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

[Palmer Dabbelt]

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@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