linux-kernel - Re: [BUG] Failed to obtain stack trace via bpf_get

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Message-Id: <20250922021531.105670-1-yangfeng59949@163.com>
Date: Mon, 22 Sep 2025 10:15:31 +0800
From: Feng Yang <yangfeng59949@....com>
To: mhiramat@...nel.org
Cc: alexei.starovoitov@...il.com,
	andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	daniel@...earbox.net,
	eddyz87@...il.com,
	haoluo@...gle.com,
	john.fastabend@...il.com,
	jolsa@...nel.org,
	kpsingh@...nel.org,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	martin.lau@...ux.dev,
	sdf@...ichev.me,
	song@...nel.org,
	yangfeng59949@....com,
	yonghong.song@...ux.dev
Subject: Re: [BUG] Failed to obtain stack trace via bpf_get_stackid on ARM64 architecture

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@...il.com> wrote:
> 
> > On Fri, Sep 19, 2025 at 12:19 AM Feng Yang <yangfeng59949@....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