of: overlay: Stop leaking resources on overlay removal

Only the overlay notifier callbacks have a chance to potentially get
hold of references to those two resources, but they are not supposed to
store them beyond OF_OVERLAY_POST_REMOVE.

Document the overlay notifier API, its constraint regarding pointer
lifetime, and then remove intentional leaks of ovcs->overlay_tree and
ovcs->fdt from free_overlay_changeset.

See also https://lkml.org/lkml/2018/4/23/1063 and following.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/overlay-notes.txt

@@ -98,6 +98,14 @@ Finally, if you need to remove all overlays in one-go, just call
 of_overlay_remove_all() which will remove every single one in the correct
 order.
 
+In addition, there is the option to register notifiers that get called on
+overlay operations. See of_overlay_notifier_register/unregister and
+enum of_overlay_notify_action for details.
+
+Note that a notifier callback is not supposed to store pointers to a device
+tree node or its content beyond OF_OVERLAY_POST_REMOVE corresponding to the
+respective node it received.
+
 Overlay DTS Format
 ------------------
 

drivers/of/overlay.c

@@ -102,12 +102,28 @@ static DEFINE_IDR(ovcs_idr);
 
 static BLOCKING_NOTIFIER_HEAD(overlay_notify_chain);
 
+/**
+ * of_overlay_notifier_register() - Register notifier for overlay operations
+ * @nb:		Notifier block to register
+ *
+ * Register for notification on overlay operations on device tree nodes. The
+ * reported actions definied by @of_reconfig_change. The notifier callback
+ * furthermore receives a pointer to the affected device tree node.
+ *
+ * Note that a notifier callback is not supposed to store pointers to a device
+ * tree node or its content beyond @OF_OVERLAY_POST_REMOVE corresponding to the
+ * respective node it received.
+ */
 int of_overlay_notifier_register(struct notifier_block *nb)
 {
 	return blocking_notifier_chain_register(&overlay_notify_chain, nb);
 }
 EXPORT_SYMBOL_GPL(of_overlay_notifier_register);
 
+/**
+ * of_overlay_notifier_register() - Unregister notifier for overlay operations
+ * @nb:		Notifier block to unregister
+ */
 int of_overlay_notifier_unregister(struct notifier_block *nb)
 {
 	return blocking_notifier_chain_unregister(&overlay_notify_chain, nb);
@@ -671,17 +687,13 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
 		of_node_put(ovcs->fragments[i].overlay);
 	}
 	kfree(ovcs->fragments);
-
 	/*
-	 * TODO
-	 *
-	 * would like to: kfree(ovcs->overlay_tree);
-	 * but can not since drivers may have pointers into this data
-	 *
-	 * would like to: kfree(ovcs->fdt);
-	 * but can not since drivers may have pointers into this data
+	 * There should be no live pointers into ovcs->overlay_tree and
+	 * ovcs->fdt due to the policy that overlay notifiers are not allowed
+	 * to retain pointers into the overlay devicetree.
 	 */
-
+	kfree(ovcs->overlay_tree);
+	kfree(ovcs->fdt);
 	kfree(ovcs);
 }