From: Ivan Maidanski <ivmai@mail.ru>
Date: Thu, 14 Dec 2017 21:54:04 +0000 (+0300)
Subject: Eliminate UBSan warning of overflow during descr subtraction in mark_from
X-Git-Tag: v7.6.2~12
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=f1221a4699f8a8031d4ebb6298225bc944495372;p=gc

Eliminate UBSan warning of overflow during descr subtraction in mark_from

There is no unsigned overflow because descr has a negative value
actually when it is subtracted from type_descr.

* mark.c (GC_mark_from): Cast descr to signed_word
in (type_descr-(descr+...)).
---

diff --git a/mark.c b/mark.c
index 6556b998..ede92c61 100644
--- a/mark.c
+++ b/mark.c
@@ -793,8 +793,8 @@ GC_INNER mse * GC_mark_from(mse *mark_stack_top, mse *mark_stack,
                 continue;
             }
             descr = *(word *)(type_descr
-                              - (descr + (GC_INDIR_PER_OBJ_BIAS
-                                          - GC_DS_PER_OBJECT)));
+                              - ((signed_word)descr + (GC_INDIR_PER_OBJ_BIAS
+                                                       - GC_DS_PER_OBJECT)));
           }
           if (0 == descr) {
               /* Can happen either because we generated a 0 descriptor  */