[3.15] gh-159739: Detect recursive untyped ctypes arguments (GH-160194) (#160350)

_as_parameter_ chains were recursion-checked for typed arguments but not for foreign calls without argtypes. A cyclic chain could exhaust the C stack and crash before the function was called. Apply the same recursion guard to untyped conversion. Backport to 3.15 and 3.14.
(cherry picked from commit 3e19e04)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Lib/test/test_ctypes/test_as_parameter.py

@@ -213,6 +213,10 @@ class A:
                 with self.assertRaises(RecursionError):
                     c_type.from_param(a)
 
+        function = CDLL(_ctypes_test.__file__)._testfunc_p_p
+        with self.assertRaisesRegex(ArgumentError, 'RecursionError'):
+            function(a)
+
 
 class AsParamWrapper:
     def __init__(self, param):

Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.b4ueJ7.rst

@@ -0,0 +1 @@
+Detect recursive ``_as_parameter_`` chains for :mod:`ctypes` calls without argument types.

Modules/_ctypes/callproc.c

@@ -91,6 +91,7 @@ module _ctypes
 #define DONT_USE_SEH
 #endif
 
+#include "pycore_ceval.h"         // _Py_EnterRecursiveCall()
 #include "pycore_runtime.h"       // _PyRuntime
 #include "pycore_global_objects.h"// _Py_ID()
 #include "pycore_traceback.h"     // _PyTraceback_Add()
@@ -744,8 +745,12 @@ static int ConvParam(ctypes_state *st,
            attribute)
         */
         if (arg) {
-            int result;
-            result = ConvParam(st, arg, index, pa);
+            if (_Py_EnterRecursiveCall(" while processing _as_parameter_")) {
+                Py_DECREF(arg);
+                return -1;
+            }
+            int result = ConvParam(st, arg, index, pa);
+            _Py_LeaveRecursiveCall();
             Py_DECREF(arg);
             return result;
         }