Re: [Qemu-devel] [PATCH v4 1/3] file-posix: Skip effectiveless OFD lock operations

[Max Reitz]

On 21.08.18 02:58, Fam Zheng wrote:
> If we know we've already locked the bytes, don't do it again; similarly
> don't unlock a byte if we haven't locked it. This doesn't change the
> behavior, but fixes a corner case explained below.
> 
> Libvirt had an error handling bug that an image can get its (ownership,
> file mode, SELinux) permissions changed (RHBZ 1584982) by mistake behind
> QEMU. Specifically, an image in use by Libvirt VM has:
> 
>     $ ls -lhZ b.img
>     -rw-r--r--. qemu qemu system_u:object_r:svirt_image_t:s0:c600,c690 b.img
> 
> Trying to attach it a second time won't work because of image locking.
> And after the error, it becomes:
> 
>     $ ls -lhZ b.img
>     -rw-r--r--. root root system_u:object_r:virt_image_t:s0 b.img
> 
> Then, we won't be able to do OFD lock operations with the existing fd.
> In other words, the code such as in blk_detach_dev:
> 
>     blk_set_perm(blk, 0, BLK_PERM_ALL, &error_abort);
> 
> can abort() QEMU, out of environmental changes.
> 
> This patch is an easy fix to this and the change is regardlessly
> reasonable, so do it.
> 
> Signed-off-by: Fam Zheng <address@hidden>
> 
> ---
> 
> v3: Don't misuse s->perm and s->shared_perm.
> v2: For s == NULL, unlock all bits. [Kevin]
> ---
>  block/file-posix.c | 54 +++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 44 insertions(+), 10 deletions(-)

Reviewed-by: Max Reitz <address@hidden>

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