Re: [PATCH] x86/ibt: make is_endbr() notrace

[Menglong Dong]

On Thu, Sep 18, 2025 at 9:05 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Sep 18, 2025 at 08:09:39PM +0800, Menglong Dong wrote:
> > is_endbr() is called in __ftrace_return_to_handler -> fprobe_return ->
> > kprobe_multi_link_exit_handler -> is_endbr.
> >
> > It is not protected by the "bpf_prog_active", so it can't be traced by
> > kprobe-multi, which can cause recurring and panic the kernel. Fix it by
> > make it notrace.
>
> This is very much a riddle wrapped in an enigma. Notably
> kprobe_multi_link_exit_handler() does not call is_endbr(). Nor is that
> cryptic next line sufficient to explain why its a problem.
>
> I suspect the is_endbr() you did mean is the one in
> arch_ftrace_get_symaddr(), but who knows.

Yeah, I mean
kprobe_multi_link_exit_handler -> ftrace_get_entry_ip ->
arch_ftrace_get_symaddr -> is_endbr
actually. And CONFIG_X86_KERNEL_IBT is enabled of course.

>
> Also, depending on compiler insanity, it is possible the thing
> out-of-lines things like __is_endbr(), getting you yet another
> __fentry__ site.

The panic happens when I run the bpf bench testing:
  ./bench kretprobe-multi-all

And skip the "is_endbr" fix this problem.

__is_endbr should be marked with "notrace" too. I slacked off
on it, as it didn't happen in my case :/

>
> Please try again.