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SubjectRe: [PATCH bpf v2] bpftool: Allow to link libbpf dynamically
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Quentin Monnet <quentin.monnet@netronome.com> writes:

> 2019-11-28 15:53 UTC+0100 ~ Toke Høiland-Jørgensen <toke@redhat.com>
>> From: Jiri Olsa <jolsa@kernel.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@kernel.org>
>> Signed-off-by: Toke Høiland-Jørgensen <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.
>>
>> 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 :)

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