On Fri, Jul 10, 2020 at 11:28 PM Song Liu <songliubraving@xxxxxx> wrote: > > > > > On Jul 10, 2020, at 8:53 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > > > 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. > > I am not aware of an easy way to tell the difference in bpf_get_stack. > But I do agree that would be much better. > Hm... Looking a bit more how all this is tied together in the kernel, I think it's actually quite easy. So, for perf_event BPF program type: 1. return a special prototype for bpf_get_stack/bpf_get_stackid, which will have this extra bit of logic for callchain. All other program types with access to bpf_get_stack/bpf_get_stackid should use the current one, probably. 2. For that special program, just like for bpf_read_branch_records(), we know that context is actually `struct bpf_perf_event_data_kern *`, and it has pt_regs, perf_sample_data and perf_event itself. 3. With that, it seems like you'll have everything you need to automatically choose a proper callchain. All this absolutely transparently to the BPF program. Am I missing something? > Thanks, > Song
