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

Feng Yang
2025-09-24T06:26:37Z

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.