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Message-Id: <20250924062536.471231-1-yangfeng59949@163.com>
Date: Wed, 24 Sep 2025 14:25:36 +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 Wed, 24 Sep 2025 00:32:15 +0900 Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> On Mon, 22 Sep 2025 10:15:31 +0800
> Feng Yang <yangfeng59949@....com> wrote:
> 
> > 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;
> > 	......
> 
> Hmm. The "trace" is returned from get_perf_callchain()
> 
> get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
> 		   u32 max_stack, bool crosstask, bool add_mark)
> {
> ...
> 
> 	if (kernel && !user_mode(regs)) {
> 		if (add_mark)
> 			perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
> 		perf_callchain_kernel(&ctx, regs);
> 	}
> 
> So this means `perf_callchain_kernel(&ctx, regs);` fails to unwind stack.
> 
> perf_callchain_kernel() -> arch_stack_walk() -> kunwind_stack_walk()
> 
> That is `kunwind_init_from_regs()` and `do_kunwind()`.
> 
> 	if (regs) {
> 		if (task != current)
> 			return -EINVAL;
> 		kunwind_init_from_regs(&state, regs);
> 	} else if (task == current) {
> 		kunwind_init_from_caller(&state);
> 	} else {
> 		kunwind_init_from_task(&state, task);
> 	}
> 
> 	return do_kunwind(&state, consume_state, cookie);
> 
> For initialization, it should be OK because it only refers pc and 
> fp(regs[29]), which are recovered by ftrace_partial_regs().
> 
> static __always_inline void
> kunwind_init_from_regs(struct kunwind_state *state,
> 		       struct pt_regs *regs)
> {
> 	kunwind_init(state, current);
> 
> 	state->regs = regs;
> 	state->common.fp = regs->regs[29];
> 	state->common.pc = regs->pc;
> 	state->source = KUNWIND_SOURCE_REGS_PC;
> }
> 
> And do_kunwind() should work increase trace->nr before return
> unless `kunwind_recover_return_address()` fails.
> 
> static __always_inline int
> do_kunwind(struct kunwind_state *state, kunwind_consume_fn consume_state,
> 	   void *cookie)
> {
> 	int ret;
> 
> 	ret = kunwind_recover_return_address(state);
> 	if (ret)
> 		return ret;
> 
> 	while (1) {
> 		if (!consume_state(state, cookie)) <--- this increases trace->nr (*).
> 			return -EINVAL;
> 		ret = kunwind_next(state);
> 		if (ret == -ENOENT)
> 			return 0;
> 		if (ret < 0)
> 			return ret;
> 	}
> }
> 
> (*) consume_state() == arch_kunwind_consume_entry() 
>   ->  data->consume_entry == callchain_trace() -> perf_callchain_store().
> 
> Hmm, can you also dump the regs and insert pr_info() to find
> which function fails?
> 
> Thanks,
> 

After testing, it was found that the stack could not be obtained because user_mode(regs) returned 1. 
Referring to the arch_ftrace_fill_perf_regs function in your email 
(https://lore.kernel.org/all/173518997908.391279.15910334347345106424.stgit@devnote2/), 
I made the following modification: by setting the value of pstate, the stack can now be obtained successfully.

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 058a99aa44bd..f2814175e958 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -159,11 +159,13 @@ ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
 {
        struct __arch_ftrace_regs *afregs = arch_ftrace_regs(fregs);
 
        memcpy(regs->regs, afregs->regs, sizeof(afregs->regs));
        regs->sp = afregs->sp;
        regs->pc = afregs->pc;
        regs->regs[29] = afregs->fp;
        regs->regs[30] = afregs->lr;
+       regs->pstate = PSR_MODE_EL1h;
        return regs;
 }
However, I'm not sure if there will be any other impacts...

By the way, during my testing, I also noticed that when executing bpf_get_stackid via kprobes or tracepoints, 
the command bpftrace -e 'kprobe:bpf_get_stackid {printf("bpf_get_stackid\n");}' produces no output. 
However, it does output something when bpf_get_stackid is invoked via uprobes. 
This phenomenon also occurs on the x86 architecture, could this be a bug as well?

Thanks.


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