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Message-ID: <20250930084645.GJ3245006@noisy.programming.kicks-ass.net>
Date: Tue, 30 Sep 2025 10:46:45 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: chenyuan_fl@....com
Cc: john.ogness@...utronix.de, rostedt@...dmis.org, bigeasy@...utronix.de,
chenyuan@...inos.cn, linux-kernel@...r.kernel.org,
linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
mhiramat@...nel.org
Subject: Re: [PATH v3] tracing: Fix race condition in kprobe initialization
causing NULL pointer dereference
On Tue, Sep 30, 2025 at 09:18:48AM +0100, chenyuan_fl@....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.
Is this something anybody cares about?
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