tracing: Fix incorrect enabling of trace events by boot cmdline

There is a problem that trace events are not properly enabled with
boot cmdline. The problem is that if we pass "trace_event=kmem:mm_page_alloc"
to the boot cmdline, it enables all kmem trace events, and not just
the page_alloc event.

This is caused by the parsing mechanism. When we parse the cmdline, the buffer
contents is modified due to tokenization. And, if we use this buffer
again, we will get the wrong result.

Unfortunately, this buffer is be accessed three times to set trace events
properly at boot time. So, we need to handle this situation.

There is already code handling ",", but we need another for ":".
This patch adds it.

Link: http://lkml.kernel.org/r/1429159484-22977-1-git-send-email-iamjoonsoo.kim@lge.com

Cc: stable@vger.kernel.org # 3.19+
Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
[ added missing return ret; ]
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -565,6 +565,7 @@ static int __ftrace_set_clr_event(struct trace_array *tr, const char *match,
 static int ftrace_set_clr_event(struct trace_array *tr, char *buf, int set)
 {
 	char *event = NULL, *sub = NULL, *match;
+	int ret;
 
 	/*
 	 * The buf format can be <subsystem>:<event-name>
@@ -590,7 +591,13 @@ static int ftrace_set_clr_event(struct trace_array *tr, char *buf, int set)
 			event = NULL;
 	}
 
-	return __ftrace_set_clr_event(tr, match, sub, event, set);
+	ret = __ftrace_set_clr_event(tr, match, sub, event, set);
+
+	/* Put back the colon to allow this to be called again */
+	if (buf)
+		*(buf - 1) = ':';
+
+	return ret;
 }
 
 /**