On 16.07.20 16:26, Kevin Wolf wrote:
> Unaligned requests will automatically be aligned to bl.request_alignment
> and we can't extend write requests to access space beyond the end of the
> image without resizing the image, so if we have the WRITE permission,
> but not the RESIZE one, it's required that the image size is aligned.
> 
> Failing to meet this requirement could cause assertion failures like
> this if RESIZE permissions weren't requested:
> 
> qemu-img: block/io.c:1910: bdrv_co_write_req_prepare: Assertion `end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed.
> 
> This was e.g. triggered by qemu-img converting to a target image with 4k
> request alignment when the image was only aligned to 512 bytes, but not
> to 4k.
> 
> Turn this into a graceful error in bdrv_check_perm() so that WRITE
> without RESIZE can only be taken if the image size is aligned. If a user
> holds both permissions and drops only RESIZE, the function will return
> an error, but bdrv_child_try_set_perm() will ignore the failure silently
> if permissions are only requested to be relaxed and just keep both
> permissions while returning success.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/block.c b/block.c
> index 35a372df57..6371928edb 100644
> --- a/block.c
> +++ b/block.c
> @@ -2025,6 +2025,22 @@ static int bdrv_check_perm(BlockDriverState *bs, BlockReopenQueue *q,
>          return -EPERM;
>      }
>  
> +    /*
> +     * Unaligned requests will automatically be aligned to bl.request_alignment
> +     * and without RESIZE we can't extend requests to write to space beyond the
> +     * end of the image, so it's required that the image size is aligned.
> +     */
> +    if ((cumulative_perms & BLK_PERM_WRITE) &&

What about WRITE_UNCHANGED?  I think this would only matter with nodes
that can have backing files (i.e., qcow2 in practice) because
WRITE_UNCHANGED is only used by COR and block jobs doing something with
a backing chain, so it shouldn’t matter in practice, but, well.

So, either way:

Reviewed-by: Max Reitz <mreitz@redhat.com>

> +        !(cumulative_perms & BLK_PERM_RESIZE))
> +    {
> +        if ((bs->total_sectors * BDRV_SECTOR_SIZE) % bs->bl.request_alignment) {
> +            error_setg(errp, "Cannot get 'write' permission without 'resize': "
> +                             "Image size is not a multiple of request "
> +                             "alignment");
> +            return -EPERM;
> +        }
> +    }
> +
>      /* Check this node */
>      if (!drv) {
>          return 0;
>