linux-kernel - Re: Getting empty callchain from perf_callchain

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Message-ID: <39AB1404-1C9A-43E9-A3EC-AED4AA26DC8C@fb.com>
Date:   Thu, 23 May 2019 08:27:50 +0000
From:   Song Liu <songliubraving@...com>
To:     Kairui Song <kasong@...hat.com>
CC:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexei Starovoitov <ast@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...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 May 22, 2019, at 11:48 PM, Kairui Song <kasong@...hat.com> wrote:
> 
> On Thu, May 23, 2019 at 7:46 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>> 
>> On Wed, May 22, 2019 at 12:45:17PM -0500, Josh Poimboeuf wrote:
>>> On Wed, May 22, 2019 at 02:49:07PM +0000, Alexei Starovoitov wrote:
>>>> The one that is broken is prog_tests/stacktrace_map.c
>>>> There we attach bpf to standard tracepoint where
>>>> kernel suppose to collect pt_regs before calling into bpf.
>>>> And that's what bpf_get_stackid_tp() is doing.
>>>> It passes pt_regs (that was collected before any bpf)
>>>> into bpf_get_stackid() which calls get_perf_callchain().
>>>> Same thing with kprobes, uprobes.
>>> 
>>> Is it trying to unwind through ___bpf_prog_run()?
>>> 
>>> If so, that would at least explain why ORC isn't working.  Objtool
>>> currently ignores that function because it can't follow the jump table.
>> 
>> Here's a tentative fix (for ORC, at least).  I'll need to make sure this
>> doesn't break anything else.
>> 
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index 242a643af82f..1d9a7cc4b836 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -1562,7 +1562,6 @@ static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
>>                BUG_ON(1);
>>                return 0;
>> }
>> -STACK_FRAME_NON_STANDARD(___bpf_prog_run); /* jump table */
>> 
>> #define PROG_NAME(stack_size) __bpf_prog_run##stack_size
>> #define DEFINE_BPF_PROG_RUN(stack_size) \
>> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
>> index 172f99195726..2567027fce95 100644
>> --- a/tools/objtool/check.c
>> +++ b/tools/objtool/check.c
>> @@ -1033,13 +1033,6 @@ static struct rela *find_switch_table(struct objtool_file *file,
>>                if (text_rela->type == R_X86_64_PC32)
>>                        table_offset += 4;
>> 
>> -               /*
>> -                * Make sure the .rodata address isn't associated with a
>> -                * symbol.  gcc jump tables are anonymous data.
>> -                */
>> -               if (find_symbol_containing(rodata_sec, table_offset))
>> -                       continue;
>> -
>>                rodata_rela = find_rela_by_dest(rodata_sec, table_offset);
>>                if (rodata_rela) {
>>                        /*
> 
> Hi Josh, this still won't fix the problem.
> 
> Problem is not (or not only) with ___bpf_prog_run, what actually went
> wrong is with the JITed bpf code.
> 
> For frame pointer unwinder, it seems the JITed bpf code will have a
> shifted "BP" register? (arch/x86/net/bpf_jit_comp.c:217), so if we can
> unshift it properly then it will work.
> 
> I tried below code, and problem is fixed (only for frame pointer
> unwinder though). Need to find a better way to detect and do any
> similar trick for bpf part, if this is a feasible way to fix it:
> 
> diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
> index 9b9fd4826e7a..2c0fa2aaa7e4 100644
> --- a/arch/x86/kernel/unwind_frame.c
> +++ b/arch/x86/kernel/unwind_frame.c
> @@ -330,8 +330,17 @@ bool unwind_next_frame(struct unwind_state *state)
>        }
> 
>        /* Move to the next frame if it's safe: */
> -       if (!update_stack_state(state, next_bp))
> -               goto bad_address;
> +       if (!update_stack_state(state, next_bp)) {
> +               // Try again with shifted BP
> +               state->bp += 5; // see AUX_STACK_SPACE
> +               next_bp = (unsigned long
> *)READ_ONCE_TASK_STACK(state->task, *state->bp);
> +               // Clean and refetch stack info, it's marked as error outed
> +               state->stack_mask = 0;
> +               get_stack_info(next_bp, state->task,
> &state->stack_info, &state->stack_mask);
> +               if (!update_stack_state(state, next_bp)) {
> +                       goto bad_address;
> +               }
> +       }
> 
>        return true;
> 
> For ORC unwinder, I think the unwinder can't find any info about the
> JITed part. Maybe if can let it just skip the JITed part and go to
> kernel context, then should be good enough.

In this case (tracepoint), the callchain bpf_get_stackid() fetches is the 
callchain at the tracepoint. So we don't need the JITed part. 

BPF program passes the regs at the tracepoint to perf_callchain_kernel(). 
However, perf_callchain_kernel() only uses regs->sp for !perf_hw_regs()
case. This is probably expected, as passing regs in doesn't really help. 

There are multiple cases in unwind_orc.c:__unwind_start(), which I don't 
understand very well. 

Does the above make sense? Did I mis-understand anything?

@Alexei, do you remember some rough time/version that ORC unwinder works
well for tracepoints? Maybe we can dig into that version to see the
difference.   

Thanks,
Song

> 
> 
> 
> 
> 
> --
> Best Regards,
> Kairui Song