gh-78878: Fix crash when creating an instance of _ctypes.CField

Lib/ctypes/test/test_struct_fields.py

@@ -46,6 +46,12 @@ class Y(X):
         Y._fields_ = []
         self.assertRaises(AttributeError, setattr, X, "_fields_", [])
 
+    def test_5(self):
+        class X(Structure):
+            _fields_ = [("x", c_int)]
+        CField = type(X.x)
+        self.assertRaises(TypeError, CField)
+
     # __set__ and __get__ should raise a TypeError in case their self
     # argument is not a ctype instance.
     def test___set__(self):

Misc/NEWS.d/next/Core and Builtins/2019-07-18-12-31-49.bpo-34697.XemM5T.rst

@@ -0,0 +1 @@
+Remove the ``PyCField_new`` which user cannot call it directly.

Modules/_ctypes/cfield.c

@@ -22,13 +22,6 @@ static void pymem_destructor(PyObject *ptr)
 /*
   PyCField_Type
 */
-static PyObject *
-PyCField_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
-{
-    CFieldObject *obj;
-    obj = (CFieldObject *)type->tp_alloc(type, 0);
-    return (PyObject *)obj;
-}
 
 /*
  * Expects the size, index and offset for the current field in *psize and
@@ -60,7 +53,7 @@ PyCField_FromDesc(PyObject *desc, Py_ssize_t index,
 #define CONT_BITFIELD 2
 #define EXPAND_BITFIELD 3
 
-    self = (CFieldObject *)_PyObject_CallNoArg((PyObject *)&PyCField_Type);
+    self = (CFieldObject *)PyCField_Type.tp_alloc((PyTypeObject *)&PyCField_Type, 0);
     if (self == NULL)
         return NULL;
     dict = PyType_stgdict(desc);
@@ -336,7 +329,7 @@ PyTypeObject PyCField_Type = {
     0,                                          /* tp_dictoffset */
     0,                                          /* tp_init */
     0,                                          /* tp_alloc */
-    PyCField_new,                               /* tp_new */
+    0,                                          /* tp_new */
     0,                                          /* tp_free */
 };
 

Modules/_ctypes/stgdict.c

@@ -248,7 +248,7 @@ MakeFields(PyObject *type, CFieldObject *descr,
             }
             continue;
         }
-        new_descr = (CFieldObject *)_PyObject_CallNoArg((PyObject *)&PyCField_Type);
+        new_descr = (CFieldObject *)PyCField_Type.tp_alloc((PyTypeObject *)&PyCField_Type, 0);
         if (new_descr == NULL) {
             Py_DECREF(fdescr);
             Py_DECREF(fieldlist);