Re: [PATH v3] tracing: Fix race condition in kprobe initialization causing NULL pointer dereference

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 30 Sep 2025 10:46:45 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Sep 30, 2025 at 09:18:48AM +0100, chenyuan_fl@163.com wrote:
> 
> > diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> > index 842383fbc03b..98b838591edc 100644
> > --- a/kernel/trace/trace_probe.h
> > +++ b/kernel/trace/trace_probe.h
> > @@ -274,19 +274,19 @@ struct event_file_link {
> >  static inline bool trace_probe_test_flag(struct trace_probe *tp,
> >  					 unsigned int flag)
> >  {
> > -	return !!(tp->event->flags & flag);
> > +	return !!(smp_load_acquire(&tp->event->flags) & flag);
> >  }
> >  
> >  static inline void trace_probe_set_flag(struct trace_probe *tp,
> >  					unsigned int flag)
> >  {
> > -	tp->event->flags |= flag;
> > +	smp_store_release(&tp->event->flags, tp->event->flags | flag);
> >  }
> >  
> >  static inline void trace_probe_clear_flag(struct trace_probe *tp,
> >  					  unsigned int flag)
> >  {
> > -	tp->event->flags &= ~flag;
> > +	smp_store_release(&tp->event->flags, tp->event->flags & ~flag);
> >  }
> 
> 
> I _think_ the clear one is superfluous. Is there anything that cares
> about stores done before the clear when the flag is found not set?
> 
> Also, code like:
> 
> static int fentry_dispatcher(struct fprobe *fp, unsigned long entry_ip,
> 			     unsigned long ret_ip, struct ftrace_regs *fregs,
> 			     void *entry_data)
> {
> 	struct trace_fprobe *tf = container_of(fp, struct trace_fprobe, fp);
> 	int ret = 0;
> 
> 	if (trace_probe_test_flag(&tf->tp, TP_FLAG_TRACE))
> 		fentry_trace_func(tf, entry_ip, fregs);
> 
> #ifdef CONFIG_PERF_EVENTS
> 	if (trace_probe_test_flag(&tf->tp, TP_FLAG_PROFILE))
> 		ret = fentry_perf_func(tf, entry_ip, fregs);
> #endif
> 	return ret;
> }
> 
> 
> Will now have two barriers; where one would suffice, eg.
> 
> 	flags = smp_load_acquire(&tp->event->flags);
> 
> 	if (flags & TP_FLAG_TRACE)
> 		fentry_trace_func(...);
> 
> 	if (flags & TP_FLAG_PROFILE)
> 		fentry_perf_func(...);
> 
> Should be just fine afaict.

Looks good to me. We should replace trace_probe_test_flag()
with trace_probe_load_flag().

Thanks,

> 
> 
> Is this something anybody cares about?


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>