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

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

[Feng Yang]

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);
        kprobe_multi_check(ctx, false);
+       bpf_printk("stackid: %d\n", id);
        return 0;
 }

./test_progs -t kprobe_multi_test/attach_api_pattern
#155/4   kprobe_multi_test/attach_api_pattern:OK
#155     kprobe_multi_test:OK
#156     kprobe_multi_testmod_test:OK
Summary: 2/1 PASSED, 0 SKIPPED, 0 FAILED

cat /sys/kernel/debug/tracing/trace
test_progs-68315   [004] ...1. 13377.097527: bpf_trace_printk: stackid: -14
......

Test Version:
6ff4a0fa3e1 ("bpf, arm64: Call bpf_jit_binary_pack_finalize() in bpf_jit_free()")
Linux localhost.localdomain 6.17.0-rc5+ #2 SMP PREEMPT_DYNAMIC Fri Sep 19 10:29:07 CST 2025 aarch64 aarch64 aarch64 GNU/Linux
clang version 17.0.6 ( 17.0.6-30.p03.ky11)
gcc (GCC) 12.3.1 (kylin 12.3.1-62.p02.ky11)
GNU Make 4.4.1

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

[Alexei Starovoitov]

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?

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

[Jiri Olsa]

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

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

[Masami Hiramatsu]

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.

struct perf_callchain_entry *
get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
		   u32 max_stack, bool crosstask, bool add_mark)
{
...
	entry = get_callchain_entry(&rctx);
	if (!entry)
		return NULL;


Thus the `get_callchain_entry(&rctx)` returns NULL. But if so,
this does not related to the ftrace_partial_regs(), because
get_callchain_entry() returns the per-cpu callchain woarking
buffer for the context, not decoding stack.

struct perf_callchain_entry *get_callchain_entry(int *rctx)
{
	int cpu;
	struct callchain_cpus_entries *entries;

	*rctx = get_recursion_context(this_cpu_ptr(callchain_recursion));
	if (*rctx == -1)
		return NULL;

	entries = rcu_dereference(callchain_cpus_entries);
	if (!entries) {
		put_recursion_context(this_cpu_ptr(callchain_recursion), *rctx);
		return NULL;
	}

	cpu = smp_processor_id();

	return (((void *)entries->cpu_entries[cpu]) +
		(*rctx * perf_callchain_entry__sizeof()));
}

What context does BPF expect, and how does it detect?

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

[Feng Yang]

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

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

[Masami Hiramatsu]

On Mon, 22 Sep 2025 10:15:31 +0800
Feng Yang <yangfeng59949@163.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@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;
> 	......

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,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

[Feng Yang]

On Wed, 24 Sep 2025 00:32:15 +0900 Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> On Mon, 22 Sep 2025 10:15:31 +0800
> Feng Yang <yangfeng59949@163.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@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;
> > 	......
> 
> 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.

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

[Masami Hiramatsu]

On Wed, 24 Sep 2025 14:25:36 +0800
Feng Yang <yangfeng59949@163.com> wrote:

> On Wed, 24 Sep 2025 00:32:15 +0900 Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > On Mon, 22 Sep 2025 10:15:31 +0800
> > Feng Yang <yangfeng59949@163.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@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;
> > > 	......
> > 
> > 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;

Good catch! 

>         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. 

That is strange. since normal kprobes passes full pt_regs.

> 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?

Yes, it must be a bug.

Thanks!

> 
> Thanks.
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

[Masami Hiramatsu]

On Wed, 24 Sep 2025 14:25:36 +0800
Feng Yang <yangfeng59949@163.com> wrote:

> 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. 

I think this is because the bpf_get_stackid is a kind of recursive
event from kprobes. Kprobe handler can not be reentered.

> 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?

Maybe if bpf_get_stackid() is kicked from uprobes, it is not recursive
call from kprobes, so it works.

So it is expected behavior, not a bug. Sorry for confusion.


Thank you,

> 
> Thanks.
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

[Feng Yang]

On Wed, 24 Sep 2025 17:04:16 +0900 Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > 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;
> 
> Good catch! 

Should I submit this patch, or will you carry out a more complete fix?

> > 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. 
> 
> I think this is because the bpf_get_stackid is a kind of recursive
> event from kprobes. Kprobe handler can not be reentered.
> 
> > 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?
> 
> Maybe if bpf_get_stackid() is kicked from uprobes, it is not recursive
> call from kprobes, so it works.
> 
> So it is expected behavior, not a bug. Sorry for confusion.
> 
> 
> Thank you,

Thank you very much for your explanation.

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

[Masami Hiramatsu]

On Wed, 24 Sep 2025 18:53:53 +0800
Feng Yang <yangfeng59949@163.com> wrote:

> On Wed, 24 Sep 2025 17:04:16 +0900 Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > > 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;
> > 
> > Good catch! 
> 
> Should I submit this patch, or will you carry out a more complete fix?


Yes, please send a fix. I think this is enoguh. Please add,

Fixes: b9b55c8912ce ("tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs")

Thank you,

> 
> > > 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. 
> > 
> > I think this is because the bpf_get_stackid is a kind of recursive
> > event from kprobes. Kprobe handler can not be reentered.
> > 
> > > 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?
> > 
> > Maybe if bpf_get_stackid() is kicked from uprobes, it is not recursive
> > call from kprobes, so it works.
> > 
> > So it is expected behavior, not a bug. Sorry for confusion.
> > 
> > 
> > Thank you,
> 
> Thank you very much for your explanation.
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

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

[Jiri Olsa]

On Wed, Sep 24, 2025 at 02:25:36PM +0800, Feng Yang wrote:

SNIP

> > 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...

nice, the test works for me with this change.. could you please send
formal patch? I can polish and send out the test [1]

thanks,
jirka


[1] https://github.com/kernel-patches/bpf/pull/9845/commits/11b31cd465a83b8719cb06331c8e81794cca40fa

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

[Masami Hiramatsu]

On Wed, 24 Sep 2025 17:42:18 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Wed, Sep 24, 2025 at 02:25:36PM +0800, Feng Yang wrote:
> 
> SNIP
> 
> > > 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...
> 
> nice, the test works for me with this change.. could you please send
> formal patch? I can polish and send out the test [1]

Yeah, and Cc to arm64 maintainers, also Closes: to this thread.

Thanks,

> 
> thanks,
> jirka
> 
> 
> [1] https://github.com/kernel-patches/bpf/pull/9845/commits/11b31cd465a83b8719cb06331c8e81794cca40fa


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>