Re: [PATCH 1/2] tracing/perf: Prevent double unregister of perf probes

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 9 Jul 2025 11:11:09 +0530
Aditya Chillara <quic_achillar@quicinc.com> wrote:

> Double perf_trace_event_unreg is allowed causing perf_refcount to go
> negative. total_ref_count also goes negative because the return value
> of tracepoint_probe_unregister is ignored.
> 
> Once total_ref_count is negative, the next call to perf_trace_event_reg
> will register perf_probe but will not allocate perf_trace_buf and sets
> it to NULL instead.
> 
> The subsequent trace_##call() will mem abort in perf_trace_buf_alloc
> because memset will be called on the NULL perf_trace_buf.
> 
> Gracefully handle the error in perf_trace_event_unreg to prevent
> double unregister.
> 
> Signed-off-by: Aditya Chillara <quic_achillar@quicinc.com>
> ---
>  kernel/trace/trace_event_perf.c | 8 ++++++--
>  kernel/trace/trace_events.c     | 3 +--
>  2 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c
> index 61e3a2620fa3c9417ac23cf5a18aeb86e7393dcc..247db88accd88eb0acf3692ea593d576519ce8b1 100644
> --- a/kernel/trace/trace_event_perf.c
> +++ b/kernel/trace/trace_event_perf.c
> @@ -154,12 +154,16 @@ static int perf_trace_event_reg(struct trace_event_call *tp_event,
>  static void perf_trace_event_unreg(struct perf_event *p_event)
>  {
>  	struct trace_event_call *tp_event = p_event->tp_event;
> -	int i;
> +	int i, ret;
>  
>  	if (--tp_event->perf_refcount > 0)
>  		return;
>  
> -	tp_event->class->reg(tp_event, TRACE_REG_PERF_UNREGISTER, NULL);
> +	ret = tp_event->class->reg(tp_event, TRACE_REG_PERF_UNREGISTER, NULL);

The only time unreg() fails is when it doesn't find a tracepoint to
unregister.

There should be no reason to check the return value of unregister if
you have your accounting correct. Thus I think you are fixing a symptom
of a bug elsewhere.

-- Steve


> +	if (ret) {
> +		++tp_event->perf_refcount;
> +		return;
> +	}
>