+/*
+ * Updates @child to change its reference to point to @new_bs, including
+ * checking and applying the necessary permisson updates both to the old node
+ * and to @new_bs.
+ *
+ * NULL is passed as @new_bs for removing the reference before freeing @child.
+ *
+ * If @new_bs is not NULL, bdrv_check_perm() must be called beforehand, as this
+ * function uses bdrv_set_perm() to update the permissions according to the new
+ * reference that @new_bs gets.
+ */
+static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
+{
+ BlockDriverState *old_bs = child->bs;
+ uint64_t perm, shared_perm;
+
+ if (old_bs) {
+ /* Update permissions for old node. This is guaranteed to succeed
+ * because we're just taking a parent away, so we're loosening
+ * restrictions. */
+ bdrv_get_cumulative_perm(old_bs, &perm, &shared_perm);
+ bdrv_check_perm(old_bs, perm, shared_perm, NULL, &error_abort);
+ bdrv_set_perm(old_bs, perm, shared_perm);
+ }
+
+ bdrv_replace_child_noperm(child, new_bs);
+
+ if (new_bs) {
+ bdrv_get_cumulative_perm(new_bs, &perm, &shared_perm);
+ bdrv_set_perm(new_bs, perm, shared_perm);
+ }
+}
+