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

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Message-ID: <CAADnVQKrnYCaUCd+BNvZQmR0-6CSu2GBa=TCCCjPLSNfb_Ddvg@mail.gmail.com>
Date: Fri, 19 Sep 2025 19:56:20 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Feng Yang <yangfeng59949@....com>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [BUG] Failed to obtain stack trace via bpf_get_stackid on ARM64 architecture

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...
I suspect the arm64 implementation wasn't really tested.
Or maybe there is some other issue.

Masami, Jiri,
thoughts?