gh-96821: Fix undefined behaviour in _testcapimodule.c (GH-96915) (GH-96927)

miss-islington · CAM-Gerlach · matthiasgoergens · web-flow · commit 3bf8e0f48797 · 2022-09-19T19:48:58.000+08:00
* gh-96821: Assert for demonstrating undefined behaviour * Fix UB (cherry picked from commit cbdeda8) Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM> Co-authored-by: Matthias Görgens <matthias.goergens@gmail.com>
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-09-18-08-47-40.gh-issue-96821.Co2iOq.rst b/Misc/NEWS.d/next/Core and Builtins/2022-09-18-08-47-40.gh-issue-96821.Co2iOq.rst
@@ -0,0 +1 @@
+Fix undefined behaviour in ``_testcapimodule.c``.
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
@@ -5576,8 +5576,10 @@ meth_fastcall_keywords(PyObject* self, PyObject* const* args,
     if (pyargs == NULL) {
         return NULL;
     }
+    assert(args != NULL || nargs == 0);
+    PyObject* const* args_offset = args == NULL ? NULL : args + nargs;
     PyObject *pykwargs = PyObject_Vectorcall((PyObject*)&PyDict_Type,
-                                              args + nargs, 0, kwargs);
+                                              args_offset, 0, kwargs);
     return Py_BuildValue("NNN", _null_to_none(self), pyargs, pykwargs);
 }
 

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Fix undefined behaviour in ``_testcapimodule.c``.
Original file line number	Diff line number	Diff line change
`@@ -5576,8 +5576,10 @@ meth_fastcall_keywords(PyObject* self, PyObject* const* args,`
`5576`	`5576`	`if (pyargs == NULL) {`
`5577`	`5577`	`return NULL;`
`5578`	`5578`	`}`
	`5579`	`+ assert(args != NULL \|\| nargs == 0);`
	`5580`	`+ PyObject* const* args_offset = args == NULL ? NULL : args + nargs;`
`5579`	`5581`	`PyObject pykwargs = PyObject_Vectorcall((PyObject)&PyDict_Type,`
`5580`		`- args + nargs, 0, kwargs);`
	`5582`	`+ args_offset, 0, kwargs);`
`5581`	`5583`	`return Py_BuildValue("NNN", _null_to_none(self), pyargs, pykwargs);`
`5582`	`5584`	`}`
`5583`	`5585`