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gh-161185: Fix strict aliasing issue in Py_SETREF() macros family #161186

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gh-161185: Fix strict aliasing issue in Py_SETREF() macros family #161186

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21 changes: 12 additions & 9 deletions Include/refcount.h
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Original file line number Diff line number Diff line change
Expand Up @@ -489,7 +489,7 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
_Py_TYPEOF(op) _tmp_old_op = (*_tmp_op_ptr); \
if (_tmp_old_op != _Py_NULL) { \
*_tmp_op_ptr = _Py_NULL; \
Py_DECREF(_tmp_old_op); \
Py_DECREF(_PyObject_CAST(_tmp_old_op)); \
} \
} while (0)
#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >= 201103L)
Expand All @@ -499,14 +499,15 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
auto _tmp_old_op = (*_tmp_op_ptr); \
if (_tmp_old_op != _Py_NULL) { \
*_tmp_op_ptr = _Py_NULL; \
Py_DECREF(_tmp_old_op); \
Py_DECREF(_PyObject_CAST(_tmp_old_op)); \
} \
} while (0)
#else
#define Py_CLEAR(op) \
do { \
PyObject **_tmp_op_ptr = _Py_CAST(PyObject**, &(op)); \
PyObject *_tmp_old_op = (*_tmp_op_ptr); \
PyObject *_tmp_old_op; \
memcpy(&_tmp_old_op, _tmp_op_ptr, sizeof(PyObject*)); \
if (_tmp_old_op != _Py_NULL) { \
PyObject *_null_ptr = _Py_NULL; \
memcpy(_tmp_op_ptr, &_null_ptr, sizeof(PyObject*)); \
Expand Down Expand Up @@ -545,21 +546,22 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
_Py_TYPEOF(&(dst)) _tmp_dst_ptr = &(dst); \
_Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
*_tmp_dst_ptr = (src); \
Py_DECREF(_tmp_old_dst); \
Py_DECREF(_PyObject_CAST(_tmp_old_dst)); \
} while (0)
#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >= 201103L)
#define Py_SETREF(dst, src) \
do { \
auto _tmp_dst_ptr = &(dst); \
auto _tmp_old_dst = (*_tmp_dst_ptr); \
*_tmp_dst_ptr = (src); \
Py_DECREF(_tmp_old_dst); \
Py_DECREF(_PyObject_CAST(_tmp_old_dst)); \
} while (0)
#else
#define Py_SETREF(dst, src) \
do { \
PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
PyObject *_tmp_old_dst; \
memcpy(&_tmp_old_dst, _tmp_dst_ptr, sizeof(PyObject*)); \
PyObject *_tmp_src = _PyObject_CAST(src); \
memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
Py_DECREF(_tmp_old_dst); \
Expand All @@ -575,21 +577,22 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
_Py_TYPEOF(&(dst)) _tmp_dst_ptr = &(dst); \
_Py_TYPEOF(dst) _tmp_old_dst = (*_tmp_dst_ptr); \
*_tmp_dst_ptr = (src); \
Py_XDECREF(_tmp_old_dst); \
Py_XDECREF(_PyObject_CAST(_tmp_old_dst)); \
} while (0)
#elif defined(__cplusplus) && (__cplusplus >= 201103L || _MSVC_LANG >= 201103L)
#define Py_XSETREF(dst, src) \
do { \
auto _tmp_dst_ptr = &(dst); \
auto _tmp_old_dst = (*_tmp_dst_ptr); \
*_tmp_dst_ptr = (src); \
Py_XDECREF(_tmp_old_dst); \
Py_XDECREF(_PyObject_CAST(_tmp_old_dst)); \
} while (0)
#else
#define Py_XSETREF(dst, src) \
do { \
PyObject **_tmp_dst_ptr = _Py_CAST(PyObject**, &(dst)); \
PyObject *_tmp_old_dst = (*_tmp_dst_ptr); \
PyObject *_tmp_old_dst; \
memcpy(&_tmp_old_dst, _tmp_dst_ptr, sizeof(PyObject*)); \
PyObject *_tmp_src = _PyObject_CAST(src); \
memcpy(_tmp_dst_ptr, &_tmp_src, sizeof(PyObject*)); \
Py_XDECREF(_tmp_old_dst); \
Expand Down
88 changes: 88 additions & 0 deletions Lib/test/test_cext/extension.c
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Original file line number Diff line number Diff line change
Expand Up @@ -140,6 +140,91 @@ test_macros(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
}


// sizeof(PyObject) in PySlot below is incompatible with opaque PyObject
#ifndef Py_TARGET_ABI3T
Py_NO_INLINE static void
pyclear_strict_aliasing(PyTypeObject **a, PyTypeObject **b)
{
Py_CLEAR(*a);
*b = NULL;
}


Py_NO_INLINE static void
setref_strict_aliasing(PyTypeObject **a, PyTypeObject **b,
PyTypeObject *first, PyTypeObject *second)
{
*a = first;
Py_SETREF(*b, second);
}


Py_NO_INLINE static void
xsetref_strict_aliasing(PyTypeObject **a, PyTypeObject **b,
PyTypeObject *first, PyTypeObject *second)
{
*a = first;
Py_XSETREF(*b, second);
}


// gh-161185: Test that Py_SETREF(), Py_XSETREF() and Py_CLEAR() macros
// have no strict aliasing issue
static PyObject *
test_setref_strict_aliasing(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
{
PySlot slots[] = {
PySlot_PTR_STATIC(Py_tp_name, STR(MODULE_NAME) ".setref_type"),
PySlot_PTR_STATIC(Py_tp_basicsize, sizeof(PyObject)),
PySlot_PTR_STATIC(Py_tp_flags, Py_TPFLAGS_DEFAULT),
PySlot_END
};
PyObject *first, *second;
Py_ssize_t first_refcnt, second_refcnt;
PyTypeObject *slot = NULL;

first = PyType_FromSlots(slots);
if (first == NULL) {
return NULL;
}
second = PyType_FromSlots(slots);
if (second == NULL) {
Py_DECREF(first);
return NULL;
}
first_refcnt = Py_REFCNT(first);
second_refcnt = Py_REFCNT(first);

// Both pointer arguments alias. The assignment makes first the old value
// that Py_SETREF() must release before storing second.
setref_strict_aliasing(&slot, &slot,
(PyTypeObject*)Py_NewRef(first),
(PyTypeObject*)Py_NewRef(second));
assert((PyObject*)slot == second);
assert(Py_REFCNT(first) == first_refcnt);
assert(Py_REFCNT(second) == (second_refcnt + 1));

pyclear_strict_aliasing(&slot, &slot);
assert(slot == NULL);
assert(Py_REFCNT(first) == first_refcnt);
assert(Py_REFCNT(second) == second_refcnt);

xsetref_strict_aliasing(&slot, &slot,
(PyTypeObject*)Py_NewRef(first),
(PyTypeObject*)Py_NewRef(second));
assert((PyObject*)slot == second);
assert(Py_REFCNT(first) == first_refcnt);
assert(Py_REFCNT(second) == (second_refcnt + 1));

Py_DECREF(first);
Py_DECREF(second);
Py_DECREF((PyObject*)slot);

Py_RETURN_NONE;
}
#endif // !Py_TARGET_ABI3T


static PyObject *
test_datetime(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
{
Expand Down Expand Up @@ -366,6 +451,9 @@ test_virtual_object(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
static PyMethodDef module_methods[] = {
{"add", test_add, METH_VARARGS, test_add_doc},
{"test_macros", test_macros, METH_NOARGS, _Py_NULL},
#ifndef Py_TARGET_ABI3T
{"test_setref_strict_aliasing", test_setref_strict_aliasing, METH_NOARGS, _Py_NULL},
#endif
{"test_datetime", test_datetime, METH_NOARGS, _Py_NULL},
{"test_unicode", test_unicode, METH_NOARGS, _Py_NULL},
#ifdef __cplusplus
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/C_API/2026-11-02-15-17-50.gh-issue-161185.4bpyDC.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix a strict aliasing issue in the :c:func:`!memcpy` implementation of
:c:macro:`Py_SETREF`, :c:macro:`Py_XSETREF` and :c:macro:`Py_CLEAR` macros.
Patch by Nathan Goldbaum and Victor Stinner.
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