gh-143005: disallow incompatible __class__ reassignment for ctypes arrays

Author: VanshAgarwal24036

Lib/test/test_ctypes/test_arrays.py

@@ -4,7 +4,7 @@
 from ctypes import (Structure, Array, ARRAY, sizeof, addressof,
                     create_string_buffer, create_unicode_buffer,
                     c_char, c_wchar, c_byte, c_ubyte, c_short, c_ushort, c_int, c_uint,
-                    c_long, c_ulonglong, c_float, c_double, c_longdouble)
+                    c_long, c_ulonglong, c_float, c_double, c_longdouble, POINTER)
 from test.support import bigmemtest, _2G, threading_helper, Py_GIL_DISABLED
 from ._support import (_CData, PyCArrayType, Py_TPFLAGS_DISALLOW_INSTANTIATION,
                        Py_TPFLAGS_IMMUTABLETYPE)
@@ -111,6 +111,46 @@ def test_ctypes_array_class_assignment_incompatible(self):
         with self.assertRaises(TypeError):
             x.__class__ = B
 
+    def test_ctypes_array_class_assignment_abstract_target(self):
+        class AbstractArray(Array):
+            pass
+        A = c_int * 3
+        a = A()
+
+        with self.assertRaises(TypeError):
+            a.__class__ = AbstractArray
+
+    def test_ctypes_array_class_assignment_non_array_instance(self):
+        p = POINTER(c_int)()
+        A = c_int * 3
+
+        with self.assertRaises(TypeError):
+            p.__class__ = A
+
+    def test_ctypes_array_class_assignment_zero_length(self):
+        A = c_long * 0
+        B = c_long * 1
+        a = A()
+
+        with self.assertRaises(TypeError):
+            a.__class__ = B
+
+    def test_ctypes_array_class_assignment_incompatible_element_type(self):
+        A = c_int * 3
+        B = c_double * 3
+        a = A()
+
+        with self.assertRaises(TypeError):
+            a.__class__ = B
+
+    def test_ctypes_array_class_assignment_signed_unsigned(self):
+        A = c_long * 3
+        B = c_ulonglong * 3
+        a = A()
+
+        with self.assertRaises(TypeError):
+            a.__class__ = B
+
     def test_step_overflow(self):
         a = (c_int * 5)()
         a[3::sys.maxsize] = (1,)

Modules/_ctypes/_ctypes.c

@@ -5018,11 +5018,11 @@ static int
 PyCArray_setattro(PyObject *self, PyObject *key, PyObject *value)
 {
     if (PyUnicode_Check(key) &&
-        PyUnicode_CompareWithASCIIString(key, "__class__") == 0)
+        _PyUnicode_EqualToASCIIString(key, "__class__"))
     {
         ctypes_state *st = get_module_state_by_def(Py_TYPE(Py_TYPE(self)));
-        StgInfo *old_info;
-        StgInfo *new_info;
+        StgInfo *old_info = NULL;
+        StgInfo *new_info = NULL;
 
         if (PyStgInfo_FromObject(st, self, &old_info) < 0) {
             return -1;
@@ -5031,7 +5031,16 @@ PyCArray_setattro(PyObject *self, PyObject *key, PyObject *value)
             return -1;
         }
 
-        /* Only care about array → array */
+        /* If one side has no storage info, disallow */
+        if (old_info == NULL || new_info == NULL) {
+            PyErr_SetString(
+                PyExc_TypeError,
+                "cannot assign incompatible ctypes array type"
+            );
+            return -1;
+        }
+
+        /* Only care about array to array */
         if (old_info->length >= 0 && new_info->length >= 0) {
             if (old_info->length != new_info->length ||
                 old_info->size != new_info->size ||