tracing: Change f_start() to take event_mutex and verify i_private != NULL

trace_format_open() and trace_format_seq_ops are racy, nothing
protects ftrace_event_call from trace_remove_event_call().

Change f_start() to take event_mutex and verify i_private != NULL,
change f_stop() to drop this lock.

This fixes nothing, but now we can change debugfs_remove("format")
callers to nullify ->i_private and fix the the problem.

Note: the usage of event_mutex is sub-optimal but simple, we can
change this later.

Link: http://lkml.kernel.org/r/20130726172543.GA3622@redhat.com

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_events.c

@@ -840,7 +840,7 @@ enum {
 
 static void *f_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	struct ftrace_event_call *call = m->private;
+	struct ftrace_event_call *call = event_file_data(m->private);
 	struct list_head *common_head = &ftrace_common_fields;
 	struct list_head *head = trace_get_fields(call);
 	struct list_head *node = v;
@@ -872,7 +872,7 @@ static void *f_next(struct seq_file *m, void *v, loff_t *pos)
 
 static int f_show(struct seq_file *m, void *v)
 {
-	struct ftrace_event_call *call = m->private;
+	struct ftrace_event_call *call = event_file_data(m->private);
 	struct ftrace_event_field *field;
 	const char *array_descriptor;
 
@@ -925,6 +925,11 @@ static void *f_start(struct seq_file *m, loff_t *pos)
 	void *p = (void *)FORMAT_HEADER;
 	loff_t l = 0;
 
+	/* ->stop() is called even if ->start() fails */
+	mutex_lock(&event_mutex);
+	if (!event_file_data(m->private))
+		return ERR_PTR(-ENODEV);
+
 	while (l < *pos && p)
 		p = f_next(m, p, &l);
 
@@ -933,6 +938,7 @@ static void *f_start(struct seq_file *m, loff_t *pos)
 
 static void f_stop(struct seq_file *m, void *p)
 {
+	mutex_unlock(&event_mutex);
 }
 
 static const struct seq_operations trace_format_seq_ops = {
@@ -944,7 +950,6 @@ static const struct seq_operations trace_format_seq_ops = {
 
 static int trace_format_open(struct inode *inode, struct file *file)
 {
-	struct ftrace_event_call *call = inode->i_private;
 	struct seq_file *m;
 	int ret;
 
@@ -953,7 +958,7 @@ static int trace_format_open(struct inode *inode, struct file *file)
 		return ret;
 
 	m = file->private_data;
-	m->private = call;
+	m->private = file;
 
 	return 0;
 }