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

From: Tom Stellard <tstellar@redhat.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>,
	Alexei Starovoitov <ast@fb.com>, bpf <bpf@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>,
	Networking <netdev@vger.kernel.org>
Subject: Re: [PATCH v2 bpf-next 06/11] libbpf: add BPF static linker APIs
Date: Tue, 8 Jun 2021 20:44:02 -0700	[thread overview]
Message-ID: <b322da84-95f3-2800-f2c8-556e9855d517@redhat.com> (raw)
In-Reply-To: <CAEf4BzbCiMkQazSe2hky=Jx6QXZiZ2jyf+AuzMJEyAv+_B7vug@mail.gmail.com>

On 6/7/21 9:08 PM, Andrii Nakryiko wrote:
> On Mon, Jun 7, 2021 at 7:41 PM Tom Stellard <tstellar@redhat.com> wrote:
>>
>> On 6/7/21 5:25 PM, Andrii Nakryiko wrote:
>>> On Mon, Jun 7, 2021 at 4:12 PM Tom Stellard <tstellar@redhat.com> wrote:
>>>>
>>>>
>>>> Hi,
>>>>
>>>>> +                               } 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)
>>
>>>> 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

>> Thanks,
>> Tom
>>
>>>>
>>>>>
>>>>> Thanks,
>>>>> Tom
>>>>>
>>>>
>>>
>>
> 