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

[Kevin Wolf <kwolf@redhat.com>]

Am 30.01.2020 um 11:40 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 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 <nirsof@gmail.com>
> 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..

What backend is this? O_DIRECT with byte alignment sounds wrong, so I
wonder if your storage really can do this or whether we just failed to
detect the actual alignment.

I guess we could change the default to pick the largest size so that the
image size is still a multiple of it. But if the image size isn't even
aligned to 512 bytes, I think refusing to open the image with O_DIRECT
feels more correct (I would be okay with doing the same with > 512 byte
images, too, if the image size isn't a multiple of the alignment).

Kevin