gh-62260: Fix ctypes.Structure subclassing with multiple layers (GH-13374)

The length field of StgDictObject for Structure class contains now
the total number of items in ffi_type_pointer.elements (excluding
the trailing null).

The old behavior of using the number of elements in the parent class can
cause the array to be truncated when it is copied, especially when there
are multiple layers of subclassing.

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_ctypes/test_structures.py

@@ -1,5 +1,5 @@
 import _ctypes_test
-import platform
+from platform import architecture as _architecture
 import struct
 import sys
 import unittest
@@ -8,6 +8,7 @@ from ctypes import (CDLL, Array, Structure, Union, POINTER, sizeof, byref, align
                     c_uint8, c_uint16, c_uint32,
                     c_short, c_ushort, c_int, c_uint,
                     c_long, c_ulong, c_longlong, c_ulonglong, c_float, c_double)
+from ctypes.util import find_library
 from struct import calcsize
 from collections import namedtuple
 from test import support
@@ -472,6 +473,66 @@ class StructureTestCase(unittest.TestCase):
         self.assertEqual(s.first, got.first)
         self.assertEqual(s.second, got.second)
 
+    def _test_issue18060(self, Vector):
+        # The call to atan2() should succeed if the
+        # class fields were correctly cloned in the
+        # subclasses. Otherwise, it will segfault.
+        if sys.platform == 'win32':
+            libm = CDLL(find_library('msvcrt.dll'))
+        else:
+            libm = CDLL(find_library('m'))
+
+        libm.atan2.argtypes = [Vector]
+        libm.atan2.restype = c_double
+
+        arg = Vector(y=0.0, x=-1.0)
+        self.assertAlmostEqual(libm.atan2(arg), 3.141592653589793)
+
+    @unittest.skipIf(_architecture() == ('64bit', 'WindowsPE'), "can't test Windows x64 build")
+    @unittest.skipUnless(sys.byteorder == 'little', "can't test on this platform")
+    def test_issue18060_a(self):
+        # This test case calls
+        # PyCStructUnionType_update_stgdict() for each
+        # _fields_ assignment, and PyCStgDict_clone()
+        # for the Mid and Vector class definitions.
+        class Base(Structure):
+            _fields_ = [('y', c_double),
+                        ('x', c_double)]
+        class Mid(Base):
+            pass
+        Mid._fields_ = []
+        class Vector(Mid): pass
+        self._test_issue18060(Vector)
+
+    @unittest.skipIf(_architecture() == ('64bit', 'WindowsPE'), "can't test Windows x64 build")
+    @unittest.skipUnless(sys.byteorder == 'little', "can't test on this platform")
+    def test_issue18060_b(self):
+        # This test case calls
+        # PyCStructUnionType_update_stgdict() for each
+        # _fields_ assignment.
+        class Base(Structure):
+            _fields_ = [('y', c_double),
+                        ('x', c_double)]
+        class Mid(Base):
+            _fields_ = []
+        class Vector(Mid):
+            _fields_ = []
+        self._test_issue18060(Vector)
+
+    @unittest.skipIf(_architecture() == ('64bit', 'WindowsPE'), "can't test Windows x64 build")
+    @unittest.skipUnless(sys.byteorder == 'little', "can't test on this platform")
+    def test_issue18060_c(self):
+        # This test case calls
+        # PyCStructUnionType_update_stgdict() for each
+        # _fields_ assignment.
+        class Base(Structure):
+            _fields_ = [('y', c_double)]
+        class Mid(Base):
+            _fields_ = []
+        class Vector(Mid):
+            _fields_ = [('x', c_double)]
+        self._test_issue18060(Vector)
+
     def test_array_in_struct(self):
         # See bpo-22273
 

Misc/NEWS.d/next/Library/2019-05-17-07-22-33.bpo-18060.5mqTQM.rst

@@ -0,0 +1,2 @@
+Fixed a class inheritance issue that can cause segfaults when deriving two or more levels of subclasses from a base class of Structure or Union.
+

Modules/_ctypes/_ctypes.c

@@ -4354,10 +4354,10 @@ _init_pos_args(PyObject *self, PyTypeObject *type,
         return index;
     }
 
-    for (i = 0;
-         i < dict->length && (i+index) < PyTuple_GET_SIZE(args);
+    for (i = index;
+         i < dict->length && i < PyTuple_GET_SIZE(args);
          ++i) {
-        PyObject *pair = PySequence_GetItem(fields, i);
+        PyObject *pair = PySequence_GetItem(fields, i - index);
         PyObject *name, *val;
         int res;
         if (!pair)
@@ -4367,7 +4367,7 @@ _init_pos_args(PyObject *self, PyTypeObject *type,
             Py_DECREF(pair);
             return -1;
         }
-        val = PyTuple_GET_ITEM(args, i + index);
+        val = PyTuple_GET_ITEM(args, i);
         if (kwds) {
             res = PyDict_Contains(kwds, name);
             if (res != 0) {
@@ -4388,7 +4388,7 @@ _init_pos_args(PyObject *self, PyTypeObject *type,
         if (res == -1)
             return -1;
     }
-    return index + dict->length;
+    return dict->length;
 }
 
 static int

Modules/_ctypes/stgdict.c

@@ -695,7 +695,7 @@ PyCStructUnionType_update_stgdict(PyObject *type, PyObject *fields, int isStruct
 
     stgdict->size = aligned_size;
     stgdict->align = total_align;
-    stgdict->length = len;      /* ADD ffi_ofs? */
+    stgdict->length = ffi_ofs + len;
 
 /*
  * The value of MAX_STRUCT_SIZE depends on the platform Python is running on.