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From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Quentin Monnet <quentin.monnet@netronome.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Jiri Olsa <jolsa@kernel.org>, lkml <linux-kernel@vger.kernel.org>,
        netdev@vger.kernel.org, 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>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>
Subject: Re: [PATCH bpf v2] bpftool: Allow to link libbpf dynamically
In-Reply-To: <497b4151-9aad-f3a9-3aff-78d665e5f750@netronome.com>
References: <20191128145316.1044912-1-toke@redhat.com> <497b4151-9aad-f3a9-3aff-78d665e5f750@netronome.com>
Date:   Thu, 28 Nov 2019 16:52:20 +0100
Message-ID: <871rtsvw0b.fsf@toke.dk>
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Quentin Monnet <quentin.monnet@netronome.com> writes:

> 2019-11-28 15:53 UTC+0100 ~ Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat=
.com>
>> From: Jiri Olsa <jolsa@kernel.org>
>>=20
>> Currently we support only static linking with kernel's libbpf
>> (tools/lib/bpf). This patch adds LIBBPF_DYNAMIC compile variable
>> that triggers libbpf detection and bpf dynamic linking:
>>=20
>>   $ make -C tools/bpf/bpftool make LIBBPF_DYNAMIC=3D1
>>=20
>> If libbpf is not installed, build (with LIBBPF_DYNAMIC=3D1) stops with:
>>=20
>>   $ make -C tools/bpf/bpftool LIBBPF_DYNAMIC=3D1
>>     Auto-detecting system features:
>>     ...                        libbfd: [ on  ]
>>     ...        disassembler-four-args: [ on  ]
>>     ...                          zlib: [ on  ]
>>     ...                        libbpf: [ OFF ]
>>=20
>>   Makefile:102: *** Error: No libbpf devel library found, please install=
-devel or libbpf-dev.
>>=20
>> Adding LIBBPF_DIR compile variable to allow linking with
>> libbpf installed into specific directory:
>>=20
>>   $ make -C tools/lib/bpf/ prefix=3D/tmp/libbpf/ install_lib install_hea=
ders
>>   $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=3D1 LIBBPF_DIR=3D/tmp/libb=
pf/
>>=20
>> It might be needed to clean build tree first because features
>> framework does not detect the change properly:
>>=20
>>   $ make -C tools/build/feature clean
>>   $ make -C tools/bpf/bpftool/ clean
>>=20
>> 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.
>>=20
>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>> Signed-off-by: Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com>
>> ---
>> v2:
>>   - Pass .a file to linker when dynamically linking, so bpftool can use
>>     private functions from libbpf without exposing them as API.
>>    =20
>>  tools/bpf/bpftool/Makefile | 38 +++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 37 insertions(+), 1 deletion(-)
>>=20
>> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
>> index 39bc6f0f4f0b..397051ed9e41 100644
>> --- a/tools/bpf/bpftool/Makefile
>> +++ b/tools/bpf/bpftool/Makefile
>> @@ -1,6 +1,15 @@
>>  # SPDX-License-Identifier: GPL-2.0-only
>> +# LIBBPF_DYNAMIC to enable libbpf dynamic linking.
>> +
>>  include ../../scripts/Makefile.include
>>  include ../../scripts/utilities.mak
>> +include ../../scripts/Makefile.arch
>> +
>> +ifeq ($(LP64), 1)
>> +  libdir_relative =3D lib64
>> +else
>> +  libdir_relative =3D lib
>> +endif
>> =20
>>  ifeq ($(srctree),)
>>  srctree :=3D $(patsubst %/,%,$(dir $(CURDIR)))
>> @@ -55,7 +64,7 @@ ifneq ($(EXTRA_LDFLAGS),)
>>  LDFLAGS +=3D $(EXTRA_LDFLAGS)
>>  endif
>> =20
>> -LIBS =3D $(LIBBPF) -lelf -lz
>> +LIBS =3D -lelf -lz
>
> Hi Toke,
>
> You don't need to remove $(LIBBPF) here, because you add it in both
> cases below (whether $(LIBBPF_DYNAMIC) is defined or not).

Oh, right, good point; will fix :)