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

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Message-Id: <20250925065641.5e066b03db123e96791d5671@kernel.org>
Date: Thu, 25 Sep 2025 06:56:41 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Jiri Olsa <olsajiri@...il.com>
Cc: Feng Yang <yangfeng59949@....com>, mhiramat@...nel.org,
 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, kpsingh@...nel.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 martin.lau@...ux.dev, sdf@...ichev.me, song@...nel.org,
 yonghong.song@...ux.dev
Subject: Re: [BUG] Failed to obtain stack trace via bpf_get_stackid on ARM64
 architecture

On Wed, 24 Sep 2025 17:42:18 +0200
Jiri Olsa <olsajiri@...il.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@...nel.org>