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

commit c5a44a1200c6eda2202434f25325e8ad19533fca upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 285589354cb7..09782d623c93 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -838,7 +838,7 @@ 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 ftrace_event_field *field;
         struct list_head *common_head = &ftrace_common_fields;
         struct list_head *head = trace_get_fields(call);
@@ -882,6 +882,11 @@ static void *f_start(struct seq_file *m, loff_t *pos)
         loff_t l = 0;
         void *p;
 
+        /* ->stop() is called even if ->start() fails */
+        mutex_lock(&event_mutex);
+        if (!event_file_data(m->private))
+                return ERR_PTR(-ENODEV);
+
         /* Start by showing the header */
         if (!*pos)
                 return (void *)FORMAT_HEADER;
@@ -896,7 +901,7 @@ static void *f_start(struct seq_file *m, 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;
 
@@ -947,6 +952,7 @@ static int f_show(struct seq_file *m, void *v)
 
 static void f_stop(struct seq_file *m, void *p)
 {
+        mutex_unlock(&event_mutex);
 }
 
 static const struct seq_operations trace_format_seq_ops = {
@@ -958,7 +964,6 @@ 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;
 
@@ -967,7 +972,7 @@ 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;
 }