netdev - csum_partial() on different archs (selftest/bpf)

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Date:   Fri, 13 Nov 2020 11:36:08 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Cc:     Tom Herbert <tom@...bertland.com>,
        Anders Roxell <anders.roxell@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: csum_partial() on different archs (selftest/bpf)

I was running the selftest/bpf on riscv, and had a closer look at one
of the failing cases:

  #14/p valid read map access into a read-only array 2 FAIL retval
65507 != -29 (run 1/1)

The test does a csum_partial() call via a BPF helper. riscv uses the
generic implementation. arm64 uses the generic csum_partial() and fail
in the same way [1]. arm (32-bit) has a arch specfic implementation,
and fail in another way (FAIL retval 131042 != -29) [2].

I mimicked the test case in a userland program, comparing the generic
csum_partial() to the x86 implementation [3], and the generic and x86
implementation does yield a different result.

x86     :    -29 : 0xffffffe3
generic :  65507 : 0x0000ffe3
arm     : 131042 : 0x0001ffe2

Who is correct? :-) It would be nice to get rid of this failed case...

Thanks,
Björn

[1] https://qa-reports.linaro.org/lkft/linux-next-master/build/next-20201112/testrun/3430401/suite/kselftest/test/bpf.test_verifier/log
[2] https://qa-reports.linaro.org/lkft/linux-mainline-master/build/v5.10-rc3-207-g585e5b17b92d/testrun/3432361/suite/kselftest/test/bpf.test_verifier/log
[3] https://gist.github.com/bjoto/dc22d593aa3ac63c2c90632de5ed82e0