Re: [PATCH RESEND] x86: checksum: Fix unaligned checksums on < i686

[Noah Goldstein]

On Sat, Jul 20, 2024 at 2:40 PM David Gow <davidgow@xxxxxxxxxx> wrote:
>
> The checksum_32 code was originally written to only handle 2-byte
> aligned buffers, but was later extended to support arbitrary alignment.
> However, the non-PPro variant doesn't apply the carry before jumping to
> the 2- or 4-byte aligned versions, which clear CF.
>
> This causes the new checksum_kunit test to fail, as it runs with a large
> number of different possible alignments and both with and without
> carries.
>
> For example:
> ./tools/testing/kunit/kunit.py run --arch i386 --kconfig_add CONFIG_M486=y checksum
> Gives:
>     KTAP version 1
>     # Subtest: checksum
>     1..3
>     ok 1 test_csum_fixed_random_inputs
>     # test_csum_all_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:267
>     Expected result == expec, but
>         result == 65281 (0xff01)
>         expec == 65280 (0xff00)
>     not ok 2 test_csum_all_carry_inputs
>     # test_csum_no_carry_inputs: ASSERTION FAILED at lib/checksum_kunit.c:314
>     Expected result == expec, but
>         result == 65535 (0xffff)
>         expec == 65534 (0xfffe)
>     not ok 3 test_csum_no_carry_inputs
>
> With this patch, it passes.
>     KTAP version 1
>     # Subtest: checksum
>     1..3
>     ok 1 test_csum_fixed_random_inputs
>     ok 2 test_csum_all_carry_inputs
>     ok 3 test_csum_no_carry_inputs
>
> I also tested it on a real 486DX2, with the same results.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: David Gow <davidgow@xxxxxxxxxx>
> ---
>
> Re-sending this from [1]. While there's an argument that the whole
> 32-bit checksum code could do with rewriting, it's:
> (a) worth fixing before someone takes the time to rewrite it, and
> (b) worth any future rewrite starting from a point where the tests pass
>
> I don't think there should be any downside to this fix: it only affects
> ancient computers, and adds a single instruction which isn't in a loop.
>
> Cheers,
> -- David
>
> [1]: https://lore.kernel.org/lkml/20230704083206.693155-2-davidgow@xxxxxxxxxx/
>
> ---
>  arch/x86/lib/checksum_32.S | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/lib/checksum_32.S b/arch/x86/lib/checksum_32.S
> index 68f7fa3e1322..a5123b29b403 100644
> --- a/arch/x86/lib/checksum_32.S
> +++ b/arch/x86/lib/checksum_32.S
> @@ -62,6 +62,7 @@ SYM_FUNC_START(csum_partial)
>         jl 8f
>         movzbl (%esi), %ebx
>         adcl %ebx, %eax
> +       adcl $0, %eax
>         roll $8, %eax
>         inc %esi
>         testl $2, %esi
> --
> 2.45.2.1089.g2a221341d9-goog
>

I'm not maintainer but LGTM.

Reviewed-by: Noah Goldstein <goldstein.w.n@xxxxxxxxx>