGFMT 4G

 # We want a null-co as the data file, because it allows us to quickly
 # "write" 2G of data without using any space.
diff --git a/tests/qemu-iotests/270.out b/tests/qemu-iotests/270.out
index c7be1110142f..6dc3b23d5bfa 100644
--- a/tests/qemu-iotests/270.out
+++ b/tests/qemu-iotests/270.out
@@ -2,7 +2,7 @@ QA output created by 270
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=4294967296
 wrote 512/512 bytes at offset 0
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT.base data_file=TEST_DIR/t.IMGFMT.orig
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=4294967296 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT data_file=TEST_DIR/t.IMGFMT.orig
 wrote 2147483136/2147483136 bytes at offset 768
 2 GiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 No errors were found on the image.
diff --git a/tests/qemu-iotests/273 b/tests/qemu-iotests/273
index 00ff79bcf8f8..41ffbf39c4ef 100755
--- a/tests/qemu-iotests/273
+++ b/tests/qemu-iotests/273
@@ -56,8 +56,8 @@ run_qemu()
 }

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
-TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base"
-_make_test_img -b "$TEST_IMG.mid"
+TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT
+_make_test_img -b "$TEST_IMG.mid" -F $IMGFMT

 run_qemu \
     -blockdev file,node-name=base,filename="$TEST_IMG.base" \
diff --git a/tests/qemu-iotests/273.out b/tests/qemu-iotests/273.out
index 684b8d6f777e..87d475850387 100644
--- a/tests/qemu-iotests/273.out
+++ b/tests/qemu-iotests/273.out
@@ -1,7 +1,7 @@
 QA output created by 273
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
+Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT
 Testing: -blockdev file,node-name=base,filename=TEST_DIR/t.IMGFMT.base -blockdev file,node-name=midf,filename=TEST_DIR/t.IMGFMT.mid -blockdev {"driver":"IMGFMT","node-name":"mid","file":"midf","backing":null} -blockdev file,node-name=topf,filename=TEST_DIR/t.IMGFMT -blockdev {"driver":"IMGFMT","file":"topf","node-name":"top","backing":null}
 {
     QMP_VERSION
diff --git a/tests/qemu-iotests/279 b/tests/qemu-iotests/279
index 75a4747e6bcd..5515d4ed0138 100755
--- a/tests/qemu-iotests/279
+++ b/tests/qemu-iotests/279
@@ -42,8 +42,8 @@ _unsupported_imgopts "subformat=monolithicFlat" \
                      "subformat=twoGbMaxExtentFlat" \

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
-TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base"
-_make_test_img -b "$TEST_IMG.mid"
+TEST_IMG="$TEST_IMG.mid" _make_test_img -b "$TEST_IMG.base" -F $IMGFMT
+_make_test_img -b "$TEST_IMG.mid" -F $IMGFMT

 echo
 echo '== qemu-img info --backing-chain =='
diff --git a/tests/qemu-iotests/279.out b/tests/qemu-iotests/279.out
index f4dc6c69cbb3..adb2e47a1a9c 100644
--- a/tests/qemu-iotests/279.out
+++ b/tests/qemu-iotests/279.out
@@ -1,7 +1,7 @@
 QA output created by 279
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
-Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid
+Formatting 'TEST_DIR/t.IMGFMT.mid', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=IMGFMT
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.mid backing_fmt=IMGFMT

 == qemu-img info --backing-chain ==
 image: TEST_DIR/t.IMGFMT
-- 
2.26.0.rc2

[PATCH v5 6/7] block: Add support to warn on backing file change without format

[Eric Blake]

For now, this is a mechanical addition; all callers pass false. But
the next patch will use it to improve 'qemu-img rebase -u' when
selecting a backing file with no format.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
---
 include/block/block.h |  4 ++--
 block.c               | 13 ++++++++++---
 block/qcow2.c         |  2 +-
 block/stream.c        |  2 +-
 blockdev.c            |  3 ++-
 qemu-img.c            |  4 ++--
 6 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index b05995fe9c5b..098fc635e865 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -351,8 +351,8 @@ BlockMeasureInfo *bdrv_measure(BlockDriver *drv, QemuOpts *opts,
 void bdrv_get_geometry(BlockDriverState *bs, uint64_t *nb_sectors_ptr);
 void bdrv_refresh_limits(BlockDriverState *bs, Error **errp);
 int bdrv_commit(BlockDriverState *bs);
-int bdrv_change_backing_file(BlockDriverState *bs,
-    const char *backing_file, const char *backing_fmt);
+int bdrv_change_backing_file(BlockDriverState *bs, const char *backing_file,
+                             const char *backing_fmt, bool warn);
 void bdrv_register(BlockDriver *bdrv);
 int bdrv_drop_intermediate(BlockDriverState *top, BlockDriverState *base,
                            const char *backing_file_str);
diff --git a/block.c b/block.c
index 2e3905c99e8c..70cc6123dd91 100644
--- a/block.c
+++ b/block.c
@@ -1322,7 +1322,8 @@ static int bdrv_backing_update_filename(BdrvChild *c, BlockDriverState *base,
     }

     ret = bdrv_change_backing_file(parent, filename,
-                                   base->drv ? base->drv->format_name : "");
+                                   base->drv ? base->drv->format_name : "",
+                                   false);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not update backing file link");
     }
@@ -4566,8 +4567,8 @@ int bdrv_check(BlockDriverState *bs,
  *            image file header
  * -ENOTSUP - format driver doesn't support changing the backing file
  */
-int bdrv_change_backing_file(BlockDriverState *bs,
-    const char *backing_file, const char *backing_fmt)
+int bdrv_change_backing_file(BlockDriverState *bs, const char *backing_file,
+                             const char *backing_fmt, bool warn)
 {
     BlockDriver *drv = bs->drv;
     int ret;
@@ -4581,6 +4582,12 @@ int bdrv_change_backing_file(BlockDriverState *bs,
         return -EINVAL;
     }

+    if (warn && backing_file && !backing_fmt) {
+        warn_report("Deprecated use of backing file without explicit "
+                    "backing format, use of this image requires "
+                    "potentially unsafe format probing");
+    }
+
     if (drv->bdrv_change_backing_file != NULL) {
         ret = drv->bdrv_change_backing_file(bs, backing_file, backing_fmt);
     } else {
diff --git a/block/qcow2.c b/block/qcow2.c
index e4b772726010..0a7e70bdf879 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -3526,7 +3526,7 @@ qcow2_co_create(BlockdevCreateOptions *create_options, Error **errp)
         }

         ret = bdrv_change_backing_file(blk_bs(blk), qcow2_opts->backing_file,
-                                       backing_format);
+                                       backing_format, false);
         if (ret < 0) {
             error_setg_errno(errp, -ret, "Could not assign backing file '%s' "
                              "with format '%s'", qcow2_opts->backing_file,
diff --git a/block/stream.c b/block/stream.c
index aa2e7af98e37..310ccbaa4cfd 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -78,7 +78,7 @@ static int stream_prepare(Job *job)
             }
         }
         bdrv_set_backing_hd(bs, base, &local_err);
-        ret = bdrv_change_backing_file(bs, base_id, base_fmt);
+        ret = bdrv_change_backing_file(bs, base_id, base_fmt, false);
         if (local_err) {
             error_report_err(local_err);
             return -EPERM;
diff --git a/blockdev.c b/blockdev.c
index fa8630cb412d..5bc9d78563ab 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3692,7 +3692,8 @@ void qmp_change_backing_file(const char *device,
     }

     ret = bdrv_change_backing_file(image_bs, backing_file,
-                               image_bs->drv ? image_bs->drv->format_name : "");
+                                   image_bs->drv ? image_bs->drv->format_name : "",
+                                   false);

     if (ret < 0) {
         error_setg_errno(errp, -ret, "Could not change backing file to '%s'",
diff --git a/qemu-img.c b/qemu-img.c
index b167376bd72e..cb8fa5a0ee8b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3644,9 +3644,9 @@ static int img_rebase(int argc, char **argv)
      * doesn't change when we switch the backing file.
      */
     if (out_baseimg && *out_baseimg) {
-        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt);
+        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, false);
     } else {
-        ret = bdrv_change_backing_file(bs, NULL, NULL);
+        ret = bdrv_change_backing_file(bs, NULL, NULL, false);
     }

     if (ret == -ENOSPC) {
-- 
2.26.0.rc2

[PATCH v5 7/7] qemu-img: Deprecate use of -b without -F

[Eric Blake]

Creating an image that requires format probing of the backing image is
inherently unsafe (we've had several CVEs over the years based on
probes leaking information to the guest on a subsequent boot, although
these days tools like libvirt are aware of the issue enough to prevent
the worst effects).  However, if our probing algorithm ever changes,
or if other tools like libvirt determine a different probe result than
we do, then subsequent use of that backing file under a different
format will present corrupted data to the guest.  Start a deprecation
clock so that future qemu-img can refuse to create unsafe backing
chains that would rely on probing.  Most warnings are intentionally
emitted from bdrv_img_create() in the block layer, but qemu-img
convert uses bdrv_create() which cannot emit its own warning without
causing spurious warnings on other code paths.  In the end, all
command-line image creation or backing file rewriting now performs a
check.

However, there is one time where probing is safe: if we probe raw,
then it is safe to record that implicitly in the image (but we still
warn, as it's better to teach the user to supply -F always than to
make them guess when it is safe).

iotest 114 specifically wants to create an unsafe image for later
amendment rather than defaulting to our new default of recording a
probed format, so it needs an update.  While touching it, expand it to
cover all of the various warnings enabled by this patch.  iotest 290
also shows a change to qcow messages; note that the fact that we now
make a probed format of 'raw' explicit now results in a double
warning, but no one should be creating new qcow images so it is not
worth cleaning up.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 docs/system/deprecated.rst | 20 ++++++++++++++++++++
 block.c                    | 21 ++++++++++++++++++++-
 qemu-img.c                 |  9 ++++++++-
 tests/qemu-iotests/114     | 12 ++++++++++++
 tests/qemu-iotests/114.out |  9 +++++++++
 tests/qemu-iotests/290.out |  5 ++++-
 6 files changed, 73 insertions(+), 3 deletions(-)

diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
index aac6be58917a..8abfd436820a 100644
--- a/docs/system/deprecated.rst
+++ b/docs/system/deprecated.rst
@@ -429,6 +429,26 @@ image).  Rather, any changes to the backing chain should be performed
 with ``qemu-img rebase -u`` either before or after the remaining
 changes being performed by amend, as appropriate.

+qemu-img backing file without format (since 5.0.0)
+''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The use of ``qemu-img create``, ``qemu-img rebase``, or ``qemu-img
+convert`` to create or modify an image that depends on a backing file
+now recommends that an explicit backing format be provided.  This is
+for safety: if QEMU probes a different format than what you thought,
+the data presented to the guest will be corrupt; similarly, presenting
+a raw image to a guest allows a potential security exploit if a future
+probe sees a non-raw image based on guest writes.
+
+To avoid the warning message, or even future refusal to create an
+unsafe image, you must pass ``-o backing_fmt=`` (or the shorthand
+``-F`` during create) to specify the intended backing format.  You may
+use ``qemu-img rebase -u`` to retroactively add a backing format to an
+existing image.  However, be aware that there are already potential
+security risks to blindly using ``qemu-img info`` to probe the format
+of an untrusted backing image, when deciding what format to add into
+an existing image.
+
 ``qemu-img convert -n -o`` (since 4.2.0)
 ''''''''''''''''''''''''''''''''''''''''

diff --git a/block.c b/block.c
index 70cc6123dd91..61871965eee4 100644
--- a/block.c
+++ b/block.c
@@ -6072,6 +6072,20 @@ void bdrv_img_create(const char *filename, const char *fmt,
                               "Could not open backing image to determine size.\n");
             goto out;
         } else {
+            if (!backing_fmt) {
+                warn_report("Deprecated use of backing file without explicit "
+                            "backing format (detected format of %s)",
+                            bs->drv->format_name);
+                if (bs->drv == &bdrv_raw) {
+                    /*
+                     * A probe of raw is always correct, so in this one
+                     * case, we can write that into the image.
+                     */
+                    backing_fmt = bs->drv->format_name;
+                    qemu_opt_set(opts, BLOCK_OPT_BACKING_FMT, backing_fmt,
+                                 NULL);
+                }
+            }
             if (size == -1) {
                 /* Opened BS, have no size */
                 size = bdrv_getlength(bs);
@@ -6085,7 +6099,12 @@ void bdrv_img_create(const char *filename, const char *fmt,
             }
             bdrv_unref(bs);
         }
-    } /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+        /* (backing_file && !(flags & BDRV_O_NO_BACKING)) */
+    } else if (backing_file && !backing_fmt) {
+        warn_report("Deprecated use of unopened backing file without "
+                    "explicit backing format, use of this image requires "
+                    "potentially unsafe format probing");
+    }

     if (size == -1) {
         error_setg(errp, "Image creation needs a size parameter");
diff --git a/qemu-img.c b/qemu-img.c
index cb8fa5a0ee8b..fec89da4e18f 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2414,6 +2414,13 @@ static int img_convert(int argc, char **argv)
         goto out;
     }

+    if (out_baseimg_param) {
+        if (!qemu_opt_get(opts, BLOCK_OPT_BACKING_FMT)) {
+            warn_report("Deprecated use of backing file without explicit "
+                        "backing format");
+        }
+    }
+
     /* Check if compression is supported */
     if (s.compressed) {
         bool encryption =
@@ -3644,7 +3651,7 @@ static int img_rebase(int argc, char **argv)
      * doesn't change when we switch the backing file.
      */
     if (out_baseimg && *out_baseimg) {
-        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, false);
+        ret = bdrv_change_backing_file(bs, out_baseimg, out_basefmt, true);
     } else {
         ret = bdrv_change_backing_file(bs, NULL, NULL, false);
     }
diff --git a/tests/qemu-iotests/114 b/tests/qemu-iotests/114
index 26104fff6c67..e65bd4f983e0 100755
--- a/tests/qemu-iotests/114
+++ b/tests/qemu-iotests/114
@@ -42,9 +42,16 @@ _unsupported_proto vxhs
 # qcow2.py does not work too well with external data files
 _unsupported_imgopts data_file

+# Intentionally specify backing file without backing format; demonstrate
+# the difference in warning messages when backing file could be probed.
+# Note that only a raw probe result will affect the resulting image.
+truncate --size=64M "$TEST_IMG.orig"
+_make_test_img -b "$TEST_IMG.orig" 64M

 TEST_IMG="$TEST_IMG.base" _make_test_img 64M
+$QEMU_IMG convert -O qcow2 -B "$TEST_IMG.orig" "$TEST_IMG.orig" "$TEST_IMG"
 _make_test_img -b "$TEST_IMG.base" 64M
+_make_test_img -u -b "$TEST_IMG.base" 64M

 # Set an invalid backing file format
 $PYTHON qcow2.py "$TEST_IMG" add-header-ext 0xE2792ACA "foo"
@@ -55,6 +62,11 @@ _img_info
 $QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | _filter_testdir
 $QEMU_IO -c "open -o backing.driver=$IMGFMT $TEST_IMG" -c "read 0 4k" | _filter_qemu_io

+# Rebase the image, to show that omitting backing format triggers a warning,
+# but probing now lets us use the backing file.
+$QEMU_IMG rebase -u -b "$TEST_IMG.base" "$TEST_IMG"
+$QEMU_IO -c "open $TEST_IMG" -c "read 0 4k" 2>&1 | _filter_qemu_io | _filter_testdir
+
 # success, all done
 echo '*** done'
 rm -f $seq.full
diff --git a/tests/qemu-iotests/114.out b/tests/qemu-iotests/114.out
index 67adef37a4f6..7c15dd53b0b5 100644
--- a/tests/qemu-iotests/114.out
+++ b/tests/qemu-iotests/114.out
@@ -1,5 +1,11 @@
 QA output created by 114
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.orig backing_fmt=raw
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864
+qemu-img: warning: Deprecated use of backing file without explicit backing format
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of IMGFMT)
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
+qemu-img: warning: Deprecated use of unopened backing file without explicit backing format, use of this image requires potentially unsafe format probing
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
@@ -11,4 +17,7 @@ qemu-io: can't open device TEST_DIR/t.qcow2: Could not open backing file: Unknow
 no file open, try 'help open'
 read 4096/4096 bytes at offset 0
 4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+qemu-img: warning: Deprecated use of backing file without explicit backing format, use of this image requires potentially unsafe format probing
+read 4096/4096 bytes at offset 0
+4 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 *** done
diff --git a/tests/qemu-iotests/290.out b/tests/qemu-iotests/290.out
index 5b4ae3196d04..f80a5d982726 100644
--- a/tests/qemu-iotests/290.out
+++ b/tests/qemu-iotests/290.out
@@ -2,6 +2,7 @@ QA output created by 290

 == qcow backed by qcow ==
 Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of IMGFMT)
 Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
@@ -26,7 +27,9 @@ cluster_size: 512
 backing file: TEST_DIR/t.IMGFMT.base

 == qcow backed by raw ==
-Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base
+qemu-img: warning: Deprecated use of backing file without explicit backing format (detected format of raw)
+qemu-img: TEST_DIR/t.IMGFMT: IMGFMT cannot store backing format; an explicit backing format of raw is unsafe
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file=TEST_DIR/t.IMGFMT.base backing_fmt=raw
 image: TEST_DIR/t.IMGFMT
 file format: IMGFMT
 virtual size: 128 MiB (134217728 bytes)
-- 
2.26.0.rc2

Re: [PATCH v5 7/7] qemu-img: Deprecate use of -b without -F

[Kevin Wolf]

Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
> Creating an image that requires format probing of the backing image is
> inherently unsafe (we've had several CVEs over the years based on
> probes leaking information to the guest on a subsequent boot, although
> these days tools like libvirt are aware of the issue enough to prevent
> the worst effects).  However, if our probing algorithm ever changes,
> or if other tools like libvirt determine a different probe result than
> we do, then subsequent use of that backing file under a different
> format will present corrupted data to the guest.  Start a deprecation
> clock so that future qemu-img can refuse to create unsafe backing
> chains that would rely on probing.  Most warnings are intentionally
> emitted from bdrv_img_create() in the block layer, but qemu-img
> convert uses bdrv_create() which cannot emit its own warning without
> causing spurious warnings on other code paths.  In the end, all
> command-line image creation or backing file rewriting now performs a
> check.
> 
> However, there is one time where probing is safe: if we probe raw,
> then it is safe to record that implicitly in the image (but we still
> warn, as it's better to teach the user to supply -F always than to
> make them guess when it is safe).

You're not stating it explicitly, but I guess the thing that you mean
that is actually unsafe is if you have a raw image, always pass
format=raw to QEMU (so the guest could write e.g. a qcow2 header), but
then create a backing file without -F, so it will be probed. This is as
bad as specifying format=raw only sometimes.

I don't like the idea of responding to this by making raw images more
convenient to use than actual image formats.

How about we approach it the other way around: The troublemaker is raw,
so let's require specifying raw explicitly, and record the probed format
implicitly in other cases. This is a bit weaker in the immediate effect
in that it doesn't protect you when you actually deal with a malicious
image, but in normal use it will point out where your scripts or
management software is too careless. The final result should be that
management tools are fixed and you'll be safe, while manual users who
can usually trust their guests aren't inconvenienced too much.

Kevin

Re: [PATCH v5 7/7] qemu-img: Deprecate use of -b without -F

[Peter Krempa]

On Tue, May 05, 2020 at 10:11:03 +0200, Kevin Wolf wrote:
> Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
> > Creating an image that requires format probing of the backing image is
> > inherently unsafe (we've had several CVEs over the years based on
> > probes leaking information to the guest on a subsequent boot, although
> > these days tools like libvirt are aware of the issue enough to prevent
> > the worst effects).  However, if our probing algorithm ever changes,
> > or if other tools like libvirt determine a different probe result than
> > we do, then subsequent use of that backing file under a different
> > format will present corrupted data to the guest.  Start a deprecation
> > clock so that future qemu-img can refuse to create unsafe backing
> > chains that would rely on probing.  Most warnings are intentionally
> > emitted from bdrv_img_create() in the block layer, but qemu-img
> > convert uses bdrv_create() which cannot emit its own warning without
> > causing spurious warnings on other code paths.  In the end, all
> > command-line image creation or backing file rewriting now performs a
> > check.
> > 
> > However, there is one time where probing is safe: if we probe raw,
> > then it is safe to record that implicitly in the image (but we still
> > warn, as it's better to teach the user to supply -F always than to
> > make them guess when it is safe).
> 
> You're not stating it explicitly, but I guess the thing that you mean
> that is actually unsafe is if you have a raw image, always pass
> format=raw to QEMU (so the guest could write e.g. a qcow2 header), but
> then create a backing file without -F, so it will be probed. This is as
> bad as specifying format=raw only sometimes.
> 
> I don't like the idea of responding to this by making raw images more
> convenient to use than actual image formats.
> 
> How about we approach it the other way around: The troublemaker is raw,
> so let's require specifying raw explicitly, and record the probed format
> implicitly in other cases. This is a bit weaker in the immediate effect
> in that it doesn't protect you when you actually deal with a malicious
> image, but in normal use it will point out where your scripts or
> management software is too careless. The final result should be that
> management tools are fixed and you'll be safe, while manual users who
> can usually trust their guests aren't inconvenienced too much.

That is definitely better than just probing. In my opinion though the
format should always be specified explicitly. No favorism of 'raw' or
any other format. In libvirt we fill-in that the format of the image is
'raw' unless the user specifies something else rather than reporting an
error. This causes continuous grief for users who forget to set the
format. (one example is that if you create a fully-allocated qcow2 image
but use it as raw and install your VM, you'll never notice until you
want to use qcow2 features. Users usually notice once they try to create
a sparse image though due to the size difference)

Re: [PATCH v5 for-5.0? 0/7] Tighten qemu-img rules on missing backing format

[Eric Blake]

On 4/3/20 12:58 PM, Eric Blake wrote:
> v4 was here:
> https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg03775.html
> In v5:
> - fix 'qemu-img convert -B' to actually warn [Kashyap]
> - squash in followups
> - a couple more iotest improvements
> 
> If we decide this is not 5.0 material, then patches 4 and 7 need a
> tweak to s/5.0/5.1/ as the start of the deprecation clock.

Ping.  I've already made the 5.1 change in my local tree, does anyone 
want to review the rest of this series before I post a v6?

> 
> Eric Blake (7):
>    sheepdog: Add trivial backing_fmt support
>    vmdk: Add trivial backing_fmt support
>    qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw
>    qcow2: Deprecate use of qemu-img amend to change backing file
>    iotests: Specify explicit backing format where sensible
>    block: Add support to warn on backing file change without format
>    qemu-img: Deprecate use of -b without -F
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org