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

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Date:   Fri, 15 Apr 2022 16:07:58 -0700
From:   Song Liu <song@...nel.org>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitrii Dolgov <9erthalion6@...il.com>,
        Quentin Monnet <quentin@...valent.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Chenbo Feng <fengc@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Daniel Wagner <daniel.wagner@...-carit.de>,
        Thomas Graf <tgraf@...g.ch>,
        Ong Boon Leong <boon.leong.ong@...el.com>,
        linux-perf-users@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next 01/11] bpf, perf: fix bpftool compilation with !CONFIG_PERF_EVENTS

On Thu, Apr 14, 2022 at 3:45 PM Alexander Lobakin <alobakin@...me> 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@...me>

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
>
>