tracing: Fix memory leak when reading set_event file

kmemleak reports the following memory leak after reading set_event file:

  # cat /sys/kernel/tracing/set_event

  # cat /sys/kernel/debug/kmemleak
  unreferenced object 0xff110001234449e0 (size 16):
  comm "cat", pid 13645, jiffies 4294981880
  hex dump (first 16 bytes):
    01 00 00 00 00 00 00 00 a8 71 e7 84 ff ff ff ff  .........q......
  backtrace (crc c43abbc):
    __kmalloc_cache_noprof+0x3ca/0x4b0
    s_start+0x72/0x2d0
    seq_read_iter+0x265/0x1080
    seq_read+0x2c9/0x420
    vfs_read+0x166/0xc30
    ksys_read+0xf4/0x1d0
    do_syscall_64+0x79/0x150
    entry_SYSCALL_64_after_hwframe+0x76/0x7e

The issue can be reproduced regardless of whether set_event is empty or
not. Here is an example about the valid content of set_event.

  # cat /sys/kernel/tracing/set_event
  sched:sched_process_fork
  sched:sched_switch
  sched:sched_wakeup
  *:*:mod:trace_events_sample

The root cause is that s_next() returns NULL when nothing is found.
This results in s_stop() attempting to free a NULL pointer because its
parameter is NULL.

Fix the issue by freeing the memory appropriately when s_next() fails
to find anything.

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20250220031528.7373-1-ahuang12@lenovo.com
Fixes: b355247 ("tracing: Cache ":mod:" events for modules not loaded yet")
Signed-off-by: Adrian Huang <ahuang12@lenovo.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Adrian Huang

kernel/trace/trace_events.c

@@ -1591,6 +1591,13 @@ s_next(struct seq_file *m, void *v, loff_t *pos)
 		return iter;
 #endif
 
+	/*
+	 * The iter is allocated in s_start() and passed via the 'v'
+	 * parameter. To stop the iterator, NULL must be returned. But
+	 * the return value is what the 'v' parameter in s_stop() receives
+	 * and frees. Free iter here as it will no longer be used.
+	 */
+	kfree(iter);
 	return NULL;
 }
 
@@ -1667,9 +1674,9 @@ static int s_show(struct seq_file *m, void *v)
 }
 #endif
 
-static void s_stop(struct seq_file *m, void *p)
+static void s_stop(struct seq_file *m, void *v)
 {
-	kfree(p);
+	kfree(v);
 	t_stop(m, NULL);
 }