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

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Message-Id: <20250924105353.840865-1-yangfeng59949@163.com>
Date: Wed, 24 Sep 2025 18:53:53 +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 17:04:16 +0900 Masami Hiramatsu (Google) <mhiramat@...nel.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.