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

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@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.

[...]

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

[Song Liu]

> On Jul 10, 2020, at 8:53 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> 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.

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. 

Thanks,
Song

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 11:28 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jul 10, 2020, at 8:53 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> >
> > 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.
>
> 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

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

[Song Liu]

> On Jul 11, 2020, at 10:06 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
> 
> On Fri, Jul 10, 2020 at 11:28 PM Song Liu <songliubraving@fb.com> wrote:
>> 
>> 
>> 
>>> On Jul 10, 2020, at 8:53 PM, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>>> 
>>> 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.
>> 
>> 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?

Good idea! A separate prototype should work here. 

Thanks,
Song