linux-kernel - Re: [PATCH bpf v2] bpftool: Allow to link libbpf dynamically

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Message-ID: <871rtsvw0b.fsf@toke.dk>
Date:   Thu, 28 Nov 2019 16:52:20 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Quentin Monnet <quentin.monnet@...ronome.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
        netdev@...r.kernel.org, bpf@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Michael Petlan <mpetlan@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH bpf v2] bpftool: Allow to link libbpf dynamically

Quentin Monnet <quentin.monnet@...ronome.com> writes:

> 2019-11-28 15:53 UTC+0100 ~ Toke Høiland-Jørgensen <toke@...hat.com>
>> From: Jiri Olsa <jolsa@...nel.org>
>> 
>> 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:
>> 
>>   $ make -C tools/bpf/bpftool make LIBBPF_DYNAMIC=1
>> 
>> If libbpf is not installed, build (with LIBBPF_DYNAMIC=1) stops with:
>> 
>>   $ make -C tools/bpf/bpftool LIBBPF_DYNAMIC=1
>>     Auto-detecting system features:
>>     ...                        libbfd: [ on  ]
>>     ...        disassembler-four-args: [ on  ]
>>     ...                          zlib: [ on  ]
>>     ...                        libbpf: [ OFF ]
>> 
>>   Makefile:102: *** Error: No libbpf devel library found, please install-devel or libbpf-dev.
>> 
>> Adding LIBBPF_DIR compile variable to allow linking with
>> libbpf installed into specific directory:
>> 
>>   $ make -C tools/lib/bpf/ prefix=/tmp/libbpf/ install_lib install_headers
>>   $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 LIBBPF_DIR=/tmp/libbpf/
>> 
>> It might be needed to clean build tree first because features
>> framework does not detect the change properly:
>> 
>>   $ make -C tools/build/feature clean
>>   $ make -C tools/bpf/bpftool/ clean
>> 
>> 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.
>> 
>> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> ---
>> v2:
>>   - Pass .a file to linker when dynamically linking, so bpftool can use
>>     private functions from libbpf without exposing them as API.
>>     
>>  tools/bpf/bpftool/Makefile | 38 +++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 37 insertions(+), 1 deletion(-)
>> 
>> 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 ($(LP64), 1)
>> +  libdir_relative = lib64
>> +else
>> +  libdir_relative = lib
>> +endif
>>  
>>  ifeq ($(srctree),)
>>  srctree := $(patsubst %/,%,$(dir $(CURDIR)))
>> @@ -55,7 +64,7 @@ ifneq ($(EXTRA_LDFLAGS),)
>>  LDFLAGS += $(EXTRA_LDFLAGS)
>>  endif
>>  
>> -LIBS = $(LIBBPF) -lelf -lz
>> +LIBS = -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 :)