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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 10 Jul 2020 20:53:03 -0700
Message-ID: <CAEf4BzaHAFNdEPp38ZnKOYTy3CfRCwaxDykS_Xir_VqDm0Kiug@mail.gmail.com>
Subject: Re: [PATCH bpf-next 1/5] bpf: block bpf_get_[stack|stackid] on
 perf_event with PEBS entries
To:     Song Liu <songliubraving@fb.com>
Cc:     open list <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Kernel Team <kernel-team@fb.com>,
        john fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Peter Ziljstra <peterz@infradead.org>
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On Fri, Jul 10, 2020 at 6:30 PM Song Liu <songliubraving@fb.com> wrote:
>
> Calling get_perf_callchain() on perf_events from PEBS entries may cause
> unwinder errors. To fix this issue, the callchain is fetched early. Such
> perf_events are marked with __PERF_SAMPLE_CALLCHAIN_EARLY.
>
> Similarly, calling bpf_get_[stack|stackid] on perf_events from PEBS may
> also cause unwinder errors. To fix this, block bpf_get_[stack|stackid] on
> these perf_events. Unfortunately, bpf verifier cannot tell whether the
> program will be attached to perf_event with PEBS entries. Therefore,
> block such programs during ioctl(PERF_EVENT_IOC_SET_BPF).
>
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---

Perhaps it's a stupid question, but why bpf_get_stack/bpf_get_stackid
can't figure out automatically that they are called from
__PERF_SAMPLE_CALLCHAIN_EARLY perf event and use different callchain,
if necessary?

It is quite suboptimal from a user experience point of view to require
two different BPF helpers depending on PEBS or non-PEBS perf events.

[...]