[Qemu-devel] [PATCH 1/3] qcow2: Check bs->drv in copy_sectors()

[Max Reitz <mreitz@redhat.com>]

Before dereferencing bs->drv for a call to its member bdrv_co_readv(),
copy_sectors() should check whether that pointer is indeed valid, since
it may have been set to NULL by e.g. a concurrent write triggering the
corruption prevention mechanism.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
To be precise, this still is a race condition. If bs->drv is set to NULL
after the check and before the call to bdrv_co_readv(), QEMU will
obviously still crash. However, in order to circumvent this behavior, we
would probably have to re-lock s->lock, check bs->drv, take the function
pointer to bdrv_co_readv() and then unlock s->lock before the function
is called. I found this rather ugly and therefore this still has a very
small chance of running into a race condition.
Therefore, I'm asking for your opinion on this, whether we can really
take this chance or should rather "do it right". In fact, if I were a
reviewer, I'd probably reject this patch and request the solution with
the function pointer (if there is no better solution), but I was afraid
to send such an ugly patch.
---
 block/qcow2-cluster.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 36c1bed..9499df9 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -380,6 +380,10 @@ static int coroutine_fn copy_sectors(BlockDriverState *bs,
 
     BLKDBG_EVENT(bs->file, BLKDBG_COW_READ);
 
+    if (!bs->drv) {
+        return -ENOMEDIUM;
+    }
+
     /* Call .bdrv_co_readv() directly instead of using the public block-layer
      * interface.  This avoids double I/O throttling and request tracking,
      * which can lead to deadlock when block layer copy-on-read is enabled.
-- 
1.9.0