Re: [PATCH] btf_encoder: Make BTF_KIND_TAG conditional

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Oct 20, 2021 at 02:27:37PM -0700, Andrii Nakryiko escreveu:
> On Wed, Oct 20, 2021 at 2:23 PM Ian Rogers <irogers@google.com> wrote:
> > On Wed, Oct 20, 2021 at 2:12 PM Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> > > On Thu, Oct 14, 2021 at 2:20 PM Ian Rogers <irogers@google.com> wrote:
> > > > @@ -648,6 +650,7 @@ static int32_t btf_encoder__add_datasec(struct btf_encoder *encoder, const char
> > > >  static int32_t btf_encoder__add_tag(struct btf_encoder *encoder, const char *value, uint32_t type,
> > > >                                     int component_idx)
> > > >  {
> > > > +#ifdef BTF_KIND_TAG /* Proxy for libbtf 6.0 */

> > > How will this work when libbpf is loaded dynamically? I believe pahole
> > > has this mode as well.

> > Well it won't have a compilation error because BTF_KIND_TAG isn't

> Great, you traded compile-time error for runtime linking error, I hope
> that trade off makes sense to Arnaldo.

This situation is tricky to handle, yeah :-\
 
> > undefined :-) Tbh, I'm not sure but it seems that you'd be limited to
> > features in the version of libbpf you compiled against.
 
> I've been consistently advocating for statically linking against
> libbpf exactly to control what APIs and features are supported. But
> people stubbornly want dynamic linking. I hope added complexity and
> feature detection makes sense in practice for pahole.

It is a pain, but fedora also have this policy.
 
> > > Also, note that libbpf now provides LIBBPF_MAJOR_VERSION and
> > > LIBBPF_MINOR_VERSION macros, starting from 0.5, so no need for
> > > guessing the version
> >
> > This was moved to a header file in:
> > https://lore.kernel.org/bpf/CAADnVQJ2qd095mvj3z9u9BXQYCe2OTDn4=Gsu9nv1tjFHc2yqQ@mail.gmail.com/T/
> >
> > But that header doesn't appear any more:
> > https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/tools/lib/bpf
> >
> > Is that a bug?
> 
> You should be checking here:
> 
> https://github.com/libbpf/libbpf/blob/master/src/libbpf_version.h

Ian, would be so kind as to follow up on this so that we get this
situation improved?

- Arnaldo