--- a/compat/backport-4.12.c
+++ b/compat/backport-4.12.c
@@ -225,6 +225,7 @@ int bp_extack_genl_register_family(struc
 
 	/* copy this since the family might access it directly */
 	family->attrbuf = copy->family.attrbuf;
+	family->mcgrp_offset = copy->family.mcgrp_offset;
 
 	mutex_lock(&copies_mutex);
 	list_add_tail(&copy->list, &copies_list);
