[PATCH] block: BDRV_O_NO_IO for backing file on creation

[Max Reitz <mreitz@redhat.com>]

When creating an image file with a backing file, we generally try to
open the backing file (unless -u was specified), mostly to verify that
it is there, but also to get the file size if none was specified for the
new image.

For neither of these things do we need data I/O, and so we can pass
BDRV_O_NO_IO when opening the backing file.  This allows us to open even
encrypted backing images without requiring the user to provide a secret.

This makes the -u switch in iotests 189 and 198 unnecessary (and the
$size parameter), so drop it, because this way we get regression tests
for this patch here.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/441
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block.c                | 6 +++++-
 tests/qemu-iotests/189 | 2 +-
 tests/qemu-iotests/198 | 2 +-
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/block.c b/block.c
index 3f456892d0..b459502632 100644
--- a/block.c
+++ b/block.c
@@ -6553,9 +6553,13 @@ void bdrv_img_create(const char *filename, const char *fmt,
         }
         assert(full_backing);
 
-        /* backing files always opened read-only */
+        /*
+         * No need to do I/O here, which allows us to open encrypted
+         * backing images without needing the secret
+         */
         back_flags = flags;
         back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
+        back_flags |= BDRV_O_NO_IO;
 
         backing_options = qdict_new();
         if (backing_fmt) {
diff --git a/tests/qemu-iotests/189 b/tests/qemu-iotests/189
index 4e463385b2..801494c6b9 100755
--- a/tests/qemu-iotests/189
+++ b/tests/qemu-iotests/189
@@ -67,7 +67,7 @@ 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" -F $IMGFMT $size
+_make_test_img --object $SECRET1 -o "encrypt.format=luks,encrypt.key-secret=sec1,encrypt.iter-time=10" -b "$TEST_IMG_BASE" -F $IMGFMT
 
 echo
 echo "== writing part of a cluster =="
diff --git a/tests/qemu-iotests/198 b/tests/qemu-iotests/198
index b333a8f281..1c93dea1f7 100755
--- a/tests/qemu-iotests/198
+++ b/tests/qemu-iotests/198
@@ -64,7 +64,7 @@ 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" -F $IMGFMT $size
+_make_test_img --object $SECRET1 -o "encrypt.format=luks,encrypt.key-secret=sec1,encrypt.iter-time=10" -b "$TEST_IMG_BASE" -F $IMGFMT
 
 echo
 echo "== writing whole image layer =="
-- 
2.31.1