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

[Stefan Hajnoczi]

On Thu, Nov 03, 2022 at 10:52:43AM +0100, Kevin Wolf wrote:
> Am 02.11.2022 um 03:49 hat Eric Biggers geschrieben:
> > 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.
> 
> In other words, STATX_DIOALIGN is unusable from the start because we
> don't know whether the information it returns is actually correct? :-/
> 
> I guess we could still use the value returned by STATX_DIOALIGN as a
> preferred value that we'll use if it survives probing, and otherwise
> fall back to the same probing we've always been doing because there was
> no (or no sane) way to get the information from the kernel.

Yes, it seems probing is required to verify the values returned by
STATX_DIOALIGN. At least until enough time passes so we can say
"STATX_DIOALIGN has been correct for X years and no one is running those
old kernels anymore".

Stefan

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