Re: request_alignment vs file size, how to fix crash?

[Vladimir Sementsov-Ogievskiy]

29.01.2020 21:01, Vladimir Sementsov-Ogievskiy wrote:
> Hi!
>
> I found a crash, which may be simply triggered for images unaligned to 
> request_alignment:
>
> # ./qemu-io --image-opts -c 'write 0 512' 
> driver=blkdebug,align=4096,image.driver=null-co,image.size=512
> qemu-io: block/io.c:1505: bdrv_aligned_pwritev: Assertion `end_sector <= 
> bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed.
> Aborted (core dumped)
>
> The problem is obvious: 512 is aligned to 4096 and becomes larger than file 
> size.
>
> I faced it after rebasing our downstream branches to newer Rhel versions. Seems 
> that after some updates of alignment detection in file-posix.c, it started to 
> detect 4096 alignment in our build environment, and iotest 152 started to crash 
> (as it operates on file of 512 bytes).
>
> My question is:
>
> What is wrong? Should we restrict images to be aligned to request_alignment, or 
> allow unaligned operations at EOF, if file is unaligned itself?


The problem started with commit

commit a6b257a08e3d72219f03e461a52152672fec0612
Author: Nir Soffer <address@hidden>
Date:   Tue Aug 13 21:21:03 2019 +0300

    file-posix: Handle undetectable alignment


It sets request_alignment to 4k, if probing of align=1 succeeded.. I think it's 
wrong logic. It leads to crashes for images unaligned to 4k.

If we force alignment to be 4k, we at least should check that file size is 
aligned to 4k. Otherwise our assumption is definitely wrong.

And still, I doubt that it's correct to force alignment to 4k, for devices 
which doesn't request any alignment..


--
Best regards,
Vladimir