[PATCH v2 2/6] tracing: Change event_enable/disable_read() to verify i_private != NULL

[Oleg Nesterov <oleg@redhat.com>]

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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/trace/trace_events.c |   31 ++++++++++++++++++++-----------
 1 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 77990c4..39cb049 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -684,19 +684,28 @@ 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");
-
 	return simple_read_from_buffer(ubuf, cnt, ppos, buf, strlen(buf));
 }
 
@@ -704,13 +713,10 @@ 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 +728,11 @@ 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;
 
-- 
1.5.5.1