tracing: only truncate ftrace files when O_TRUNC is set

The current code will truncate the ftrace files contents if O_APPEND
is not set and the file is opened in write mode. This is incorrect.
It should only truncate the file if O_TRUNC is set. Otherwise
if one of these files is opened by a C program with fopen "r+",
it will incorrectly truncate the file.

Reported-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/ftrace.c

@@ -1662,7 +1662,7 @@ ftrace_regex_open(struct inode *inode, struct file *file, int enable)
 
 	mutex_lock(&ftrace_regex_lock);
 	if ((file->f_mode & FMODE_WRITE) &&
-	    !(file->f_flags & O_APPEND))
+	    (file->f_flags & O_TRUNC))
 		ftrace_filter_reset(enable);
 
 	if (file->f_mode & FMODE_READ) {
@@ -2577,7 +2577,7 @@ ftrace_graph_open(struct inode *inode, struct file *file)
 
 	mutex_lock(&graph_lock);
 	if ((file->f_mode & FMODE_WRITE) &&
-	    !(file->f_flags & O_APPEND)) {
+	    (file->f_flags & O_TRUNC)) {
 		ftrace_graph_count = 0;
 		memset(ftrace_graph_funcs, 0, sizeof(ftrace_graph_funcs));
 	}

kernel/trace/trace.c

@@ -2031,7 +2031,7 @@ static int tracing_open(struct inode *inode, struct file *file)
 
 	/* If this file was open for write, then erase contents */
 	if ((file->f_mode & FMODE_WRITE) &&
-	    !(file->f_flags & O_APPEND)) {
+	    (file->f_flags & O_TRUNC)) {
 		long cpu = (long) inode->i_private;
 
 		if (cpu == TRACE_PIPE_ALL_CPU)

kernel/trace/trace_events.c

@@ -376,7 +376,7 @@ ftrace_event_seq_open(struct inode *inode, struct file *file)
 	const struct seq_operations *seq_ops;
 
 	if ((file->f_mode & FMODE_WRITE) &&
-	    !(file->f_flags & O_APPEND))
+	    (file->f_flags & O_TRUNC))
 		ftrace_clear_events();
 
 	seq_ops = inode->i_private;