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

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

[David Gow]

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@google.com>
---

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@google.com/

---
 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

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

[Noah Goldstein]

On Sat, Jul 20, 2024 at 2:40 PM David Gow <davidgow@google.com> 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@google.com>
> ---
>
> 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@google.com/
>
> ---
>  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@gmail.com>

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

[David Gow]

[-- Attachment #1: Type: text/plain, Size: 2922 bytes --]

On Sat, 20 Jul 2024 at 14:40, David Gow <davidgow@google.com> 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@google.com>
> ---
>
> 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@google.com/
>
> ---

Dragging this back to the surface as it's the last remaining test
failure on my KUnit test grid.

Is there any particular reason this can't go in as-is? It still
applies cleanly here and fixes the tests, but I'm happy to re-send it
/ make changes if you'd prefer.

If you'd rather it go in via the KUnit tree, that should be fine, too.

Thanks,
-- David

>  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
>

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