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
