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

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Date:   Tue, 8 Jun 2021 21:41:38 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Tom Stellard <tstellar@...hat.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...com>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 06/11] libbpf: add BPF static linker APIs

On Tue, Jun 8, 2021 at 8:44 PM Tom Stellard <tstellar@...hat.com> wrote:
>
> On 6/7/21 9:08 PM, Andrii Nakryiko wrote:
> > On Mon, Jun 7, 2021 at 7:41 PM Tom Stellard <tstellar@...hat.com> wrote:
> >>
> >> On 6/7/21 5:25 PM, Andrii Nakryiko wrote:
> >>> On Mon, Jun 7, 2021 at 4:12 PM Tom Stellard <tstellar@...hat.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
>

See README.rst in selftests/bpf:

Kernel function call test and Clang version
===========================================

Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support
to generate extern function in BTF.  It was introduced in `Clang 13`__.

Without it, the error from compiling bpf selftests looks like:

.. code-block:: console

  libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2

__ https://reviews.llvm.org/D93563


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