tracing: Cache ":mod:" events for modules not loaded yet

When the :mod: command is written into /sys/kernel/tracing/set_event (or
that file within an instance), if the module specified after the ":mod:"
is not yet loaded, it will store that string internally. When the module
is loaded, it will enable the events as if the module was loaded when the
string was written into the set_event file.

This can also be useful to enable events that are in the init section of
the module, as the events are enabled before the init section is executed.

This also works on the kernel command line:

 trace_event=:mod:<module>

Will enable the events for <module> when it is loaded.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/20250116143533.514730995@goodmis.org
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: rostedt

Documentation/admin-guide/kernel-parameters.txt

@@ -6858,6 +6858,14 @@
 			comma-separated list of trace events to enable. See
 			also Documentation/trace/events.rst
 
+			To enable modules, use :mod: keyword:
+
+			trace_event=:mod:<module>
+
+			The value before :mod: will only enable specific events
+			that are part of the module. See the above mentioned
+			document for more information.
+
 	trace_instance=[instance-info]
 			[FTRACE] Create a ring buffer instance early in boot up.
 			This will be listed in:

Documentation/trace/events.rst

@@ -60,7 +60,9 @@ a specific module::
 
 	# echo ':mod:<module>' > /sys/kernel/tracing/set_event
 
-Will enable all events in the module ``<module>``.
+Will enable all events in the module ``<module>``.  If the module is not yet
+loaded, the string will be saved and when a module is that matches ``<module>``
+is loaded, then it will apply the enabling of events then.
 
 The text before ``:mod:`` will be parsed to specify specific events that the
 module creates::

kernel/trace/ftrace.c

@@ -4930,23 +4930,6 @@ static int ftrace_hash_move_and_update_ops(struct ftrace_ops *ops,
 	return __ftrace_hash_move_and_update_ops(ops, orig_hash, hash, enable);
 }
 
-static bool module_exists(const char *module)
-{
-	/* All modules have the symbol __this_module */
-	static const char this_mod[] = "__this_module";
-	char modname[MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 2];
-	unsigned long val;
-	int n;
-
-	n = snprintf(modname, sizeof(modname), "%s:%s", module, this_mod);
-
-	if (n > sizeof(modname) - 1)
-		return false;
-
-	val = module_kallsyms_lookup_name(modname);
-	return val != 0;
-}
-
 static int cache_mod(struct trace_array *tr,
 		     const char *func, char *module, int enable)
 {

kernel/trace/trace.c

@@ -9407,6 +9407,10 @@ trace_array_create_systems(const char *name, const char *systems,
 	INIT_LIST_HEAD(&tr->hist_vars);
 	INIT_LIST_HEAD(&tr->err_log);
 
+#ifdef CONFIG_MODULES
+	INIT_LIST_HEAD(&tr->mod_events);
+#endif
+
 	if (allocate_trace_buffers(tr, trace_buf_size) < 0)
 		goto out_free_tr;
 
@@ -9823,6 +9827,24 @@ late_initcall_sync(trace_eval_sync);
 
 
 #ifdef CONFIG_MODULES
+
+bool module_exists(const char *module)
+{
+	/* All modules have the symbol __this_module */
+	static const char this_mod[] = "__this_module";
+	char modname[MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 2];
+	unsigned long val;
+	int n;
+
+	n = snprintf(modname, sizeof(modname), "%s:%s", module, this_mod);
+
+	if (n > sizeof(modname) - 1)
+		return false;
+
+	val = module_kallsyms_lookup_name(modname);
+	return val != 0;
+}
+
 static void trace_module_add_evals(struct module *mod)
 {
 	if (!mod->num_trace_evals)
@@ -10535,6 +10557,10 @@ __init static int tracer_alloc_buffers(void)
 #endif
 	ftrace_init_global_array_ops(&global_trace);
 
+#ifdef CONFIG_MODULES
+	INIT_LIST_HEAD(&global_trace.mod_events);
+#endif
+
 	init_trace_flags_index(&global_trace);
 
 	register_tracer(&nop_trace);

kernel/trace/trace.h

@@ -400,6 +400,9 @@ struct trace_array {
 	cpumask_var_t		pipe_cpumask;
 	int			ref;
 	int			trace_ref;
+#ifdef CONFIG_MODULES
+	struct list_head	mod_events;
+#endif
 #ifdef CONFIG_FUNCTION_TRACER
 	struct ftrace_ops	*ops;
 	struct trace_pid_list	__rcu *function_pids;
@@ -434,6 +437,15 @@ enum {
 	TRACE_ARRAY_FL_BOOT	= BIT(1),
 };
 
+#ifdef CONFIG_MODULES
+bool module_exists(const char *module);
+#else
+static inline bool module_exists(const char *module)
+{
+	return false;
+}
+#endif
+
 extern struct list_head ftrace_trace_arrays;
 
 extern struct mutex trace_types_lock;