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

Feng Yang
2025-09-24T10:54:48Z

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?

> > 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.