gh-118561: Fix crash involving list.extend in free-threaded build (#1…

…18723) The `list_preallocate_exact` function did not zero initialize array contents. In the free-threaded build, this could expose uninitialized memory to concurrent readers between the call to `list_preallocate_exact` and the filling of the array contents with items.
python · colesbury · May 10, 2024 · May 9, 2024 · May 9, 2024 · May 9, 2024
commit fa80517cbd7e82f701cd3b33f39aa134464b7ebd
diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-05-07-16-57-56.gh-issue-118561.wNMKVd.rst b/Misc/NEWS.d/next/Core and Builtins/2024-05-07-16-57-56.gh-issue-118561.wNMKVd.rst
@@ -0,0 +1,2 @@
+Fix race condition in free-threaded build where :meth:`list.extend` could expose
+uninitialied memory to concurrent readers.
diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -192,14 +192,15 @@ list_preallocate_exact(PyListObject *self, Py_ssize_t size)
         return -1;
     }
     items = array->ob_item;
+    memset(items, 0, size * sizeof(PyObject *));
 #else
     items = PyMem_New(PyObject*, size);
     if (items == NULL) {
         PyErr_NoMemory();
         return -1;
     }
 #endif
-    self->ob_item = items;
+    FT_ATOMIC_STORE_PTR_RELEASE(self->ob_item, items);
     self->allocated = size;
     return 0;
 }
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix race condition in free-threaded build where :meth:`list.extend` could expose
		uninitialied memory to concurrent readers.