Re: [PATCH bpf-next 1/5] bpf: block bpf_get_[stack|stackid] on perf_event with PEBS entries

[Andrii Nakryiko]

On Fri, Jul 10, 2020 at 6:30 PM Song Liu <songliubraving@xxxxxx> 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@xxxxxx>
> ---

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.

[...]