Re: [PATCH bpf-next 02/11] bpf: always emit struct bpf_perf_link BTF

[Alexander Lobakin <alobakin@pm.me>]

From: Song Liu <song@kernel.org>
Date: Fri, 15 Apr 2022 16:24:41 -0700

> On Thu, Apr 14, 2022 at 3:45 PM Alexander Lobakin <alobakin@pm.me> wrote:
> >
> > When building bpftool with !CONFIG_PERF_EVENTS:
> >
> > skeleton/pid_iter.bpf.c:47:14: error: incomplete definition of type 'struct bpf_perf_link'
> >         perf_link = container_of(link, struct bpf_perf_link, link);
> >                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:74:22: note: expanded from macro 'container_of'
> >                 ((type *)(__mptr - offsetof(type, member)));    \
> >                                    ^~~~~~~~~~~~~~~~~~~~~~
> > tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:68:60: note: expanded from macro 'offsetof'
> >  #define offsetof(TYPE, MEMBER)  ((unsigned long)&((TYPE *)0)->MEMBER)
> >                                                   ~~~~~~~~~~~^
> > skeleton/pid_iter.bpf.c:44:9: note: forward declaration of 'struct bpf_perf_link'
> >         struct bpf_perf_link *perf_link;
> >                ^
> >
> > &bpf_perf_link is being defined and used only under the ifdef.
> > Move it out of the block and explicitly emit a BTF to fix
> > compilation.
> >
> > Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output")
> > Signed-off-by: Alexander Lobakin <alobakin@pm.me>
>
> Similar to v1, this fix is weird to me. I hope we have can fix it in user
> space.

I've been thinking on this, but userspace is not provided with any
autoconf.h definitions (only selftests have them), so its code must
be sort of universal.
Both this and 01/11 are compile time and due to imcomplete and/or
absent BTF struct declarations. I'm not familiar with
bpf_core_field_exists(), and it might be that it's able to solve
01/11, but not this one. &bpf_perf_link must be present
unconditionally, otherwise it won't be defined in the generated
vmlinux.h at all.

Thanks,
Al