Re: [PATCHv2 bpf-next 09/11] bpf: Remove trace_in argument from __bpf_get_stack

On Fri, Jul 31, 2026 at 6:33 AM Jiri Olsa <[email protected]> wrote:
>
> On Thu, Jul 30, 2026 at 04:06:14PM -0700, Andrii Nakryiko wrote:
>
> SNIP
>
> > >  BPF_CALL_4(bpf_get_stack_pe, struct bpf_perf_event_data_kern *, ctx,
> > >            void *, buf, u32, size, u64, flags)
> > >  {
> > > @@ -947,7 +970,7 @@ BPF_CALL_4(bpf_get_stack_pe, struct bpf_perf_event_data_kern *, ctx,
> > >         int err = -EINVAL;
> > >
> > >         if (!(event->attr.sample_type & PERF_SAMPLE_CALLCHAIN))
> > > -               return __bpf_get_stack(regs, NULL, NULL, buf, size, flags, false /* !may_fault */);
> > > +               return __bpf_get_stack(regs, NULL, buf, size, flags, false /* !may_fault */);
> > >
> > >         if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
> > >                                BPF_F_USER_BUILD_ID)))
> > > @@ -966,7 +989,7 @@ BPF_CALL_4(bpf_get_stack_pe, struct bpf_perf_event_data_kern *, ctx,
> > >
> > >         if (kernel) {
> > >                 trace->nr = nr_kernel;
> >
> > this whole count_kernel_ip() logic above, why do we have it? I don't
> > think we can have combined user and kernel stack trace, so how can we
> > end up with PERF_CONTEXT_USER "ip" at all? I might be missing
>
> the callchain comes from perf event's callchain and that can
> have both kernel and user part

in general, but bpf_get_stack and bpf_get_stackid never request both
kernel and user stack trace in one go and we set add_mark to false, so
we can't have PERF_CONTEXT_USER, which is why I am asking what's the
deal with this nr_kernel calculation.

If it's unnecessary, we should just drop that part and not have that
extra unnecessary complication

>
> > something subtle here, but can you please check again if this whole
> > trace->nr calculation/override is even necessary.
>
> as I wrote in the other reply I don't think we need to change
> trace->nr directly, just pass the needed nr/count as argument
>
> jirka
>
> >
> > > -               err = __bpf_get_stack(regs, NULL, trace, buf, size, flags, false /* !may_fault */);
> > > +               err = __bpf_get_stack_pe(trace, buf, size, flags);
> > >
> > >         } else { /* user */
> > >                 u64 skip = flags & BPF_F_SKIP_FIELD_MASK;
> > > @@ -974,9 +997,8 @@ BPF_CALL_4(bpf_get_stack_pe, struct bpf_perf_event_data_kern *, ctx,
> > >                 skip += nr_kernel;
> > >                 if (skip > BPF_F_SKIP_FIELD_MASK)
> > >                         goto clear;
> > > -
> > >                 flags = (flags & ~BPF_F_SKIP_FIELD_MASK) | skip;
> > > -               err = __bpf_get_stack(regs, NULL, trace, buf, size, flags, false /* !may_fault */);
> > > +               err = __bpf_get_stack_pe(trace, buf, size, flags);
> > >         }
> > >
> > >         /* restore nr */
> > > --
> > > 2.54.0
> > >