Re: [BUG] Failed to obtain stack trace via bpf_get_stackid on ARM64 architecture

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Sep 19, 2025 at 07:56:20PM -0700, Alexei Starovoitov wrote:
> On Fri, Sep 19, 2025 at 12:19 AM Feng Yang <yangfeng59949@163.com> wrote:
> >
> > When I use bpf_program__attach_kprobe_multi_opts to hook a BPF program that contains the bpf_get_stackid function on the arm64 architecture,
> > I find that the stack trace cannot be obtained. The trace->nr in __bpf_get_stackid is 0, and the function returns -EFAULT.
> >
> > For example:
> > diff --git a/tools/testing/selftests/bpf/progs/kprobe_multi.c b/tools/testing/selftests/bpf/progs/kprobe_multi.c
> > index 9e1ca8e34913..844fa88cdc4c 100644
> > --- a/tools/testing/selftests/bpf/progs/kprobe_multi.c
> > +++ b/tools/testing/selftests/bpf/progs/kprobe_multi.c
> > @@ -36,6 +36,15 @@ __u64 kretprobe_test6_result = 0;
> >  __u64 kretprobe_test7_result = 0;
> >  __u64 kretprobe_test8_result = 0;
> >
> > +typedef __u64 stack_trace_t[2];
> > +
> > +struct {
> > +       __uint(type, BPF_MAP_TYPE_STACK_TRACE);
> > +       __uint(max_entries, 1024);
> > +       __type(key, __u32);
> > +       __type(value, stack_trace_t);
> > +} stacks SEC(".maps");
> > +
> >  static void kprobe_multi_check(void *ctx, bool is_return)
> >  {
> >         if (bpf_get_current_pid_tgid() >> 32 != pid)
> > @@ -100,7 +109,9 @@ int test_kretprobe(struct pt_regs *ctx)
> >  SEC("kprobe.multi")
> >  int test_kprobe_manual(struct pt_regs *ctx)
> >  {
> > +       int id = bpf_get_stackid(ctx, &stacks, 0);
> 
> ftrace_partial_regs() supposed to work on x86 and arm64,
> but since multi-kprobe is the only user...
> I suspect the arm64 implementation wasn't really tested.
> Or maybe there is some other issue.
> 
> Masami, Jiri,
> thoughts?

hi,
I did quick test for kprobe multi and I can reproduce the issue
on arm64 with ci:
  https://github.com/kernel-patches/bpf/pull/9809

but can't really tell what's missing on arm side.. cc-ing Will

thanks,
jirka