PyList_GET_ITEM() now checks the index

vstinner · vstinner · commit 255edc37f31d · 2023-06-28T01:52:13.000+02:00
diff --git a/Doc/whatsnew/3.13.rst b/Doc/whatsnew/3.13.rst
@@ -441,6 +441,12 @@ New Features
   ``NULL`` if the referent is no longer live.
   (Contributed by Victor Stinner in :gh:`105927`.)
 
+* In debug mode and on :option:`Python built with assertions <--with-assertions>`,
+  :c:func:`PyList_GET_ITEM` now checks the index with an assertion.
+  If the assertion fails, make sure that the list size is set before calling
+  :c:func:`PyList_GET_ITEM`.
+  (Contributed by Victor Stinner.)
+
 Porting to Python 3.13
 ----------------------
 
diff --git a/Include/cpython/listobject.h b/Include/cpython/listobject.h
@@ -41,6 +41,8 @@ static inline Py_ssize_t PyList_GET_SIZE(PyObject *op) {
 static inline void
 PyList_SET_ITEM(PyObject *op, Py_ssize_t index, PyObject *value) {
     PyListObject *list = _PyList_CAST(op);
+    assert(0 <= index);
+    assert(index < Py_SIZE(list));
     list->ob_item[index] = value;
 }
 #define PyList_SET_ITEM(op, index, value) \
diff --git a/Include/internal/pycore_list.h b/Include/internal/pycore_list.h
@@ -49,8 +49,8 @@ _PyList_AppendTakeRef(PyListObject *self, PyObject *newitem)
     Py_ssize_t allocated = self->allocated;
     assert((size_t)len + 1 < PY_SSIZE_T_MAX);
     if (allocated > len) {
-        PyList_SET_ITEM(self, len, newitem);
         Py_SET_SIZE(self, len + 1);
+        PyList_SET_ITEM(self, len, newitem);
         return 0;
     }
     return _PyList_AppendTakeRefListResize(self, newitem);
diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -953,8 +953,9 @@ list_extend(PyListObject *self, PyObject *iterable)
         }
         if (Py_SIZE(self) < self->allocated) {
             /* steals ref */
-            PyList_SET_ITEM(self, Py_SIZE(self), item);
-            Py_SET_SIZE(self, Py_SIZE(self) + 1);
+            Py_ssize_t len = Py_SIZE(self);
+            Py_SET_SIZE(self, len + 1);
+            PyList_SET_ITEM(self, len, item);
         }
         else {
             if (_PyList_AppendTakeRef(self, item) < 0)

Original file line number	Diff line number	Diff line change
`@@ -41,6 +41,8 @@ static inline Py_ssize_t PyList_GET_SIZE(PyObject *op) {`
`41`	`41`	`static inline void`
`42`	`42`	`PyList_SET_ITEM(PyObject op, Py_ssize_t index, PyObject value) {`
`43`	`43`	`PyListObject *list = _PyList_CAST(op);`
	`44`	`+ assert(0 <= index);`
	`45`	`+ assert(index < Py_SIZE(list));`
`44`	`46`	`list->ob_item[index] = value;`
`45`	`47`	`}`
`46`	`48`	`#define PyList_SET_ITEM(op, index, value) \`
Original file line number	Diff line number	Diff line change
`@@ -953,8 +953,9 @@ list_extend(PyListObject self, PyObject iterable)`
`953`	`953`	`}`
`954`	`954`	`if (Py_SIZE(self) < self->allocated) {`
`955`	`955`	`/* steals ref */`
`956`		`- PyList_SET_ITEM(self, Py_SIZE(self), item);`
`957`		`- Py_SET_SIZE(self, Py_SIZE(self) + 1);`
	`956`	`+ Py_ssize_t len = Py_SIZE(self);`
	`957`	`+ Py_SET_SIZE(self, len + 1);`
	`958`	`+ PyList_SET_ITEM(self, len, item);`
`958`	`959`	`}`
`959`	`960`	`else {`
`960`	`961`	`if (_PyList_AppendTakeRef(self, item) < 0)`