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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 13:36:42 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 |
On 17.07.20 13:32, Kevin Wolf wrote:
> Am 17.07.2020 um 13:02 hat Max Reitz geschrieben:
>> 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 basically just replacing the line with this?
>
> if ((cumulative_perms & (BLK_PERM_WRITE | BDRV_PERM_WRITE_UNCHANGED)) &&
>
> I can do that while applying if it is what you mean.
Yes. :)
>> So, either way:
>>
>> Reviewed-by: Max Reitz <mreitz@redhat.com>
>
> Thanks!
>
> Kevin
>
>>> + !(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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