Code: ftrace_regex_lseek(struct file *file, loff_t offset, int whence)
 {
 	loff_t ret;
 
	if (file->f_mode & FMODE_READ)
		ret = seq_lseek(file, offset, whence);
	else
		file->f_pos = ret = 1;

	return ret;
}

Commit Message: tracing: Fix possible NULL pointer dereferences

Currently set_ftrace_pid and set_graph_function files use seq_lseek
for their fops.  However seq_open() is called only for FMODE_READ in
the fops->open() so that if an user tries to seek one of those file
when she open it for writing, it sees NULL seq_file and then panic.

It can be easily reproduced with following command:

  $ cd /sys/kernel/debug/tracing
  $ echo 1234 | sudo tee -a set_ftrace_pid

In this example, GNU coreutils' tee opens the file with fopen(, "a")
and then the fopen() internally calls lseek().

Link: http://lkml.kernel.org/r/1365663302-2170-1-git-send-email-namhyung@kernel.org

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: stable@vger.kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
CWE ID: 
ftrace_regex_lseek(struct file *file, loff_t offset, int whence)
ftrace_filter_lseek(struct file *file, loff_t offset, int whence)
 {
 	loff_t ret;
 
	if (file->f_mode & FMODE_READ)
		ret = seq_lseek(file, offset, whence);
	else
		file->f_pos = ret = 1;

	return ret;
}
