Re: [PATCH 1/2] perf tools: Add more weak libbpf functions

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Nov 09, 2021 at 03:33:04PM -0800, Andrii Nakryiko wrote:
> On Tue, Nov 9, 2021 at 10:50 AM Ian Rogers <irogers@google.com> wrote:
> >
> > On Tue, Nov 9, 2021 at 6:07 AM Jiri Olsa <jolsa@redhat.com> wrote:
> > >
> > > We hit the window where perf uses libbpf functions, that did not
> > > make it to the official libbpf release yet and it's breaking perf
> > > build with dynamicly linked libbpf.
> > >
> > > Fixing this by providing the new interface as weak functions which
> > > calls the original libbpf functions. Fortunatelly the changes were
> > > just renames.
> >
> > Could we just provide these functions behind a libbpf version #if ?
> > Weak symbols break things in subtle ways, under certain circumstances
> > the weak symbol is preferred over the strong due to lazy object file
> > resolution:
> > https://maskray.me/blog/2021-06-20-symbol-processing#archive-processing
> > This bit me last week, but in general you get away with it as the lazy
> > object file will get processed in an archive exposing the strong
> > symbol. With an #if you either get a linker error for 2 definitions or
> > 0 definitions, and it's clear what is broken.
> >
> > In the past we had problems due to constant propagation from weak
> > const variables, where #if was the solution:
> > https://lore.kernel.org/lkml/20191001003623.255186-1-irogers@google.com/
> >
> > There was some recent conversation on libbpf version for pahole here:
> > https://lore.kernel.org/bpf/CAP-5=fUc3LtU0WYg-Py9Jf+9picaWHJdSw=sdOMA54uY3p1pdw@mail.gmail.com/T/
> > https://lore.kernel.org/bpf/20211021183330.460681-1-irogers@google.com/
> >
> > Thanks,
> > Ian
> >
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >  tools/perf/util/bpf-event.c | 27 +++++++++++++++++++++++++++
> > >  1 file changed, 27 insertions(+)
> > >
> > > diff --git a/tools/perf/util/bpf-event.c b/tools/perf/util/bpf-event.c
> > > index 4d3b4cdce176..ceb96360fd12 100644
> > > --- a/tools/perf/util/bpf-event.c
> > > +++ b/tools/perf/util/bpf-event.c
> > > @@ -33,6 +33,33 @@ struct btf * __weak btf__load_from_kernel_by_id(__u32 id)
> > >         return err ? ERR_PTR(err) : btf;
> > >  }
> > >
> > > +struct bpf_program * __weak
> > > +bpf_object__next_program(const struct bpf_object *obj, struct bpf_program *prev)
> > > +{
> > > +#pragma GCC diagnostic push
> > > +#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
> > > +       return bpf_program__next(prev, obj);
> > > +#pragma GCC diagnostic pop
> > > +}
> > > +
> > > +struct bpf_map * __weak
> > > +bpf_object__next_map(const struct bpf_object *obj, const struct bpf_map *prev)
> > > +{
> > > +#pragma GCC diagnostic push
> > > +#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
> > > +       return bpf_map__next(prev, obj);
> > > +#pragma GCC diagnostic pop
> > > +}
> > > +
> > > +const void * __weak
> > > +btf__raw_data(const struct btf *btf_ro, __u32 *size)
> > > +{
> > > +#pragma GCC diagnostic push
> > > +#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
> > > +       return btf__get_raw_data(btf_ro, size);
> 
> you can still use old variants for the time being, if you want. Were
> new APIs used accidentally? Libbpf maintains a guarantee that if some
> API is deprecated in favor of the new one, there will be at least one
> full libbpf release where both APIs are available and not marked as
> deprecated.

we could use old api instead of btf__raw_data, we could just revert
the perf change

but bpf_object__next_program and bpf_object__next_map are used through
macros like bpf_object__for_each_map or bpf_object__for_each_program,
so we'd need to define 'old versions' of them

jirka

> 
> 
> > > +#pragma GCC diagnostic pop
> > > +}
> > > +
> > >  static int snprintf_hex(char *buf, size_t size, unsigned char *data, size_t len)
> > >  {
> > >         int ret = 0;
> > > --
> > > 2.31.1
> > >
>