tracing/filters: NIL-terminate user input filter

Make sure messages from user space are NIL-terminated strings,
otherwise we could dump random memory while reading filter file.

Try this:
 # echo 'parent_comm ==' > events/sched/sched_process_fork/filter
 # cat events/sched/sched_process_fork/filter
 parent_comm == �
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Tom Zanussi <tzanussi@gmail.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <49E04C32.6060508@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_events.c

@@ -503,6 +503,7 @@ event_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
 
 	if (copy_from_user(&buf, ubuf, cnt))
 		return -EFAULT;
+	buf[cnt] = '\0';
 
 	pred = kzalloc(sizeof(*pred), GFP_KERNEL);
 	if (!pred)
@@ -569,6 +570,7 @@ subsystem_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
 
 	if (copy_from_user(&buf, ubuf, cnt))
 		return -EFAULT;
+	buf[cnt] = '\0';
 
 	pred = kzalloc(sizeof(*pred), GFP_KERNEL);
 	if (!pred)