Re: [PATCH v2 bpf-next 06/11] libbpf: add BPF static linker APIs

On 6/7/21 9:08 PM, Andrii Nakryiko wrote:

On Mon, Jun 7, 2021 at 7:41 PM Tom Stellard [off-list ref] wrote:

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On 6/7/21 5:25 PM, Andrii Nakryiko wrote:

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On Mon, Jun 7, 2021 at 4:12 PM Tom Stellard [off-list ref] wrote:

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Hi,

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+                               } else {
+                                       pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
+                                       return -EINVAL;
+                               }

Kernel build of commit 324c92e5e0ee are failing for me with this error
message:

/builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/bpf/bpftool/bpftool gen object /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/bind_perm.linked1.o /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/bind_perm.o
libbpf: relocation against STT_SECTION in non-exec section is not supported!

What information can I provide to help debug this failure?

Can you please send that bind_perm.o file? Also what's your `clang
--version` output?

clang version 12.0.0 (Fedora 12.0.0-2.fc35)

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I suspect this might be due to Clang commit 6a2ea84600ba ("BPF: Add
more relocation kinds"), but I get a different error on 324c92e5e0ee.
So meanwhile you might try applying 9f0c317f6aa1 ("libbpf: Add support
for new llvm bpf relocations") from bpf-next/master and check if that
helps. But please do share bind_perm.o, just to double-check what's
going on.

Here is bind_perm.o: https://fedorapeople.org/~tstellar/bind_perm.o

So somehow you end up with .eh_frame section in BPF object file, which
shouldn't ever happen. So there must be something that you are doing
differently (compiler flags or something else) that makes Clang
produce .eh_frame. So we need to figure out why .eh_frame gets
generated. Not sure how, but maybe you have some ideas of what might
be different about your build.

Thanks for the pointer.  The problem was that in the Fedora kernel builds,
we enable -funwind-tables by default on all architectures, which is why the
.eh_frame section was there.  I fixed our clang builds, but I'm now getting
a new error when I run: CC=clang make -C tools/testing/selftests/bpf  V=1


/builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/tools/sbin/bpftool gen skeleton /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/bpf_cubic.linked3.o name bpf_cubic > /builddir/build/BUILD/kernel-5.13-rc4-61-g324c92e5e0ee/linux-5.13.0-0.rc4.20210603git324c92e5e0ee.35.fc35.x86_64/tools/testing/selftests/bpf/bpf_cubic.skel.h
libbpf: failed to find BTF for extern 'tcp_cong_avoid_ai' [27] section: -2

Here is the bpf_cubic.lined3.o object file: https://fedorapeople.org/~tstellar/bpf_cubic.linked3.o

-Tom

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Thanks,
Tom

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Thanks,
Tom