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

Feng Yang
2025-09-22T02:16:29Z

On Sun, 21 Sep 2025 22:30:37 +0900 Masami Hiramatsu wrote:

> On Fri, 19 Sep 2025 19:56:20 -0700
> Alexei Starovoitov <alexei.starovoitov@gmail.com> 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...
>
> It should be able to unwind stack. It saves sp, pc, lr, fp.
>
> regs->sp = afregs->sp;
> regs->pc = afregs->pc;
> regs->regs[29] = afregs->fp;
> regs->regs[30] = afregs->lr;
>
> > I suspect the arm64 implementation wasn't really tested.
> > Or maybe there is some other issue.
>
> It depends on how bpf_get_stackid() works. Some registers for that
> function may not be saved.
>
> If it returns -EFAULT, the get_perf_callchain() returns NULL.
>

During my test, the reason for returning -EFAULT was that trace->nr was 0.

static long __bpf_get_stackid(struct bpf_map *map,
struct perf_callchain_entry *trace, u64 flags)
{
struct bpf_stack_map *smap = container_of(map, struct bpf_stack_map, map);
struct stack_map_bucket *bucket, *new_bucket, *old_bucket;
u32 skip = flags & BPF_F_SKIP_FIELD_MASK;
u32 hash, id, trace_nr, trace_len;
bool user = flags & BPF_F_USER_STACK;
u64 *ips;
bool hash_matches;

if (trace->nr <= skip)
/* skipping more than usable stack trace */
return -EFAULT;
......
}

thanks