Re: [PATCH v2] tracing/user_events: Clear copied tracing state before fork duplication

[Jérémy Jean]

On 2026-08-27 14:19, Steven Rostedt wrote:
> On Thu, 27 Aug 2026 13:09:44 +0100
> Bradley Morgan <brads@xxxxxxxxxxxxxx> wrote:
>
> > >> @@ -865,9 +865,12 @@ void user_event_mm_remove(struct task_struct *t)
> > >>
> > >>  void user_event_mm_dup(struct task_struct *t, struct user_event_mm
> > >*old_mm)
> > >>  {
> > >> -	struct user_event_mm *mm = user_event_mm_alloc(t);
> > >> +	struct user_event_mm *mm;
> > >
> > >Why this change?
> > >
> > >>  	struct user_event_enabler *enabler;
> > >>
> > >> +	t->user_event_mm = NULL;
> > >> +	mm = user_event_mm_alloc(t);
> > >
> > >I don't see why you moved the mm assignment down here. The
> > >t->user_event_mm
> > >is not used in user_event_mm_alloc().
> >
> > Uff, not wrong, I must be dummy dumb dumb, well, I base my reviews off
> >
> > Does this fix the bug? And is this a small fix?
>
> The bug is fixed because it needs to NULL out that value. I asked from 
> v1
> to move that change to this function. But this function only needs to 
> add
> that line before the return. It doesn't need to modify anything else in
> that function.
>
> That is, something like this:
>
> diff --git a/kernel/trace/trace_events_user.c 
> b/kernel/trace/trace_events_user.c
> index 8c82ecb735f4..6b89d225b189 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -868,6 +868,9 @@ void user_event_mm_dup(struct task_struct *t, 
> struct user_event_mm *old_mm)
>  	struct user_event_mm *mm = user_event_mm_alloc(t);
>  	struct user_event_enabler *enabler;
>
> +	/* On failure, do not free parent's copy */
> +	t->user_event_mm = NULL;
> +
>  	if (!mm)
>  		return;
>
>
> -- Steve

Hello Steve,

Thanks for this. I thought that my previous version was okay
so that it was not required to read the details of
user_event_mm_alloc() to get convinced whether user_event_mm is
accessed or not, but it's true that in the end, this is not
required. Your fix is simpler. Do you want me to send a v3
with that simplification and the same changelog as in the v2?

Regards,
Jérémy