From 94b39ceb736997eb924dc509ea0ef55f4a19c706 Mon Sep 17 00:00:00 2001
From: Martin Panter <vadmium+py@gmail.com>
Date: Sat, 14 Jan 2017 06:30:37 +0000
Subject: [PATCH] Issue #1621: Overflow should not be possible in listextend()

---
 Objects/listobject.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Objects/listobject.c b/Objects/listobject.c
index dcd7b5efe5b..05dddfc6d7c 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -804,6 +804,9 @@ listextend(PyListObject *self, PyObject *b)
             Py_RETURN_NONE;
         }
         m = Py_SIZE(self);
+        /* It should not be possible to allocate a list large enough to cause
+        an overflow on any relevant platform */
+        assert(m < PY_SSIZE_T_MAX - n);
         if (list_resize(self, m + n) < 0) {
             Py_DECREF(b);
             return NULL;