of: overlay: do not free changeset when of_overlay_apply returns error

New unittests for overlay notifiers reveal a memory leak in
of_overlay_apply() when a notifier returns an error for action
OF_OVERLAY_POST_APPLY.  The pr_err() message is:

   OF: ERROR: memory leak, expected refcount 1 instead of 3,
   of_node_get()/of_node_put() unbalanced - destroy cset entry: attach
   overlay node /testcase-data/overlay-node/test-bus/test-unittest17

Change the error path to no longer call free_overlay_changeset(),
and document that the caller of of_overlay_fdt_apply() may choose
to remove the overlay.

Update the unittest that triggered the error to expect the changed
return values and to call of_overlay_remove().
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220502181742.1402826-4-frowand.list@gmail.com

drivers/of/overlay.c

@@ -952,6 +952,25 @@ static int of_overlay_apply(struct overlay_changeset *ovcs)
 	return ret;
 }
 
+/*
+ * of_overlay_fdt_apply() - Create and apply an overlay changeset
+ * @overlay_fdt:	pointer to overlay FDT
+ * @overlay_fdt_size:	number of bytes in @overlay_fdt
+ * @ret_ovcs_id:	pointer for returning created changeset id
+ *
+ * Creates and applies an overlay changeset.
+ *
+ * See of_overlay_apply() for important behavior information.
+ *
+ * Return: 0 on success, or a negative error number.  *@ret_ovcs_id is set to
+ * the value of overlay changeset id, which can be passed to of_overlay_remove()
+ * to remove the overlay.
+ *
+ * On error return, the changeset may be partially applied.  This is especially
+ * likely if an OF_OVERLAY_POST_APPLY notifier returns an error.  In this case
+ * the caller should call of_overlay_remove() with the value in *@ret_ovcs_id.
+ */
+
 int of_overlay_fdt_apply(const void *overlay_fdt, u32 overlay_fdt_size,
 			 int *ret_ovcs_id)
 {
@@ -1019,15 +1038,19 @@ int of_overlay_fdt_apply(const void *overlay_fdt, u32 overlay_fdt_size,
 	ovcs->overlay_mem = overlay_mem;
 
 	ret = of_overlay_apply(ovcs);
-	if (ret < 0)
-		goto err_free_ovcs;
+	/*
+	 * If of_overlay_apply() error, calling free_overlay_changeset() may
+	 * result in a memory leak if the apply partly succeeded, so do NOT
+	 * goto err_free_ovcs.  Instead, the caller of of_overlay_fdt_apply()
+	 * can call of_overlay_remove();
+	 */
 
 	mutex_unlock(&of_mutex);
 	of_overlay_mutex_unlock();
 
 	*ret_ovcs_id = ovcs->id;
 
-	return 0;
+	return ret;
 
 err_free_ovcs:
 	free_overlay_changeset(ovcs);

drivers/of/unittest.c

@@ -2845,7 +2845,7 @@ static void __init of_unittest_overlay_notify(void)
 
 	EXPECT_END(KERN_INFO, "OF: overlay: overlay changeset pre-apply notifier error -16, target: /testcase-data/overlay-node/test-bus");
 
-	unittest(!ovcs_id, "ovcs_id created for overlay_16\n");
+	unittest(ovcs_id, "ovcs_id not created for overlay_16\n");
 
 	/* ---  overlay 17  --- */
 
@@ -2856,7 +2856,13 @@ static void __init of_unittest_overlay_notify(void)
 
 	EXPECT_END(KERN_INFO, "OF: overlay: overlay changeset post-apply notifier error -17, target: /testcase-data/overlay-node/test-bus");
 
-	unittest(!ovcs_id, "ovcs_id created for overlay_17\n");
+	unittest(ovcs_id, "ovcs_id not created for overlay_17\n");
+
+	if (ovcs_id) {
+		ret = of_overlay_remove(&ovcs_id);
+		unittest(!ret,
+			"overlay_17 of_overlay_remove(), ret = %d\n", ret);
+	}
 
 	/* ---  overlay 18  --- */