block: Do not unref bs->file on error in BD's open
The block layer takes care of removing the bs->file child if the block driver's bdrv_open()/bdrv_file_open() implementation fails. The block driver therefore does not need to do so, and indeed should not unless it sets bs->file to NULL afterwards -- because if this is not done, the bdrv_unref_child() in bdrv_open_inherit() will dereference the freed memory block at bs->file afterwards, which is not good. We can now decide whether to add a "bs->file = NULL;" after each of the offending bdrv_unref_child() invocations, or just drop them altogether. The latter is simpler, so let's do that. Cc: qemu-stable <qemu-stable@nongnu.org> Signed-off-by: NMax Reitz <mreitz@redhat.com> Reviewed-by: NEric Blake <eblake@redhat.com> Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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