linux-kernel - Re: [PATCH bpf v3] bpftool: Allow to link libbpf dynamically

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Message-ID: <87v9r3um22.fsf@toke.dk>
Date:   Fri, 29 Nov 2019 09:24:53 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        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 v3] bpftool: Allow to link libbpf dynamically

Jiri Olsa <jolsa@...hat.com> writes:

> On Thu, Nov 28, 2019 at 05:07:12PM +0100, Toke Høiland-Jørgensen wrote:
>
> SNIP
>
>>  ifeq ($(srctree),)
>>  srctree := $(patsubst %/,%,$(dir $(CURDIR)))
>> @@ -63,6 +72,19 @@ RM ?= rm -f
>>  FEATURE_USER = .bpftool
>>  FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
>>  FEATURE_DISPLAY = libbfd disassembler-four-args zlib
>> +ifdef LIBBPF_DYNAMIC
>> +  FEATURE_TESTS   += libbpf
>> +  FEATURE_DISPLAY += libbpf
>> +
>> +  # for linking with debug library run:
>> +  # make LIBBPF_DYNAMIC=1 LIBBPF_DIR=/opt/libbpf
>> +  ifdef LIBBPF_DIR
>> +    LIBBPF_CFLAGS  := -I$(LIBBPF_DIR)/include
>> +    LIBBPF_LDFLAGS := -L$(LIBBPF_DIR)/$(libdir_relative)
>> +    FEATURE_CHECK_CFLAGS-libbpf  := $(LIBBPF_CFLAGS)
>> +    FEATURE_CHECK_LDFLAGS-libbpf := $(LIBBPF_LDFLAGS)
>> +  endif
>> +endif
>>  
>>  check_feat := 1
>>  NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
>> @@ -88,6 +110,18 @@ ifeq ($(feature-reallocarray), 0)
>>  CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
>>  endif
>>  
>> +ifdef LIBBPF_DYNAMIC
>> +  ifeq ($(feature-libbpf), 1)
>> +    # bpftool uses non-exported functions from libbpf, so just add the dynamic
>> +    # version of libbpf and let the linker figure it out
>> +    LIBS    := -lbpf $(LIBS)
>
> nice, so linker will pick up the missing symbols and we
> don't need to check on particular libbpf version then

Yup, exactly. I verified with objdump that the end result is a
dynamically linked bpftool with LIBBPF_DYNAMIC is set, and a statically
linked one if it isn't; so the linker seems to be smart enough to just
figure out how to do the right thing :)

-Toke