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>

1 files changed, 26 insertions, 0 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index cb85ee4a8807..87402b6e8c58 100644
--- a/kernel/trace/trace.c
+++ b/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);