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Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is retu
From: |
Eric Biggers |
Subject: |
Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned |
Date: |
Tue, 1 Nov 2022 19:49:20 -0700 |
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
- [PATCH 0/2] file-posix: alignment probing improvements, Stefan Hajnoczi, 2022/11/01
- [PATCH 2/2] file-posix: add statx(STATX_DIOALIGN) support, Stefan Hajnoczi, 2022/11/01
- [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Stefan Hajnoczi, 2022/11/01
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Eric Biggers, 2022/11/02
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned,
Eric Biggers <=
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Stefan Hajnoczi, 2022/11/02
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Kevin Wolf, 2022/11/03
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Stefan Hajnoczi, 2022/11/03
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Eric Biggers, 2022/11/03
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Eric Biggers, 2022/11/03
- Re: [PATCH 1/2] file-posix: fix Linux alignment probing when EIO is returned, Stefan Hajnoczi, 2022/11/03