From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
To: Feng Yang <yangfeng59949@163.com>
Cc: alexei.starovoitov@gmail.com, andrii@kernel.org, ast@kernel.org,
bpf@vger.kernel.org, daniel@iogearbox.net, eddyz87@gmail.com,
haoluo@google.com, john.fastabend@gmail.com, jolsa@kernel.org,
kpsingh@kernel.org, linux-kernel@vger.kernel.org,
linux-trace-kernel@vger.kernel.org, martin.lau@linux.dev,
sdf@fomichev.me, song@kernel.org, yonghong.song@linux.dev
Subject: Re: [BUG] Failed to obtain stack trace via bpf_get_stackid on ARM64 architecture
Date: Thu, 25 Sep 2025 07:00:22 +0900 [thread overview]
Message-ID: <20250925070022.42ef1398e0f61c797222679c@kernel.org> (raw)
In-Reply-To: <20250924105353.840865-1-yangfeng59949@163.com>
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>
next prev parent reply other threads:[~2025-09-24 22:00 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-19 7:19 [BUG] Failed to obtain stack trace via bpf_get_stackid on ARM64 architecture Feng Yang
2025-09-20 2:56 ` Alexei Starovoitov
2025-09-20 22:17 ` Jiri Olsa
2025-09-21 13:30 ` Masami Hiramatsu
2025-09-22 2:15 ` Feng Yang
2025-09-23 15:32 ` Masami Hiramatsu
2025-09-24 6:25 ` Feng Yang
2025-09-24 7:31 ` Masami Hiramatsu
2025-09-24 8:04 ` Masami Hiramatsu
2025-09-24 10:53 ` Feng Yang
2025-09-24 22:00 ` Masami Hiramatsu [this message]
2025-09-24 15:42 ` Jiri Olsa
2025-09-24 21:56 ` Masami Hiramatsu
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