PyList_GET_ITEM() now checks the index

python · Jun 27, 2023 · 255edc3 · 255edc3
1 parent 6b5166f
commit 255edc3
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Showing 4 changed files with 12 additions and 3 deletions.
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)