misc: pass miscdevice through file's private_data

Make the miscdevice accessible through the file's private_data.

Previously, this was done only when an open() file operation had been
registered. If no custom open() file operation was defined,
private_data was set to NULL.

This subtle quirk was confusing, to the point where kernel code
registered *empty* file open operations to have private_data point to
the misc device structure and avoid duplicating that logic.

And it could easily lead to bugs, where the addition or removal of a
custom open() file operation surprisingly changes the initial value of
a file's private_data structure.

To resolve this, we now place the miscdevice in the file's private_data
member unconditionally when open() is called.
Signed-off-by: Tom Van Braeckel <tomvanbraeckel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/misc.c

@@ -140,12 +140,17 @@ static int misc_open(struct inode * inode, struct file * file)
 			goto fail;
 	}
 
+	/*
+	 * Place the miscdevice in the file's
+	 * private_data so it can be used by the
+	 * file operations, including f_op->open below
+	 */
+	file->private_data = c;
+
 	err = 0;
 	replace_fops(file, new_fops);
-	if (file->f_op->open) {
-		file->private_data = c;
+	if (file->f_op->open)
 		err = file->f_op->open(inode,file);
-	}
 fail:
 	mutex_unlock(&misc_mtx);
 	return err;