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

[David Gow <davidgow@google.com>]

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