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

[Song Liu]

On Thu, Apr 14, 2022 at 3:45 PM Alexander Lobakin <alobakin@xxxxx> 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.
>
> Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output")
> Signed-off-by: Alexander Lobakin <alobakin@xxxxx>

While I can't think of a real failure with this approach, it does feel
weird to me. Can we fix this with bpf_core_field_exists()?

Thanks,
Song


> ---
>  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
>
>