Re: [PATCH 1/4] selftests/bpf: Elide a check for LLVM versions that can't compile it

[Nick Desaulniers]

On Mon, Jan 27, 2020 at 6:14 PM 'Palmer Dabbelt' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> 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.

Do older versions of clang still work? Should there be a lower bounds?

>
> 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@xxxxxxxxx/T/
>
> Fixes: 20a9ad2e7136 ("selftests/bpf: add CO-RE relocs array tests")
> Signed-off-by: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> ---
>  .../testing/selftests/bpf/progs/test_core_reloc_arrays.c  | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/test_core_reloc_arrays.c b/tools/testing/selftests/bpf/progs/test_core_reloc_arrays.c
> index bf67f0fdf743..c9a3e0585a84 100644
> --- a/tools/testing/selftests/bpf/progs/test_core_reloc_arrays.c
> +++ b/tools/testing/selftests/bpf/progs/test_core_reloc_arrays.c
> @@ -40,15 +40,23 @@ int test_core_arrays(void *ctx)
>         /* in->a[2] */
>         if (BPF_CORE_READ(&out->a2, &in->a[2]))
>                 return 1;
> +#if defined(__clang__) && (__clang_major__ < 10) && (__clang_minor__ < 1)
> +# warning "clang 9.0 SEGVs on multidimensional arrays, see https://reviews.llvm.org/D69438";
> +#else
>         /* in->b[1][2][3] */
>         if (BPF_CORE_READ(&out->b123, &in->b[1][2][3]))
>                 return 1;
> +#endif
>         /* in->c[1].c */
>         if (BPF_CORE_READ(&out->c1c, &in->c[1].c))
>                 return 1;
> +#if defined(__clang__) && (__clang_major__ < 10) && (__clang_minor__ < 1)
> +# warning "clang 9.0 SEGVs on multidimensional arrays, see https://reviews.llvm.org/D69438";
> +#else
>         /* in->d[0][0].d */
>         if (BPF_CORE_READ(&out->d00d, &in->d[0][0].d))
>                 return 1;
> +#endif
>
>         return 0;
>  }
> --
> 2.25.0.341.g760bfbb309-goog
>
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-- 
Thanks,
~Nick Desaulniers