Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes: > On Tue, Sep 14, 2021 at 12:02 PM Arnaldo Carvalho de Melo > <acme@xxxxxxxxxx> wrote: >> >> Em Tue, Sep 14, 2021 at 11:28:28AM -0700, Andrii Nakryiko escreveu: >> > On Tue, Sep 14, 2021 at 11:21 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote: >> > > >> > > On Tue, Sep 14, 2021 at 10:00:04AM -0700, Andrii Nakryiko wrote: >> > > > Perf code re-implements libbpf's btf__load_from_kernel_by_id() API as >> > > > a weak function, presumably to dynamically link against old version of >> > > > libbpf shared library. Unfortunately this causes compilation warning >> > > > when perf is compiled against libbpf v0.6+. >> > > > >> > > > For now, just ignore deprecation warning, but there might be a better >> > > > solution, depending on perf's needs. >> > > >> > > HI, >> > > the problem we tried to solve is when perf is using symbols >> > > which are not yet available in released libbpf.. but it all >> > > linkes in default perf build because it's linked statically >> > > libbpf.a in the tree >> > > >> > >> > If you are always statically linking libbpf into perf, there is no >> > need to implement this __weak shim. Libbpf is never going to deprecate >> > an API if a new/replacement API hasn't been at least in a previous >> > released version. So in this case btf__load_from_kernel_by_id() was >> > added in libbpf 0.5, and btf__get_from_id() was marked deprecated in >> > libbpf 0.6 (not yet released, of course). So with that, do you still >> > think we need this __weak re-implementation? >> > >> > I was wondering if this was done to make latest perf code compile >> > against some old libbpf source code or dynamically linked against old >> > libbpf. But if that's not the case, the fix should be a removal of >> > __weak btf__load_from_kernel_by_id(). >> >> It was made to build against the libbpf that comes with fedora 34, the >> distro I'm using, which is: >> >> ⬢[acme@toolbox perf]$ sudo dnf install libbpf-devel >> Package libbpf-devel-2:0.4.0-1.fc34.x86_64 is already installed. >> Dependencies resolved. >> Nothing to do. >> Complete! >> ⬢[acme@toolbox perf]$ cat /etc/redhat-release >> Fedora release 34 (Thirty Four) >> >> And we have 'make -C tools/perf build-test' that has one entry to build >> with LIBBPF_EXTERNAL=1, i.e. using whatever libbpf-devel package is >> installed in the distro, in addtion to statically linking with the >> libbpf in the kernel sources. >> >> That is done because several distros are linking perf with the libbpf >> they ship. >> >> When I merged the latest upstream this test failed, and I realized that >> some files in tools/perf/ had changed to make use of a new function and >> that was the reason for the build test failure. >> >> So I tried to provide a transition help for these cases, initially as a >> feature test that would look if that new function was available and if >> not, provide the fallback, but then ended up following Jiri's suggestion >> for a __weak function, as that involved less coding. >> > > Ok, that's cool, then my "fix" should be fine for now. Can you please > land it in perf/core to unblock Stephen's (cc'ed) build failure when > merging perf and bpf-next trees? > > Also it's good to keep in mind that libbpf is now providing > LIBBPF_MAJOR_VERSION and LIBBPF_MINOR_VERSION macro, so when > statically linking you should be able to use that to detect libbpf > version. For shared library cases we should probably also add runtime > APIs (e.g., int libbpf_major_version(void), int > libbpf_minor_version(void), const char *libbpf_version(void)) so that > you can do more detection based on libbpf version at runtime. Let me > know if it's something that would be helpful. Yes, please! We're currently using this horror to be able to print the libbpf version being used by xdp-tools: https://github.com/xdp-project/xdp-tools/blob/master/lib/util/util.c#L100 Would be awesome to have an API function we could just call instead :) -Toke
