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* [PATCH 0/2] qemu-img convert: Unshare write permission for source
@ 2021-04-13 13:23 Kevin Wolf
  2021-04-13 13:23 ` [PATCH 1/2] block: Add BDRV_O_NO_SHARE for blk_new_open() Kevin Wolf
  2021-04-13 13:23 ` [PATCH 2/2] qemu-img convert: Unshare write permission for source Kevin Wolf
  0 siblings, 2 replies; 5+ messages in thread
From: Kevin Wolf @ 2021-04-13 13:23 UTC (permalink / raw)
  To: qemu-block; +Cc: kwolf, xuwei, qemu-devel

Kevin Wolf (2):
  block: Add BDRV_O_NO_SHARE for blk_new_open()
  qemu-img convert: Unshare write permission for source

 include/block/block.h |  1 +
 block/block-backend.c | 19 +++++++++++++------
 qemu-img.c            |  2 +-
 3 files changed, 15 insertions(+), 7 deletions(-)

-- 
2.30.2



^ permalink raw reply	[flat|nested] 5+ messages in thread

* [PATCH 1/2] block: Add BDRV_O_NO_SHARE for blk_new_open()
  2021-04-13 13:23 [PATCH 0/2] qemu-img convert: Unshare write permission for source Kevin Wolf
@ 2021-04-13 13:23 ` Kevin Wolf
  2021-04-13 13:41   ` Eric Blake
  2021-04-13 13:23 ` [PATCH 2/2] qemu-img convert: Unshare write permission for source Kevin Wolf
  1 sibling, 1 reply; 5+ messages in thread
From: Kevin Wolf @ 2021-04-13 13:23 UTC (permalink / raw)
  To: qemu-block; +Cc: kwolf, xuwei, qemu-devel

Normally, blk_new_open() just shares all permissions. This was fine
originally when permissions only protected against uses in the same
process because no other part of the code would actually get to access
the block nodes opened with blk_new_open(). However, since we use it for
file locking now, unsharing permissions becomes desirable.

Add a new BDRV_O_NO_SHARE flag that is used in blk_new_open() to unshare
any permissions that can be unshared.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 include/block/block.h |  1 +
 block/block-backend.c | 19 +++++++++++++------
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/block/block.h b/include/block/block.h
index b3f6e509d4..735db05a39 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -101,6 +101,7 @@ typedef struct HDGeometry {
     uint32_t cylinders;
 } HDGeometry;
 
+#define BDRV_O_NO_SHARE    0x0001 /* don't share permissons */
 #define BDRV_O_RDWR        0x0002
 #define BDRV_O_RESIZE      0x0004 /* request permission for resizing the node */
 #define BDRV_O_SNAPSHOT    0x0008 /* open the file read only and save writes in a snapshot */
diff --git a/block/block-backend.c b/block/block-backend.c
index 413af51f3b..b4746541ac 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -398,15 +398,19 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
     BlockBackend *blk;
     BlockDriverState *bs;
     uint64_t perm = 0;
+    uint64_t shared = BLK_PERM_ALL;
 
-    /* blk_new_open() is mainly used in .bdrv_create implementations and the
-     * tools where sharing isn't a concern because the BDS stays private, so we
-     * just request permission according to the flags.
+    /*
+     * blk_new_open() is mainly used in .bdrv_create implementations and the
+     * tools where sharing isn't a major concern because the BDS stays private
+     * and the file is generally not supposed to be used by a second process,
+     * so we just request permission according to the flags.
      *
      * The exceptions are xen_disk and blockdev_init(); in these cases, the
      * caller of blk_new_open() doesn't make use of the permissions, but they
      * shouldn't hurt either. We can still share everything here because the
-     * guest devices will add their own blockers if they can't share. */
+     * guest devices will add their own blockers if they can't share.
+     */
     if ((flags & BDRV_O_NO_IO) == 0) {
         perm |= BLK_PERM_CONSISTENT_READ;
         if (flags & BDRV_O_RDWR) {
@@ -416,8 +420,11 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
     if (flags & BDRV_O_RESIZE) {
         perm |= BLK_PERM_RESIZE;
     }
+    if (flags & BDRV_O_NO_SHARE) {
+        shared = BLK_PERM_WRITE_UNCHANGED;
+    }
 
-    blk = blk_new(qemu_get_aio_context(), perm, BLK_PERM_ALL);
+    blk = blk_new(qemu_get_aio_context(), perm, shared);
     bs = bdrv_open(filename, reference, options, flags, errp);
     if (!bs) {
         blk_unref(blk);
@@ -426,7 +433,7 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
 
     blk->root = bdrv_root_attach_child(bs, "root", &child_root,
                                        BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
-                                       blk->ctx, perm, BLK_PERM_ALL, blk, errp);
+                                       blk->ctx, perm, shared, blk, errp);
     if (!blk->root) {
         blk_unref(blk);
         return NULL;
-- 
2.30.2



^ permalink raw reply	[flat|nested] 5+ messages in thread

* [PATCH 2/2] qemu-img convert: Unshare write permission for source
  2021-04-13 13:23 [PATCH 0/2] qemu-img convert: Unshare write permission for source Kevin Wolf
  2021-04-13 13:23 ` [PATCH 1/2] block: Add BDRV_O_NO_SHARE for blk_new_open() Kevin Wolf
@ 2021-04-13 13:23 ` Kevin Wolf
  2021-04-13 13:43   ` Eric Blake
  1 sibling, 1 reply; 5+ messages in thread
From: Kevin Wolf @ 2021-04-13 13:23 UTC (permalink / raw)
  To: qemu-block; +Cc: kwolf, xuwei, qemu-devel

For a successful conversion of an image, we must make sure that its
content doesn't change during the conversion.

A special case of this is using the same image file both as the source
and as the destination. If both input and output format are raw, the
operation would just be useless work, with other formats it is a sure
way to destroy the image. This will now fail because the image file
can't be opened a second time for the output when opening it for the
input has already acquired file locks to unshare BLK_PERM_WRITE.

Nevertheless, if there is some reason in a special case why it is
actually okay to allow writes to the image while it is being converted,
-U can still be used to force sharing all permissions.

Note that for most image formats, BLK_PERM_WRITE would already be
unshared by the format driver, so this only really makes a difference
for raw source images (but any output format).

Reported-by: Xueqiang Wei <xuwei@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 qemu-img.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/qemu-img.c b/qemu-img.c
index babb5573ab..a5993682aa 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2146,7 +2146,7 @@ static void set_rate_limit(BlockBackend *blk, int64_t rate_limit)
 
 static int img_convert(int argc, char **argv)
 {
-    int c, bs_i, flags, src_flags = 0;
+    int c, bs_i, flags, src_flags = BDRV_O_NO_SHARE;
     const char *fmt = NULL, *out_fmt = NULL, *cache = "unsafe",
                *src_cache = BDRV_DEFAULT_CACHE, *out_baseimg = NULL,
                *out_filename, *out_baseimg_param, *snapshot_name = NULL;
-- 
2.30.2



^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH 1/2] block: Add BDRV_O_NO_SHARE for blk_new_open()
  2021-04-13 13:23 ` [PATCH 1/2] block: Add BDRV_O_NO_SHARE for blk_new_open() Kevin Wolf
@ 2021-04-13 13:41   ` Eric Blake
  0 siblings, 0 replies; 5+ messages in thread
From: Eric Blake @ 2021-04-13 13:41 UTC (permalink / raw)
  To: Kevin Wolf, qemu-block; +Cc: xuwei, qemu-devel

On 4/13/21 8:23 AM, Kevin Wolf wrote:
> Normally, blk_new_open() just shares all permissions. This was fine
> originally when permissions only protected against uses in the same
> process because no other part of the code would actually get to access
> the block nodes opened with blk_new_open(). However, since we use it for
> file locking now, unsharing permissions becomes desirable.
> 
> Add a new BDRV_O_NO_SHARE flag that is used in blk_new_open() to unshare
> any permissions that can be unshared.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  include/block/block.h |  1 +
>  block/block-backend.c | 19 +++++++++++++------
>  2 files changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/include/block/block.h b/include/block/block.h
> index b3f6e509d4..735db05a39 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -101,6 +101,7 @@ typedef struct HDGeometry {
>      uint32_t cylinders;
>  } HDGeometry;
>  
> +#define BDRV_O_NO_SHARE    0x0001 /* don't share permissons */
>  #define BDRV_O_RDWR        0x0002

Huh. We originally created BDRV_O_RDONLY = 0 and BDRV_O_RDWR = 2 back in
commit 83f6409109 (in 2006); I see no reason why we skipped 1 at that
time (maybe to allow BDRV_O_WRONLY? but who would use it), so exploiting
it now seems fine.

Reviewed-by: Eric Blake <eblake@redhat.com>

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



^ permalink raw reply	[flat|nested] 5+ messages in thread

* Re: [PATCH 2/2] qemu-img convert: Unshare write permission for source
  2021-04-13 13:23 ` [PATCH 2/2] qemu-img convert: Unshare write permission for source Kevin Wolf
@ 2021-04-13 13:43   ` Eric Blake
  0 siblings, 0 replies; 5+ messages in thread
From: Eric Blake @ 2021-04-13 13:43 UTC (permalink / raw)
  To: Kevin Wolf, qemu-block; +Cc: xuwei, qemu-devel

On 4/13/21 8:23 AM, Kevin Wolf wrote:
> For a successful conversion of an image, we must make sure that its
> content doesn't change during the conversion.
> 
> A special case of this is using the same image file both as the source
> and as the destination. If both input and output format are raw, the
> operation would just be useless work, with other formats it is a sure
> way to destroy the image. This will now fail because the image file
> can't be opened a second time for the output when opening it for the
> input has already acquired file locks to unshare BLK_PERM_WRITE.
> 
> Nevertheless, if there is some reason in a special case why it is
> actually okay to allow writes to the image while it is being converted,
> -U can still be used to force sharing all permissions.
> 
> Note that for most image formats, BLK_PERM_WRITE would already be
> unshared by the format driver, so this only really makes a difference
> for raw source images (but any output format).
> 
> Reported-by: Xueqiang Wei <xuwei@redhat.com>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  qemu-img.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

> 
> diff --git a/qemu-img.c b/qemu-img.c
> index babb5573ab..a5993682aa 100644
> --- a/qemu-img.c
> +++ b/qemu-img.c
> @@ -2146,7 +2146,7 @@ static void set_rate_limit(BlockBackend *blk, int64_t rate_limit)
>  
>  static int img_convert(int argc, char **argv)
>  {
> -    int c, bs_i, flags, src_flags = 0;
> +    int c, bs_i, flags, src_flags = BDRV_O_NO_SHARE;
>      const char *fmt = NULL, *out_fmt = NULL, *cache = "unsafe",
>                 *src_cache = BDRV_DEFAULT_CACHE, *out_baseimg = NULL,
>                 *out_filename, *out_baseimg_param, *snapshot_name = NULL;
> 

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



^ permalink raw reply	[flat|nested] 5+ messages in thread

end of thread, other threads:[~2021-04-13 13:46 UTC | newest]

Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-04-13 13:23 [PATCH 0/2] qemu-img convert: Unshare write permission for source Kevin Wolf
2021-04-13 13:23 ` [PATCH 1/2] block: Add BDRV_O_NO_SHARE for blk_new_open() Kevin Wolf
2021-04-13 13:41   ` Eric Blake
2021-04-13 13:23 ` [PATCH 2/2] qemu-img convert: Unshare write permission for source Kevin Wolf
2021-04-13 13:43   ` Eric Blake

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