2019-11-28 17:07 UTC+0100 ~ Toke Høiland-Jørgensen [off-list ref]

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 <redacted>
---
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 <redacted>