Re: [PATCH v3 7/7] perf: Make perf_pmu_unregister() useable

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Mar 10, 2025 at 09:05:45PM +0530, Ravi Bangoria wrote:
> > @@ -207,6 +207,7 @@ static void perf_ctx_unlock(struct perf_
> >  }
> >  
> >  #define TASK_TOMBSTONE ((void *)-1L)
> > +#define EVENT_TOMBSTONE ((void *)-1L)
> >  
> >  static bool is_kernel_event(struct perf_event *event)
> >  {
> > @@ -2348,6 +2349,11 @@ static void perf_child_detach(struct per
> >  
> >  	sync_child_event(event);
> >  	list_del_init(&event->child_list);
> > +	/*
> > +	 * Cannot set to NULL, as that would confuse the situation vs
> > +	 * not being a child event. See for example unaccount_event().
> > +	 */
> > +	event->parent = EVENT_TOMBSTONE;
> 
> This will cause issues where we do `event = event->parent`. No? For ex:
> 
>   perf_pmu_unregister()
>     ...
>       perf_event_exit_event()
>         perf_event_wakeup()
>           ring_buffer_wakeup()
>             if (event->parent)
>               event = event->parent;

It will. However, if we do not do this, we have a potential
use-after-free, and people seem to take a dim view of those too :-)

I had convinced myself all paths that were doing this 'event =
event->parent' were unreachable by the time we set the TOMBSTONE, but
clearly I missed one.

I suppose we can do something like so, not really pretty though.

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13722,9 +13722,12 @@ perf_event_exit_event(struct perf_event
 {
 	struct perf_event *parent_event = event->parent;
 	unsigned long detach_flags = DETACH_EXIT;
+	bool parent_dead = false;
 
-	if (parent_event == EVENT_TOMBSTONE)
+	if (parent_event == EVENT_TOMBSTONE) {
 		parent_event = NULL;
+		parent_dead = true;
+	}
 
 	if (parent_event) {
 		/*
@@ -13748,6 +13751,9 @@ perf_event_exit_event(struct perf_event
 
 	perf_remove_from_context(event, detach_flags);
 
+	if (parent_dead)
+		return;
+
 	/*
 	 * Child events can be freed.
 	 */