tracing/user_events: Remove user_ns walk for groups

During discussions it was suggested that user_ns is not a good place to
try to attach a tracing namespace. The current code has stubs to enable
that work that are very likely to change and incur a performance cost.

Remove the user_ns walk when creating a group and determining the system
name to use, since it's unlikely user_ns will be used in the future.

Link: https://lore.kernel.org/all/20230601-urenkel-holzofen-cd9403b9cadd@brauner/
Link: https://lore.kernel.org/linux-trace-kernel/20230601224928.301-1-beaub@linux.microsoft.com

Suggested-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: beaubelgrave

kernel/trace/trace_events_user.c

@@ -182,21 +182,11 @@ static void user_event_group_destroy(struct user_event_group *group)
 	kfree(group);
 }
 
-static char *user_event_group_system_name(struct user_namespace *user_ns)
+static char *user_event_group_system_name(void)
 {
 	char *system_name;
 	int len = sizeof(USER_EVENTS_SYSTEM) + 1;
 
-	if (user_ns != &init_user_ns) {
-		/*
-		 * Unexpected at this point:
-		 * We only currently support init_user_ns.
-		 * When we enable more, this will trigger a failure so log.
-		 */
-		pr_warn("user_events: Namespace other than init_user_ns!\n");
-		return NULL;
-	}
-
 	system_name = kmalloc(len, GFP_KERNEL);
 
 	if (!system_name)
@@ -207,34 +197,12 @@ static char *user_event_group_system_name(struct user_namespace *user_ns)
 	return system_name;
 }
 
-static inline struct user_event_group
-*user_event_group_from_user_ns(struct user_namespace *user_ns)
-{
-	if (user_ns == &init_user_ns)
-		return init_group;
-
-	return NULL;
-}
-
 static struct user_event_group *current_user_event_group(void)
 {
-	struct user_namespace *user_ns = current_user_ns();
-	struct user_event_group *group = NULL;
-
-	while (user_ns) {
-		group = user_event_group_from_user_ns(user_ns);
-
-		if (group)
-			break;
-
-		user_ns = user_ns->parent;
-	}
-
-	return group;
+	return init_group;
 }
 
-static struct user_event_group
-*user_event_group_create(struct user_namespace *user_ns)
+static struct user_event_group *user_event_group_create(void)
 {
 	struct user_event_group *group;
 
@@ -243,7 +211,7 @@ static struct user_event_group
 	if (!group)
 		return NULL;
 
-	group->system_name = user_event_group_system_name(user_ns);
+	group->system_name = user_event_group_system_name();
 
 	if (!group->system_name)
 		goto error;
@@ -2603,7 +2571,7 @@ static int __init trace_events_user_init(void)
 	if (!fault_cache)
 		return -ENOMEM;
 
-	init_group = user_event_group_create(&init_user_ns);
+	init_group = user_event_group_create();
 
 	if (!init_group) {
 		kmem_cache_destroy(fault_cache);