Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Nov 01, 2022 at 07:27:16PM -0700, Eric Biggers wrote:
> On Tue, Nov 01, 2022 at 03:00:30PM -0400, Stefan Hajnoczi wrote:
> > Linux dm-crypt returns errno EIO from unaligned O_DIRECT pread(2) calls.
> 
> Citation needed.  For direct I/O to block devices, the kernel's block layer
> checks the alignment before the I/O is actually submitted to the underlying
> block device.  See
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/fops.c?h=v6.1-rc3#n306
> 
> > Buglink: https://gitlab.com/qemu-project/qemu/-/issues/1290
> 
> That "bug" seems to be based on a misunderstanding of the kernel source code,
> and not any actual testing.
> 
> I just tested it, and the error code is EINVAL.
> 

I think I see what's happening.  The kernel code was broken just a few months
ago, in v6.0 by the commit "block: relax direct io memory alignment"
(https://git.kernel.org/linus/b1a000d3b8ec582d).  Now the block layer lets DIO
through when the user buffer is only aligned to the device's dma_alignment.  But
a dm-crypt device has a dma_alignment of 512 even when the crypto sector size
(and thus also the logical block size) is 4096.  So there is now a case where
misaligned DIO can reach dm-crypt, when that shouldn't be possible.

It also means that STATX_DIOALIGN will give the wrong value for
stx_dio_mem_align in the above case, 512 instead of 4096.  This is because
STATX_DIOALIGN for block devices relies on the dma_alignment.

I'll raise this on the linux-block and dm-devel mailing lists.  It would be nice
if people reported kernel bugs instead of silently working around them...

- Eric