Re: [PATCH bpf-next 01/11] bpf, perf: fix bpftool compilation with !CONFIG_PERF_EVENTS

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Apr 19, 2022 at 2:04 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Apr 14, 2022 at 10:44:48PM +0000, Alexander Lobakin wrote:
> > When CONFIG_PERF_EVENTS is not set, struct perf_event remains empty.
> > However, the structure is being used by bpftool indirectly via BTF.
> > This leads to:
> >
> > skeleton/pid_iter.bpf.c:49:30: error: no member named 'bpf_cookie' in 'struct perf_event'
> >         return BPF_CORE_READ(event, bpf_cookie);
> >                ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
> >
> > ...
> >
> > skeleton/pid_iter.bpf.c:49:9: error: returning 'void' from a function with incompatible result type '__u64' (aka 'unsigned long long')
> >         return BPF_CORE_READ(event, bpf_cookie);
> >                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Tools and samples can't use any CONFIG_ definitions, so the fields
> > used there should always be present.
> > Move CONFIG_BPF_SYSCALL block out of the CONFIG_PERF_EVENTS block
> > to make it available unconditionally.
>
> Urgh, this is nasty.. did you verify nothing relies on that structure
> actually being empty?
>
> Also, why are we changing kernel headers to fix some daft userspace
> issue?
>

I agree, this is quite ugly. And I think it's not necessary at all.

BPF CO-RE, which bpftool relies on here, allows to have bpftool's own
minimal definition of struct perf_event with bpf_cookie field and not
rely on UAPI headers having full definition. Something like this:

struct perf_event___local {
    u64 bpf_cookie;
} __attribute__((preserve_access_index));

Then use `struct perf_event___local` (note the three underscores, they
are important) instead of struct perf_event in BPF code.

And we'll have to do the same for struct bpf_perf_link, I presume?

> > Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output")
> > Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> > ---
> >  include/linux/perf_event.h | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index af97dd427501..b1d5715b8b34 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -762,12 +762,14 @@ struct perf_event {
> >       u64                             (*clock)(void);
> >       perf_overflow_handler_t         overflow_handler;
> >       void                            *overflow_handler_context;
> > +#endif /* CONFIG_PERF_EVENTS */
> >  #ifdef CONFIG_BPF_SYSCALL
> >       perf_overflow_handler_t         orig_overflow_handler;
> >       struct bpf_prog                 *prog;
> >       u64                             bpf_cookie;
> >  #endif
> >
> > +#ifdef CONFIG_PERF_EVENTS
> >  #ifdef CONFIG_EVENT_TRACING
> >       struct trace_event_call         *tp_event;
> >       struct event_filter             *filter;
> > --
> > 2.35.2
> >
> >