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From:   Song Liu <song@kernel.org>
Date:   Fri, 15 Apr 2022 16:20:44 -0700
Message-ID: <CAPhsuW4SZS9p6OQ0yAaOo_BdNT3HeA4T9pLCCEiazc+HmJBZKQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next 01/11] bpf, perf: fix bpftool compilation with !CONFIG_PERF_EVENTS
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
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On Fri, Apr 15, 2022 at 4:07 PM Song Liu <song@kernel.org> wrote:
>
> On Thu, Apr 14, 2022 at 3:45 PM Alexander Lobakin <alobakin@pm.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@pm.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()?

Hmm.. the error happens at compile time, so I guess it is not very easy.

Andrii,
Do you have some recommendation on this?

Song