[Python-checkins] bpo-44707: Fix an undefined behavior of the null pointer arithmetic (GH-27292) (GH-27443)

[ambv]

https://github.com/python/cpython/commit/12fc0d28fcdeca32314d0755d3b30c7d96907440
commit: 12fc0d28fcdeca32314d0755d3b30c7d96907440
branch: 3.9
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: ambv <lukasz at langa.pl>
date: 2021-07-29T14:05:43+02:00
summary:

bpo-44707: Fix an undefined behavior of the null pointer arithmetic (GH-27292) (GH-27443)

(cherry picked from commit e5c8ddb1714fb51ab1defa24352c98e0f01205dc)

Co-authored-by: Serhiy Storchaka <storchaka at gmail.com>

files:
M Objects/listobject.c

diff --git a/Objects/listobject.c b/Objects/listobject.c
index f9eb37064a9c9..6c94ba57cfe8e 100644
--- a/Objects/listobject.c
+++ b/Objects/listobject.c
@@ -165,9 +165,10 @@ PyList_New(Py_ssize_t size)
 static PyObject *
 list_new_prealloc(Py_ssize_t size)
 {
+    assert(size > 0);
     PyListObject *op = (PyListObject *) PyList_New(0);
-    if (size == 0 || op == NULL) {
-        return (PyObject *) op;
+    if (op == NULL) {
+        return NULL;
     }
     assert(op->ob_item == NULL);
     op->ob_item = PyMem_New(PyObject *, size);
@@ -446,6 +447,9 @@ list_slice(PyListObject *a, Py_ssize_t ilow, Py_ssize_t ihigh)
     PyObject **src, **dest;
     Py_ssize_t i, len;
     len = ihigh - ilow;
+    if (len <= 0) {
+        return PyList_New(0);
+    }
     np = (PyListObject *) list_new_prealloc(len);
     if (np == NULL)
         return NULL;
@@ -500,6 +504,9 @@ list_concat(PyListObject *a, PyObject *bb)
     if (Py_SIZE(a) > PY_SSIZE_T_MAX - Py_SIZE(b))
         return PyErr_NoMemory();
     size = Py_SIZE(a) + Py_SIZE(b);
+    if (size == 0) {
+        return PyList_New(0);
+    }
     np = (PyListObject *) list_new_prealloc(size);
     if (np == NULL) {
         return NULL;