linux-kernel - Re: Getting empty callchain from perf_callchain

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Message-ID: <c881767d-b6f3-c53e-5c70-556d09ea8d89@fb.com>
Date:   Fri, 17 May 2019 21:06:25 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Song Liu <songliubraving@...com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     Kairui Song <kasong@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: Getting empty callchain from perf_callchain_kernel()

On 5/17/19 11:40 AM, Song Liu wrote:
> +Alexei, Daniel, and bpf
> 
>> On May 17, 2019, at 2:10 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Fri, May 17, 2019 at 04:15:39PM +0800, Kairui Song wrote:
>>> Hi, I think the actual problem is that bpf_get_stackid_tp (and maybe
>>> some other bfp functions) is now broken, or, strating an unwind
>>> directly inside a bpf program will end up strangely. It have following
>>> kernel message:
>>
>> Urgh, what is that bpf_get_stackid_tp() doing to get the regs? I can't
>> follow.
> 
> I guess we need something like the following? (we should be able to
> optimize the PER_CPU stuff).
> 
> Thanks,
> Song
> 
> 
> diff --git i/kernel/trace/bpf_trace.c w/kernel/trace/bpf_trace.c
> index f92d6ad5e080..c525149028a7 100644
> --- i/kernel/trace/bpf_trace.c
> +++ w/kernel/trace/bpf_trace.c
> @@ -696,11 +696,13 @@ static const struct bpf_func_proto bpf_perf_event_output_proto_tp = {
>          .arg5_type      = ARG_CONST_SIZE_OR_ZERO,
>   };
> 
> +static DEFINE_PER_CPU(struct pt_regs, bpf_stackid_tp_regs);
>   BPF_CALL_3(bpf_get_stackid_tp, void *, tp_buff, struct bpf_map *, map,
>             u64, flags)
>   {
> -       struct pt_regs *regs = *(struct pt_regs **)tp_buff;
> +       struct pt_regs *regs = this_cpu_ptr(&bpf_stackid_tp_regs);
> 
> +       perf_fetch_caller_regs(regs);

No. pt_regs is already passed in. It's the first argument.
If we call perf_fetch_caller_regs() again the stack trace will be wrong.
bpf prog should not see itself, interpreter or all the frames in between.