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Re: [PATCH for-5.1 v2 1/2] block: Require aligned image size to avoid as
From: |
Max Reitz |
Subject: |
Re: [PATCH for-5.1 v2 1/2] block: Require aligned image size to avoid assertion failure |
Date: |
Fri, 17 Jul 2020 13:02:02 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 |
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;
>
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