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>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index ee6a733..21db0de 100644 (file)
--- 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,
 };