tracing: Fix panic when lseek() called on "trace" opened for writing

The file_ops struct for the "trace" special file defined llseek as seq_lseek().
However, if the file was opened for writing only, seq_open() was not called,
and the seek would dereference a null pointer, file->private_data.

This patch introduces a new wrapper for seq_lseek() which checks if the file
descriptor is opened for reading first. If not, it does nothing.

Cc: <stable@kernel.org>
Signed-off-by: Slava Pestov <slavapestov@google.com>
LKML-Reference: <1290640396-24179-1-git-send-email-slavapestov@google.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ee6a7339cf0..21db0deb5c7 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2339,11 +2339,19 @@ tracing_write_stub(struct file *filp, const char __user *ubuf,
         return count;
 }
 
+static loff_t tracing_seek(struct file *file, loff_t offset, int origin)
+{
+        if (file->f_mode & FMODE_READ)
+                return seq_lseek(file, offset, origin);
+        else
+                return 0;
+}
+
 static const struct file_operations tracing_fops = {
         .open           = tracing_open,
         .read           = seq_read,
         .write          = tracing_write_stub,
-        .llseek         = seq_lseek,
+        .llseek         = tracing_seek,
         .release        = tracing_release,
 };