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

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Date:   Fri, 13 Nov 2020 13:24:40 +0100
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Tom Herbert <tom@...bertland.com>,
        Anders Roxell <anders.roxell@...il.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: csum_partial() on different archs (selftest/bpf)

Hi,

On Fri, Nov 13, 2020 at 11:36:08AM +0100, Björn Töpel wrote:
> 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].

It's worse than that, because arm64, parisc, alpha and others implement
do_csum(), called by the generic csum_partial(), and those all return a
16-bit value.

It would be good to firstly formalize the size of the value returned by
the bpf_csum_diff() helper, because it's not clear from the doc (and the
helper returns a s64).

Then homogenizing the csum_partial() implementations is difficult. One way
forward, without having to immediately rewrite all arch-specific
implementations, would be to replace csum_partial() and do_csum() with
csum_partial_32(), csum_partial_16(), do_csum_32() and do_csum_16(). That
way we can use a generic implementation of the 32-bit variant if the
arch-specific implementation is 16-bit.

Thanks,
Jean

> 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