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

On Tue, 30 Sep 2025 10:46:45 +0200
Peter Zijlstra [off-list ref] wrote:

On Tue, Sep 30, 2025 at 09:18:48AM +0100, chenyuan_fl@163.com wrote:

quoted

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) [off-list ref]