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

[Google]

On Wed, 24 Sep 2025 18:53:53 +0800
Feng Yang <yangfeng59949@xxxxxxx> wrote:

> On Wed, 24 Sep 2025 17:04:16 +0900 Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> 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@xxxxxxxxxx>