From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
lkml <linux-kernel@vger.kernel.org>,
Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
Ingo Molnar <mingo@kernel.org>,
Namhyung Kim <namhyung@kernel.org>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Michael Petlan <mpetlan@redhat.com>,
Jesper Dangaard Brouer <brouer@redhat.com>,
Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
Quentin Monnet <quentin.monnet@netronome.com>
Subject: Re: [PATCHv4 0/6] perf/bpftool: Allow to link libbpf dynamically
Date: Mon, 02 Dec 2019 22:15:35 +0100 [thread overview]
Message-ID: <87wobepgy0.fsf@toke.dk> (raw)
In-Reply-To: <CAEf4BzY_D9JHjuU6K=ciS70NSy2UvSm_uf1NfN_tmFz1445Jiw@mail.gmail.com>
Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> On Mon, Dec 2, 2019 at 5:19 AM Jiri Olsa <jolsa@kernel.org> wrote:
>>
>> hi,
>> adding support to link bpftool with libbpf dynamically,
>> and config change for perf.
>>
>> It's now possible to use:
>> $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1
>>
>> which will detect libbpf devel package and if found, link it with bpftool.
>>
>> It's possible to use arbitrary installed libbpf:
>> $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 LIBBPF_DIR=/tmp/libbpf/
>>
>> I based this change on top of Arnaldo's perf/core, because
>> it contains libbpf feature detection code as dependency.
>>
>> Also available in:
>> git://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git
>> libbpf/dyn
>>
>> v4 changes:
>> - based on Toke's v3 post, there's no need for additional API exports:
>>
>> Since bpftool uses bits of libbpf that are not exported as public API in
>> the .so version, we also pass in libbpf.a to the linker, which allows it to
>> pick up the private functions from the static library without having to
>> expose them as ABI.
>
> Whoever understands how this is supposed to work, can you please
> explain? From reading this, I think what we **want** is:
>
> - all LIBBPF_API-exposed APIs should be dynamically linked against libbpf.so;
> - everything else used from libbpf (e.g., netlink APIs), should come
> from libbpf.a.
>
> Am I getting the idea right?
>
> If yes, are we sure it actually works like that in practice? I've
> compiled with LIBBPF_DYNAMIC=1, and what I see is that libelf, libc,
> zlib, etc functions do have relocations against them in ".rela.plt"
> section. None of libbpf exposed APIs, though, have any of such
> relocations. Which to me suggests that they are just statically linked
> against libbpf.a and libbpf.so is just recorded in ELF as a dynamic
> library dependency because of this extra -lbpf flag. Which kind of
> defeats the purpose of this whole endeavor, no?
>
> I'm no linker expert, though, so I apologize if I got it completely
> wrong, would really appreciate someone to detail this a bit more.
> Thanks!
Ah, that is my mistake: I was getting dynamic libbpf symbols with this
approach, but that was because I had the version of libbpf.so in my
$LIBDIR that had the patch to expose the netlink APIs as versioned
symbols; so it was just pulling in everything from the shared library.
So what I was going for was exactly what you described above; but it
seems that doesn't actually work. Too bad, and sorry for wasting your
time on this :/
-Toke
next prev parent reply other threads:[~2019-12-02 21:15 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-02 13:18 [PATCHv4 0/6] perf/bpftool: Allow to link libbpf dynamically Jiri Olsa
2019-12-02 13:18 ` [PATCH 1/6] perf tools: Allow to specify libbpf install directory Jiri Olsa
2019-12-02 13:18 ` [PATCH 2/6] bpftool: Allow to link libbpf dynamically Jiri Olsa
2019-12-02 13:18 ` [PATCH 3/6] bpftool: Rename BPF_DIR Makefile variable to LIBBPF_SRC_DIR Jiri Olsa
2019-12-02 13:18 ` [PATCH 4/6] bpftool: Rename LIBBPF_OUTPUT Makefile variable to LIBBPF_BUILD_OUTPUT Jiri Olsa
2019-12-02 13:18 ` [PATCH 5/6] bpftool: Rename LIBBPF_PATH Makefile variable to LIBBPF_BUILD_PATH Jiri Olsa
2019-12-02 13:18 ` [PATCH 6/6] selftests, bpftool: Add build test for libbpf dynamic linking Jiri Olsa
2019-12-02 15:38 ` Quentin Monnet
2019-12-02 19:41 ` [PATCHv4 0/6] perf/bpftool: Allow to link libbpf dynamically Andrii Nakryiko
2019-12-02 21:15 ` Toke Høiland-Jørgensen [this message]
2019-12-04 5:52 ` Alexei Starovoitov
2019-12-04 9:01 ` Jiri Olsa
2019-12-04 10:57 ` Toke Høiland-Jørgensen
2019-12-04 17:39 ` Alexei Starovoitov
2019-12-04 18:27 ` Daniel Borkmann
2019-12-04 20:22 ` Toke Høiland-Jørgensen
2019-12-04 21:16 ` Andrii Nakryiko
2019-12-04 21:54 ` Jakub Kicinski
2019-12-04 23:39 ` Alexei Starovoitov
2019-12-05 0:23 ` Jakub Kicinski
2019-12-05 0:29 ` David Miller
2019-12-05 1:25 ` Alexei Starovoitov
2019-12-05 1:09 ` Alexei Starovoitov
2019-12-05 2:10 ` Jakub Kicinski
2019-12-05 3:17 ` Alexei Starovoitov
2019-12-05 4:26 ` Jakub Kicinski
2019-12-05 6:44 ` Alexei Starovoitov
2019-12-05 8:35 ` Jesper Dangaard Brouer
2019-12-05 12:09 ` Michal Rostecki
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