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

On 27 August 2026 13:27:44 BST, "Jérémy Jean"
[off-list ref] wrote:

On 2026-08-27 14:19, Steven Rostedt wrote:

quoted

On Thu, 27 Aug 2026 13:09:44 +0100
Bradley Morgan [off-list ref] wrote:

quoted

quoted

quoted

@@ -865,9 +865,12 @@ void user_event_mm_remove(struct task_struct

*t)

quoted

quoted

quoted

quoted

 void user_event_mm_dup(struct task_struct *t, struct user_event_mm

*old_mm)

quoted

 {
-	struct user_event_mm *mm = user_event_mm_alloc(t);
+	struct user_event_mm *mm;

Why this change?

quoted

 	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

quoted

to move that change to this function. But this function only needs to

add

quoted

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

quoted

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)

quoted

 	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

I wouldn't mind, add my tag, I review differently from Steven, so Steven
may not be happy at me :(

I review on


1: does this do what it's intended
2: does it fix X?
3: Any comments, any new functions used instead, any way to get the line count shorter?


And others, I apologise if I'm wrong. I just review in a different style.

--- Thanks!

https://lore.kernel.org/all/EE579805-42F2-4C58-B752-F28779EEB717@grrlz.net/ (local)