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

[Quentin Monnet <quentin.monnet@netronome.com>]

2019-11-28 17:07 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 libbpf-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>
> ---
> v3:
>   - Keep $(LIBBPF) in $LIBS, and just add -lbpf on top
>   - Fix typo in error message
> v2:
>   - Pass .a file to linker when dynamically linking, so bpftool can use
>     private functions from libbpf without exposing them as API.

Thanks for the changes!

Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>