[media] dvbdev: split dvb_unregister_device()

dvb_unregister_device() has a major problem: it combines unregistering
with memory disposal. Sometimes, it is necessary to unregister a
device, but no memory can be freed yet, because a process still has a
(stale) file handle. Therefore, we need to split
dvb_unregister_device(). This will allow sanitizing a few callers.

With my new design, dvb_unregister_device() appears misnamed, but to
reduce patch noise, I'm not renaming it just yet.

Signed-off-by: Max Kellermann <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dvbdev.c

···
 EXPORT_SYMBOL(dvb_register_device);
 
 
-void dvb_unregister_device(struct dvb_device *dvbdev)
+void dvb_remove_device(struct dvb_device *dvbdev)
 {
 	if (!dvbdev)
 		return;
···
 	device_destroy(dvb_class, MKDEV(DVB_MAJOR, dvbdev->minor));
 
 	list_del (&dvbdev->list_head);
+}
+EXPORT_SYMBOL(dvb_remove_device);
+
+
+void dvb_free_device(struct dvb_device *dvbdev)
+{
+	if (!dvbdev)
+		return;
+
 	kfree (dvbdev->fops);
 	kfree (dvbdev);
+}
+EXPORT_SYMBOL(dvb_free_device);
+
+
+void dvb_unregister_device(struct dvb_device *dvbdev)
+{
+	dvb_remove_device(dvbdev);
+	dvb_free_device(dvbdev);
 }
 EXPORT_SYMBOL(dvb_unregister_device);
 

drivers/media/dvb-core/dvbdev.h

···
 			int demux_sink_pads);
 
 /**
+ * dvb_remove_device - Remove a registered DVB device
+ *
+ * This does not free memory.  To do that, call dvb_free_device().
+ *
+ * @dvbdev:	pointer to struct dvb_device
+ */
+void dvb_remove_device(struct dvb_device *dvbdev);
+
+/**
+ * dvb_free_device - Free memory occupied by a DVB device.
+ *
+ * Call dvb_unregister_device() before calling this function.
+ *
+ * @dvbdev:	pointer to struct dvb_device
+ */
+void dvb_free_device(struct dvb_device *dvbdev);
+
+/**
  * dvb_unregister_device - Unregisters a DVB device
+ *
+ * This is a combination of dvb_remove_device() and dvb_free_device().
+ * Using this function is usually a mistake, and is often an indicator
+ * for a use-after-free bug (when a userspace process keeps a file
+ * handle to a detached device).
  *
  * @dvbdev:	pointer to struct dvb_device
  */