linux-kernel - Re: [PATCH] selftests/bpf: Elide a check for LLVM versions that can't compile it

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Date:   Fri, 24 Jan 2020 21:27:11 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Palmer Dabbelt <palmerdabbelt@...gle.com>,
        linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     shuah@...nel.org, ast@...nel.org, daniel@...earbox.net,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
        john.fastabend@...il.com,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        kernel-team@...roid.com, Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH] selftests/bpf: Elide a check for LLVM versions that can't compile it

Palmer Dabbelt <palmerdabbelt@...gle.com> writes:

> The current stable LLVM BPF backend fails to compile the BPF selftests
> due to a compiler bug.  The bug has been fixed in trunk, but that fix
> hasn't landed in the binary packages I'm using yet (Fedora arm64).
> Without this workaround the tests don't compile for me.
>
> This patch triggers a preprocessor warning on LLVM versions that
> definitely have the bug.  The test may be conservative (ie, I'm not sure
> if 9.1 will have the fix), but it should at least make the current set
> of stable releases work together.
>
> See https://reviews.llvm.org/D69438 for more information on the fix.  I
> obtained the workaround from
> https://lore.kernel.org/linux-kselftest/aed8eda7-df20-069b-ea14-f06628984566@gmail.com/T/
>
> Fixes: 20a9ad2e7136 ("selftests/bpf: add CO-RE relocs array tests")
> Signed-off-by: Palmer Dabbelt <palmerdabbelt@...gle.com>

Having to depend on the latest trunk llvm to compile the selftests is
definitely unfortunate. I believe there are some tests that won't work
at all without trunk llvm (the fentry/fexit stuff comes to mind;
although I'm not sure if they'll fail to compile, just fail to run?).
Could we extend this type of checking to any such case?

-Toke