Re: [Qemu-devel] [PATCH 1/1] parallels: wrong call to bdrv_truncate

[Kevin Wolf <kwolf@redhat.com>]

[ Cc: qemu-block ]

Am 27.03.2017 um 16:38 hat Denis V. Lunev geschrieben:
> Parallels driver should not call bdrv_truncate if the image was opened
> in the read-only mode. Without the patch
>     qemu-img check harddisk.hds
> asserts with
>     bdrv_truncate: Assertion `child->perm & BLK_PERM_RESIZE' failed.
> 
> Parameters used on the write path are not needed if the image is opened
> in the read-only mode.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Reported-by: Edgar Kaziahmedov <edos@virtuozzo.mipt.ru>
> CC: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  block/parallels.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/parallels.c b/block/parallels.c
> index 6bf9375..4173b3f 100644
> --- a/block/parallels.c
> +++ b/block/parallels.c
> @@ -687,7 +687,8 @@ static int parallels_open(BlockDriverState *bs, QDict *options, int flags,
>      if (local_err != NULL) {
>          goto fail_options;
>      }
> -    if (!bdrv_has_zero_init(bs->file->bs) ||
> +
> +    if (!(flags & BDRV_O_RESIZE) || !bdrv_has_zero_init(bs->file->bs) ||
>              bdrv_truncate(bs->file, bdrv_getlength(bs->file->bs)) != 0) {
>          s->prealloc_mode = PRL_PREALLOC_MODE_FALLOCATE;
>      }

Relying on BDRV_O_RESIZE in block drivers is wrong. It is set in some
paths (specifically the users of blk_new_open), but not in others. We
should probably have filtered out the flag before passing it to the
drivers.

As a concrete example, if you're using -blockdev, the bdrv_truncate()
call won't be executed after applying this patch.

I think the correct way would be to check bdrv_is_read_only() instead.

Kevin