From a074af0a88905c7c6600870c3774118511c179d4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?B=C3=A9n=C3=A9dikt=20Tran?=
 <10796600+picnixz@users.noreply.github.com>
Date: Mon, 28 Apr 2025 11:37:35 +0200
Subject: [PATCH] avoid NULL + 0 arithmetic in `list_extend_*` functions

---
 Objects/listobject.c | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/Objects/listobject.c b/Objects/listobject.c
index 2ac4ce095fcadd..7648c1dfe9f0a8 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -1315,9 +1315,15 @@ list_extend_set(PyListObject *self, PySetObject *other)
 {
     Py_ssize_t m = Py_SIZE(self);
     Py_ssize_t n = PySet_GET_SIZE(other);
-    if (list_resize(self, m + n) < 0) {
+    Py_ssize_t r = m + n;
+    if (r == 0) {
+        return 0;
+    }
+    if (list_resize(self, r) < 0) {
         return -1;
     }
+
+    assert(self->ob_item != NULL);
     /* populate the end of self with iterable's items */
     Py_ssize_t setpos = 0;
     Py_hash_t hash;
@@ -1327,7 +1333,7 @@ list_extend_set(PyListObject *self, PySetObject *other)
         FT_ATOMIC_STORE_PTR_RELEASE(*dest, key);
         dest++;
     }
-    Py_SET_SIZE(self, m + n);
+    Py_SET_SIZE(self, r);
     return 0;
 }
 
@@ -1337,10 +1343,15 @@ list_extend_dict(PyListObject *self, PyDictObject *dict, int which_item)
     // which_item: 0 for keys and 1 for values
     Py_ssize_t m = Py_SIZE(self);
     Py_ssize_t n = PyDict_GET_SIZE(dict);
-    if (list_resize(self, m + n) < 0) {
+    Py_ssize_t r = m + n;
+    if (r == 0) {
+        return 0;
+    }
+    if (list_resize(self, r) < 0) {
         return -1;
     }
 
+    assert(self->ob_item != NULL);
     PyObject **dest = self->ob_item + m;
     Py_ssize_t pos = 0;
     PyObject *keyvalue[2];
@@ -1351,7 +1362,7 @@ list_extend_dict(PyListObject *self, PyDictObject *dict, int which_item)
         dest++;
     }
 
-    Py_SET_SIZE(self, m + n);
+    Py_SET_SIZE(self, r);
     return 0;
 }
 
@@ -1360,10 +1371,15 @@ list_extend_dictitems(PyListObject *self, PyDictObject *dict)
 {
     Py_ssize_t m = Py_SIZE(self);
     Py_ssize_t n = PyDict_GET_SIZE(dict);
-    if (list_resize(self, m + n) < 0) {
+    Py_ssize_t r = m + n;
+    if (r == 0) {
+        return 0;
+    }
+    if (list_resize(self, r) < 0) {
         return -1;
     }
 
+    assert(self->ob_item != NULL);
     PyObject **dest = self->ob_item + m;
     Py_ssize_t pos = 0;
     Py_ssize_t i = 0;
@@ -1379,7 +1395,7 @@ list_extend_dictitems(PyListObject *self, PyDictObject *dict)
         i++;
     }
 
-    Py_SET_SIZE(self, m + n);
+    Py_SET_SIZE(self, r);
     return 0;
 }