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

---

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