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

[Daniel Borkmann]

On 11/29/19 3:00 PM, Toke HÃiland-JÃrgensen wrote:
> Daniel Borkmann <daniel@xxxxxxxxxxxxx> writes:
> > On 11/28/19 5:07 PM, Toke HÃiland-JÃrgensen wrote:
> > > From: Jiri Olsa <jolsa@xxxxxxxxxx>
[...]
> > >    ifeq ($(srctree),)
> > >    srctree := $(patsubst %/,%,$(dir $(CURDIR)))
> > > @@ -63,6 +72,19 @@ RM ?= rm -f
> > >    FEATURE_USER = .bpftool
> > >    FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
> > >    FEATURE_DISPLAY = libbfd disassembler-four-args zlib
> > > +ifdef LIBBPF_DYNAMIC
> > > +  FEATURE_TESTS   += libbpf
> > > +  FEATURE_DISPLAY += libbpf
> > > +
> > > +  # for linking with debug library run:
> > > +  # make LIBBPF_DYNAMIC=1 LIBBPF_DIR=/opt/libbpf
> >
> > The Makefile already has BPF_DIR which points right now to
> > '$(srctree)/tools/lib/bpf/' and LIBBPF_PATH for the final one and
> > where $(LIBBPF_PATH)libbpf.a is expected to reside. Can't we improve
> > the Makefile to reuse and work with these instead of adding yet
> > another LIBBPF_DIR var which makes future changes in this area more
> > confusing? The libbpf build spills out libbpf.{a,so*} by default
> > anyway.
>
> I see what you mean; however, LIBBPF_DIR is meant to be specifically an
> override for the dynamic library, not just the path to libbpf.
>
> Would it be less confusing to overload the LIBBPF_DYNAMIC variable
> instead? I.e.,
>
> make LIBBPF_DYNAMIC=1
>
> would dynamically link against the libbpf installed in the system, but
>
> make LIBBPF_DYNAMIC=/opt/libbpf
>
> would override that and link against whatever is in /opt/libbpf instead?
> WDYT?

Hm, given perf tool has similar LIB*_DIR vars in place for its libs, it probably
makes sense to stick with this convention as well then. Perhaps better in this
case to just rename s/BPF_DIR/BPF_SRC_DIR/, s/LIBBPF_OUTPUT/LIBBPF_BUILD_OUTPUT/,
and s/LIBBPF_PATH/LIBBPF_BUILD_PATH/ to make their purpose more clear.

One thing that would be good to do as well for this patch is to:

 i) Document both LIBBPF_DYNAMIC and LIBBPF_DIR in the Makefile comment you
    added at the top along with a simple usage example.

ii) Extend tools/testing/selftests/bpf/test_bpftool_build.sh with a dynamic
    linking test case, e.g. building libbpf into a temp dir and pointing
    LIBBPF_DIR to it for bpftool LIBBPF_DYNAMIC=1 build.

Thanks,
Daniel