Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes: > On Mon, Dec 2, 2019 at 5:19 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: >> >> hi, >> adding support to link bpftool with libbpf dynamically, >> and config change for perf. >> >> It's now possible to use: >> $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 >> >> which will detect libbpf devel package and if found, link it with bpftool. >> >> It's possible to use arbitrary installed libbpf: >> $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 LIBBPF_DIR=/tmp/libbpf/ >> >> I based this change on top of Arnaldo's perf/core, because >> it contains libbpf feature detection code as dependency. >> >> Also available in: >> git://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git >> libbpf/dyn >> >> v4 changes: >> - based on Toke's v3 post, there's no need for additional API exports: >> >> 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. > > Whoever understands how this is supposed to work, can you please > explain? From reading this, I think what we **want** is: > > - all LIBBPF_API-exposed APIs should be dynamically linked against libbpf.so; > - everything else used from libbpf (e.g., netlink APIs), should come > from libbpf.a. > > Am I getting the idea right? > > If yes, are we sure it actually works like that in practice? I've > compiled with LIBBPF_DYNAMIC=1, and what I see is that libelf, libc, > zlib, etc functions do have relocations against them in ".rela.plt" > section. None of libbpf exposed APIs, though, have any of such > relocations. Which to me suggests that they are just statically linked > against libbpf.a and libbpf.so is just recorded in ELF as a dynamic > library dependency because of this extra -lbpf flag. Which kind of > defeats the purpose of this whole endeavor, no? > > I'm no linker expert, though, so I apologize if I got it completely > wrong, would really appreciate someone to detail this a bit more. > Thanks! Ah, that is my mistake: I was getting dynamic libbpf symbols with this approach, but that was because I had the version of libbpf.so in my $LIBDIR that had the patch to expose the netlink APIs as versioned symbols; so it was just pulling in everything from the shared library. So what I was going for was exactly what you described above; but it seems that doesn't actually work. Too bad, and sorry for wasting your time on this :/ -Toke
