tracing: Add show_event_filters to expose active event filters

Currently, to audit active Ftrace event filters, userspace must
recursively traverse the events/ directory and read each individual
filter file. This is inefficient for monitoring tools and debugging.

Introduce "show_event_filters" at the trace root directory. This file
displays all events that currently have a filter applied, alongside the
actual filter string, in a consolidated system:event [tab] filter
format.

The implementation reuses the existing trace_event_file iterators to
ensure atomic traversal of the event list and utilises guard(rcu)() for
automatic, scope-based protection when accessing volatile filter
strings.

Link: https://patch.msgid.link/20260105142939.2655342-2-atomlin@atomlin.com
Signed-off-by: Aaron Tomlin <atomlin@atomlin.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Documentation/trace/ftrace.rst

@@ -684,6 +684,14 @@ of ftrace. Here is a list of some of the key files:
 
 	See events.rst for more information.
 
+  show_event_filters:
+
+	A list of events that have filters. This shows the
+	system/event pair along with the filter that is attached to
+	the event.
+
+	See events.rst for more information.
+
   available_events:
 
 	A list of events that can be enabled in tracing.

kernel/trace/trace_events.c

@@ -1662,6 +1662,32 @@ static void t_stop(struct seq_file *m, void *p)
 	mutex_unlock(&event_mutex);
 }
 
+/**
+ * t_show_filters - seq_file callback to display active event filters
+ * @m: The seq_file interface for formatted output
+ * @v: The current trace_event_file being iterated
+ *
+ * Identifies and prints active filters for the current event file in the
+ * iteration. If a filter is applied to the current event and, if so,
+ * prints the system name, event name, and the filter string.
+ */
+static int t_show_filters(struct seq_file *m, void *v)
+{
+	struct trace_event_file *file = v;
+	struct trace_event_call *call = file->event_call;
+	struct event_filter *filter;
+
+	guard(rcu)();
+	filter = rcu_dereference(file->filter);
+	if (!filter || !filter->filter_string)
+		return 0;
+
+	seq_printf(m, "%s:%s\t%s\n", call->class->system,
+		   trace_event_name(call), filter->filter_string);
+
+	return 0;
+}
+
 #ifdef CONFIG_MODULES
 static int s_show(struct seq_file *m, void *v)
 {
@@ -2489,6 +2515,7 @@ ftrace_event_npid_write(struct file *filp, const char __user *ubuf,
 
 static int ftrace_event_avail_open(struct inode *inode, struct file *file);
 static int ftrace_event_set_open(struct inode *inode, struct file *file);
+static int ftrace_event_show_filters_open(struct inode *inode, struct file *file);
 static int ftrace_event_set_pid_open(struct inode *inode, struct file *file);
 static int ftrace_event_set_npid_open(struct inode *inode, struct file *file);
 static int ftrace_event_release(struct inode *inode, struct file *file);
@@ -2507,6 +2534,13 @@ static const struct seq_operations show_set_event_seq_ops = {
 	.stop = s_stop,
 };
 
+static const struct seq_operations show_show_event_filters_seq_ops = {
+	.start = t_start,
+	.next = t_next,
+	.show = t_show_filters,
+	.stop = t_stop,
+};
+
 static const struct seq_operations show_set_pid_seq_ops = {
 	.start = p_start,
 	.next = p_next,
@@ -2536,6 +2570,13 @@ static const struct file_operations ftrace_set_event_fops = {
 	.release = ftrace_event_release,
 };
 
+static const struct file_operations ftrace_show_event_filters_fops = {
+	.open = ftrace_event_show_filters_open,
+	.read = seq_read,
+	.llseek = seq_lseek,
+	.release = seq_release,
+};
+
 static const struct file_operations ftrace_set_event_pid_fops = {
 	.open = ftrace_event_set_pid_open,
 	.read = seq_read,
@@ -2680,6 +2721,20 @@ ftrace_event_set_open(struct inode *inode, struct file *file)
 	return ret;
 }
 
+/**
+ * ftrace_event_show_filters_open - open interface for set_event_filters
+ * @inode: The inode of the file
+ * @file: The file being opened
+ *
+ * Connects the set_event_filters file to the sequence operations
+ * required to iterate over and display active event filters.
+ */
+static int
+ftrace_event_show_filters_open(struct inode *inode, struct file *file)
+{
+	return ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
+}
+
 static int
 ftrace_event_set_pid_open(struct inode *inode, struct file *file)
 {
@@ -4400,6 +4455,9 @@ create_event_toplevel_files(struct dentry *parent, struct trace_array *tr)
 	if (!entry)
 		return -ENOMEM;
 
+	trace_create_file("show_event_filters", TRACE_MODE_READ, parent, tr,
+			  &ftrace_show_event_filters_fops);
+
 	nr_entries = ARRAY_SIZE(events_entries);
 
 	e_events = eventfs_create_events_dir("events", parent, events_entries,