Re: [PATCH v2] perf/core: Fix group leader use-after-free after sibling detach

[Peter Zijlstra]

Finally got to look at this. In principle this seems okay, but while
staring at it, I had a few questions, see below.

On Tue, Jun 30, 2026 at 12:42:12AM +0530, Aditya Chillara wrote:
> ---
>  kernel/events/core.c | 62 +++++++++++++++++++++++++++++++++++-----------------
>  1 file changed, 42 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 954c36e28101..744643ada948 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2253,6 +2253,8 @@ static void put_event(struct perf_event *event);
>  static void __event_disable(struct perf_event *event,
>  			    struct perf_event_context *ctx,
>  			    enum perf_event_state state);
> +static void event_sched_out(struct perf_event *event,
> +			    struct perf_event_context *ctx);
>  
>  static void perf_put_aux_event(struct perf_event *event)
>  {
> @@ -2343,6 +2345,44 @@ static inline struct list_head *get_event_list(struct perf_event *event)
>  				    &event->pmu_ctx->flexible_active;
>  }
>  
> +/* @sibling must already be unlinked from its old leader's sibling_list. */
> +static void perf_promote_sibling_to_leader(struct perf_event *sibling,
> +					   struct perf_event_context *ctx,
> +					   int group_caps)
> +{
> +	/*
> +	 * Events that have PERF_EV_CAP_SIBLING require being part of
> +	 * a group and cannot exist on their own, schedule them out
> +	 * and move them into the ERROR state. Also see
> +	 * _perf_event_enable(), it will not be able to recover this
> +	 * ERROR state.
> +	 */
> +	if (sibling->event_caps & PERF_EV_CAP_SIBLING) {
> +		event_sched_out(sibling, ctx);
> +
> +		/*
> +		 * The guards keep this correct even when @sibling is already
> +		 * disabled (see __perf_remove_from_context()).
> +		 */
> +		if (sibling->state > PERF_EVENT_STATE_OFF)
> +			perf_cgroup_event_disable(sibling, ctx);
> +		if (sibling->state > PERF_EVENT_STATE_ERROR)
> +			perf_event_set_state(sibling, PERF_EVENT_STATE_ERROR);
> +	}

The below code used __event_disable(); and this change is not
mentioned in the Changelog. Why was this changed?

> +
> +	sibling->group_leader = sibling;
> +	sibling->group_caps = group_caps;
> +
> +	if (sibling->attach_state & PERF_ATTACH_CONTEXT) {
> +		add_event_to_groups(sibling, ctx);
> +
> +		if (sibling->state == PERF_EVENT_STATE_ACTIVE)
> +			list_add_tail(&sibling->active_list, get_event_list(sibling));
> +	}
> +
> +	perf_event__header_size(sibling);
> +}
> +
>  static void perf_group_detach(struct perf_event *event)
>  {
>  	struct perf_event *leader = event->group_leader;
> @@ -2368,6 +2408,7 @@ static void perf_group_detach(struct perf_event *event)
>  		list_del_init(&event->sibling_list);
>  		event->group_leader->nr_siblings--;
>  		event->group_leader->group_generation++;

Here we can do 's/event->group_//'

Also, this case 'leader != event' we remove one sibling from a group and
decrement leader->nr_siblings...

> +		perf_promote_sibling_to_leader(event, ctx, event->event_caps);
>  		goto out;
>  	}
>  
> @@ -2377,29 +2418,10 @@ static void perf_group_detach(struct perf_event *event)
>  	 * to whatever list we are on.
>  	 */
>  	list_for_each_entry_safe(sibling, tmp, &event->sibling_list, sibling_list) {
> -
> -		/*
> -		 * Events that have PERF_EV_CAP_SIBLING require being part of
> -		 * a group and cannot exist on their own, schedule them out
> -		 * and move them into the ERROR state. Also see
> -		 * _perf_event_enable(), it will not be able to recover this
> -		 * ERROR state.
> -		 */
> -		if (sibling->event_caps & PERF_EV_CAP_SIBLING)
> -			__event_disable(sibling, ctx, PERF_EVENT_STATE_ERROR);
> -
> -		sibling->group_leader = sibling;
>  		list_del_init(&sibling->sibling_list);
>  
>  		/* Inherit group flags from the previous leader */
> -		sibling->group_caps = event->group_caps;
> -
> -		if (sibling->attach_state & PERF_ATTACH_CONTEXT) {
> -			add_event_to_groups(sibling, event->ctx);
> -
> -			if (sibling->state == PERF_EVENT_STATE_ACTIVE)
> -				list_add_tail(&sibling->active_list, get_event_list(sibling));
> -		}
> +		perf_promote_sibling_to_leader(sibling, ctx, event->group_caps);
>  
>  		WARN_ON_ONCE(sibling->ctx != event->ctx);
>  	}

This is the case 'leader == event' (per not being the other case), and
this we remove all siblings, but then do not set leader->nr_siblings =
0, should we ?