tracing: Change event_enable/disable_read() to verify i_private != NULL

tracing_open_generic_file() is racy, ftrace_event_file can be
already freed by rmdir or trace_remove_event_call().

Change event_enable_read() and event_disable_read() to read and
verify "file = i_private" under event_mutex.

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

Link: http://lkml.kernel.org/r/20130726172536.GA3612@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>

1 files changed, 20 insertions, 10 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index c2d13c528c3c..3dfa8419d0dc 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -684,15 +684,25 @@ static ssize_t
 event_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
                   loff_t *ppos)
 {
-        struct ftrace_event_file *file = filp->private_data;
+        struct ftrace_event_file *file;
+        unsigned long flags;
         char buf[4] = "0";
 
-        if (file->flags & FTRACE_EVENT_FL_ENABLED &&
-            !(file->flags & FTRACE_EVENT_FL_SOFT_DISABLED))
+        mutex_lock(&event_mutex);
+        file = event_file_data(filp);
+        if (likely(file))
+                flags = file->flags;
+        mutex_unlock(&event_mutex);
+
+        if (!file)
+                return -ENODEV;
+
+        if (flags & FTRACE_EVENT_FL_ENABLED &&
+            !(flags & FTRACE_EVENT_FL_SOFT_DISABLED))
                 strcpy(buf, "1");
 
-        if (file->flags & FTRACE_EVENT_FL_SOFT_DISABLED ||
-            file->flags & FTRACE_EVENT_FL_SOFT_MODE)
+        if (flags & FTRACE_EVENT_FL_SOFT_DISABLED ||
+            flags & FTRACE_EVENT_FL_SOFT_MODE)
                 strcat(buf, "*");
 
         strcat(buf, "\n");
@@ -704,13 +714,10 @@ static ssize_t
 event_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
                    loff_t *ppos)
 {
-        struct ftrace_event_file *file = filp->private_data;
+        struct ftrace_event_file *file;
         unsigned long val;
         int ret;
 
-        if (!file)
-                return -EINVAL;
-
         ret = kstrtoul_from_user(ubuf, cnt, 10, &val);
         if (ret)
                 return ret;
@@ -722,8 +729,11 @@ event_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
         switch (val) {
         case 0:
         case 1:
+                ret = -ENODEV;
                 mutex_lock(&event_mutex);
-                ret = ftrace_event_enable_disable(file, val);
+                file = event_file_data(filp);
+                if (likely(file))
+                        ret = ftrace_event_enable_disable(file, val);
                 mutex_unlock(&event_mutex);
                 break;