gh-132097: use a macro for semantically casting function pointers (#132406)

Author: picnixz

Diff for: Include/internal/pycore_emscripten_trampoline.h

@@ -37,17 +37,18 @@ _PyEM_TrampolineCall(PyCFunctionWithKeywords func,
                      PyObject* kw);
 
 #define _PyCFunction_TrampolineCall(meth, self, args) \
-    _PyEM_TrampolineCall( \
-        (*(PyCFunctionWithKeywords)(void(*)(void))(meth)), (self), (args), NULL)
+    _PyEM_TrampolineCall(*_PyCFunctionWithKeywords_CAST(meth), (self), (args), NULL)
 
 #define _PyCFunctionWithKeywords_TrampolineCall(meth, self, args, kw) \
     _PyEM_TrampolineCall((meth), (self), (args), (kw))
 
-#define descr_set_trampoline_call(set, obj, value, closure) \
-    ((int)_PyEM_TrampolineCall((PyCFunctionWithKeywords)(set), (obj), (value), (PyObject*)(closure)))
+#define descr_set_trampoline_call(set, obj, value, closure)                 \
+    ((int)_PyEM_TrampolineCall(_PyCFunctionWithKeywords_CAST(set), (obj),   \
+                               (value), (PyObject*)(closure)))
 
-#define descr_get_trampoline_call(get, obj, closure) \
-    _PyEM_TrampolineCall((PyCFunctionWithKeywords)(get), (obj), (PyObject*)(closure), NULL)
+#define descr_get_trampoline_call(get, obj, closure)                \
+    _PyEM_TrampolineCall(_PyCFunctionWithKeywords_CAST(get), (obj), \
+                         (PyObject*)(closure), NULL)
 
 
 #else // defined(__EMSCRIPTEN__) && defined(PY_CALL_TRAMPOLINE)

Diff for: Include/methodobject.h

@@ -33,7 +33,7 @@ typedef PyObject *(*PyCMethod)(PyObject *, PyTypeObject *, PyObject *const *,
 typedef PyCFunctionFast _PyCFunctionFast;
 typedef PyCFunctionFastWithKeywords _PyCFunctionFastWithKeywords;
 
-// Cast an function to the PyCFunction type to use it with PyMethodDef.
+// Cast a function to the PyCFunction type to use it with PyMethodDef.
 //
 // This macro can be used to prevent compiler warnings if the first parameter
 // uses a different pointer type than PyObject* (ex: METH_VARARGS and METH_O
@@ -49,8 +49,17 @@ typedef PyCFunctionFastWithKeywords _PyCFunctionFastWithKeywords;
 // used to prevent a compiler warning. If the function has a single parameter,
 // it triggers an undefined behavior when Python calls it with 2 parameters
 // (bpo-33012).
-#define _PyCFunction_CAST(func) \
-    _Py_CAST(PyCFunction, _Py_CAST(void(*)(void), (func)))
+#define _PyCFunction_CAST(func)                         \
+    _Py_FUNC_CAST(PyCFunction, func)
+// The macros below are given for semantic convenience, allowing users
+// to see whether a cast to suppress an undefined behavior is necessary.
+// Note: At runtime, the original function signature must be respected.
+#define _PyCFunctionFast_CAST(func)                     \
+    _Py_FUNC_CAST(PyCFunctionFast, func)
+#define _PyCFunctionWithKeywords_CAST(func)             \
+    _Py_FUNC_CAST(PyCFunctionWithKeywords, func)
+#define _PyCFunctionFastWithKeywords_CAST(func)         \
+    _Py_FUNC_CAST(PyCFunctionFastWithKeywords, func)
 
 PyAPI_FUNC(PyCFunction) PyCFunction_GetFunction(PyObject *);
 PyAPI_FUNC(PyObject *) PyCFunction_GetSelf(PyObject *);

Diff for: Include/pyport.h

@@ -36,6 +36,16 @@
 // Macro to use the more powerful/dangerous C-style cast even in C++.
 #define _Py_CAST(type, expr) ((type)(expr))
 
+// Cast a function to another function type T.
+//
+// The macro first casts the function to the "void func(void)" type
+// to prevent compiler warnings.
+//
+// Note that using this cast only prevents the compiler from emitting
+// warnings, but does not prevent an undefined behavior at runtime if
+// the original function signature is not respected.
+#define _Py_FUNC_CAST(T, func) _Py_CAST(T, _Py_CAST(void(*)(void), (func)))
+
 // Static inline functions should use _Py_NULL rather than using directly NULL
 // to prevent C++ compiler warnings. On C23 and newer and on C++11 and newer,
 // _Py_NULL is defined as nullptr.

Diff for: Objects/descrobject.c

@@ -519,7 +519,7 @@ wrapperdescr_raw_call(PyWrapperDescrObject *descr, PyObject *self,
     wrapperfunc wrapper = descr->d_base->wrapper;
 
     if (descr->d_base->flags & PyWrapperFlag_KEYWORDS) {
-        wrapperfunc_kwds wk = (wrapperfunc_kwds)(void(*)(void))wrapper;
+        wrapperfunc_kwds wk = _Py_FUNC_CAST(wrapperfunc_kwds, wrapper);
         return (*wk)(self, args, descr->d_wrapped, kwds);
     }
 

Diff for: Objects/methodobject.c

@@ -567,7 +567,7 @@ cfunction_call(PyObject *func, PyObject *args, PyObject *kwargs)
     PyObject *result;
     if (flags & METH_KEYWORDS) {
         result = _PyCFunctionWithKeywords_TrampolineCall(
-            (*(PyCFunctionWithKeywords)(void(*)(void))meth),
+            *_PyCFunctionWithKeywords_CAST(meth),
             self, args, kwargs);
     }
     else {

Diff for: Objects/typeobject.c

@@ -10807,7 +10807,8 @@ static pytype_slotdef slotdefs[] = {
            "__repr__($self, /)\n--\n\nReturn repr(self)."),
     TPSLOT(__hash__, tp_hash, slot_tp_hash, wrap_hashfunc,
            "__hash__($self, /)\n--\n\nReturn hash(self)."),
-    FLSLOT(__call__, tp_call, slot_tp_call, (wrapperfunc)(void(*)(void))wrap_call,
+    FLSLOT(__call__, tp_call, slot_tp_call,
+           _Py_FUNC_CAST(wrapperfunc, wrap_call),
            "__call__($self, /, *args, **kwargs)\n--\n\nCall self as a function.",
            PyWrapperFlag_KEYWORDS),
     TPSLOT(__str__, tp_str, slot_tp_str, wrap_unaryfunc,
@@ -10844,7 +10845,8 @@ static pytype_slotdef slotdefs[] = {
     TPSLOT(__delete__, tp_descr_set, slot_tp_descr_set,
            wrap_descr_delete,
            "__delete__($self, instance, /)\n--\n\nDelete an attribute of instance."),
-    FLSLOT(__init__, tp_init, slot_tp_init, (wrapperfunc)(void(*)(void))wrap_init,
+    FLSLOT(__init__, tp_init, slot_tp_init,
+           _Py_FUNC_CAST(wrapperfunc, wrap_init),
            "__init__($self, /, *args, **kwargs)\n--\n\n"
            "Initialize self.  See help(type(self)) for accurate signature.",
            PyWrapperFlag_KEYWORDS),

Diff for: Python/bytecodes.c

@@ -4170,7 +4170,7 @@ dummy_func(
                 DECREF_INPUTS();
                 ERROR_IF(true, error);
             }
-            PyObject *res_o = ((PyCFunctionFast)(void(*)(void))cfunc)(
+            PyObject *res_o = _PyCFunctionFast_CAST(cfunc)(
                 PyCFunction_GET_SELF(callable_o),
                 args_o,
                 total_args);
@@ -4202,8 +4202,7 @@ dummy_func(
             STAT_INC(CALL, hit);
             /* res = func(self, arguments, nargs, kwnames) */
             PyCFunctionFastWithKeywords cfunc =
-                (PyCFunctionFastWithKeywords)(void(*)(void))
-                PyCFunction_GET_FUNCTION(callable_o);
+                _PyCFunctionFastWithKeywords_CAST(PyCFunction_GET_FUNCTION(callable_o));
 
             STACKREFS_TO_PYOBJECTS(arguments, total_args, args_o);
             if (CONVERSION_FAILED(args_o)) {
@@ -4371,7 +4370,7 @@ dummy_func(
                 ERROR_IF(true, error);
             }
             PyCFunctionFastWithKeywords cfunc =
-                (PyCFunctionFastWithKeywords)(void(*)(void))meth->ml_meth;
+                _PyCFunctionFastWithKeywords_CAST(meth->ml_meth);
             PyObject *res_o = cfunc(self, (args_o + 1), nargs, NULL);
             STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
             assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));
@@ -4450,8 +4449,7 @@ dummy_func(
                 DECREF_INPUTS();
                 ERROR_IF(true, error);
             }
-            PyCFunctionFast cfunc =
-                (PyCFunctionFast)(void(*)(void))meth->ml_meth;
+            PyCFunctionFast cfunc = _PyCFunctionFast_CAST(meth->ml_meth);
             PyObject *res_o = cfunc(self, (args_o + 1), nargs);
             STACKREFS_TO_PYOBJECTS_CLEANUP(args_o);
             assert((res_o != NULL) ^ (_PyErr_Occurred(tstate) != NULL));