linux-kernel - Re: [PATCH bpf-next 1/5] bpf: block bpf_get_[stack|stackid] on perf

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Date:   Sat, 11 Jul 2020 06:28:45 +0000
From:   Song Liu <songliubraving@...com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
CC:     open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>
Subject: Re: [PATCH bpf-next 1/5] bpf: block bpf_get_[stack|stackid] on
 perf_event with PEBS entries



> On Jul 10, 2020, at 8:53 PM, Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:
> 
> On Fri, Jul 10, 2020 at 6:30 PM Song Liu <songliubraving@...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@...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