gh-156204: Guard recursion in PyErr_GivenExceptionMatches

Lib/test/test_exceptions.py

@@ -2806,6 +2806,32 @@ def test_except_star_invalid_exception_type(self):
             except (ValueError, 42):
                 pass
 
+    @cpython_only
+    @unittest.skipIf(_testcapi is None, "requires _testcapi")
+    def test_given_exception_matches_nested_tuple(self):
+        # Nested tuples are searched recursively.
+        self.assertTrue(
+            _testcapi.err_givenexceptionmatches(ValueError(), ((ValueError,),)))
+        self.assertFalse(
+            _testcapi.err_givenexceptionmatches(TypeError(), ((ValueError,),)))
+
+    @cpython_only
+    @unittest.skipIf(_testcapi is None, "requires _testcapi")
+    @support.skip_emscripten_stack_overflow()
+    @support.skip_wasi_stack_overflow()
+    @support.run_with_limited_c_stack(depth=500_000)
+    def test_given_exception_matches_deeply_nested_tuple(self):
+        # gh-156204: PyErr_GivenExceptionMatches() used to exhaust the C stack
+        # and crash the interpreter on deeply nested tuples of exception types.
+        tup = (ValueError,)
+        for _ in range(500_000):
+            tup = (tup,)
+
+        with support.catch_unraisable_exception() as cm:
+            self.assertFalse(
+                _testcapi.err_givenexceptionmatches(ValueError(), tup))
+            self.assertIsInstance(cm.unraisable.exc_value, RecursionError)
+
 
 class PEP626Tests(unittest.TestCase):
 

Misc/NEWS.d/next/Core_and_Builtins/2026-08-21-18-49-12.gh-issue-156204.ZusA7e.rst

@@ -0,0 +1,3 @@
+Fix a crash in :c:func:`PyErr_GivenExceptionMatches` when evaluating deeply
+nested exception tuples. The recursion is now bounded, and exceeding the
+limit is reported as an unraisable exception.

Modules/_testcapi/exceptions.c

@@ -54,6 +54,26 @@ err_restore(PyObject *self, PyObject *args) {
     return NULL;
 }
 
+/*[clinic input]
+_testcapi.err_givenexceptionmatches
+    err: object
+    exc: object
+    /
+
+Test PyErr_GivenExceptionMatches().
+[clinic start generated code]*/
+
+static PyObject *
+_testcapi_err_givenexceptionmatches_impl(PyObject *module, PyObject *err,
+                                         PyObject *exc)
+/*[clinic end generated code: output=e40994ab6dd75001 input=7b8ef542df07575b]*/
+{
+    assert(!PyErr_Occurred());
+    int res = PyErr_GivenExceptionMatches(err, exc);
+    assert(!PyErr_Occurred());
+    return PyBool_FromLong(res);
+}
+
 /*[clinic input]
 _testcapi.exception_print
     exception as exc: object
@@ -552,6 +572,7 @@ static PyMethodDef test_methods[] = {
     _TESTCAPI_MAKE_EXCEPTION_WITH_DOC_METHODDEF
     _TESTCAPI_EXC_SET_OBJECT_METHODDEF
     _TESTCAPI_EXC_SET_OBJECT_FETCH_METHODDEF
+    _TESTCAPI_ERR_GIVENEXCEPTIONMATCHES_METHODDEF
     _TESTCAPI_ERR_SETSTRING_METHODDEF
     _TESTCAPI_ERR_SETFROMERRNOWITHFILENAME_METHODDEF
     _TESTCAPI_RAISE_EXCEPTION_METHODDEF

Python/errors.c

@@ -4,6 +4,7 @@
 #include "Python.h"
 #include "pycore_audit.h"         // _PySys_Audit()
 #include "pycore_call.h"          // _PyObject_CallNoArgs()
+#include "pycore_ceval.h"         // _Py_ReachedRecursionLimitWithMargin()
 #include "pycore_fileutils.h"     // _PyFile_Flush
 #include "pycore_initconfig.h"    // _PyStatus_ERR()
 #include "pycore_pyerrors.h"      // _PyErr_Format()
@@ -337,17 +338,27 @@ PyErr_GivenExceptionMatches(PyObject *err, PyObject *exc)
         return 0;
     }
     if (PyTuple_Check(exc)) {
-        Py_ssize_t i, n;
-        n = PyTuple_Size(exc);
-        for (i = 0; i < n; i++) {
+        PyThreadState *tstate = _PyThreadState_GET();
+        if (_Py_ReachedRecursionLimitWithMargin(tstate, 2)) {
+            PyObject *exc_value = _PyErr_GetRaisedException(tstate);
+            _PyErr_SetString(tstate, PyExc_RecursionError,
+                             "maximum recursion depth exceeded while "
+                             "checking exception tuple");
+            PyErr_FormatUnraisable("Exception ignored while "
+                                   "checking exception tuple");
+            _PyErr_SetRaisedException(tstate, exc_value);
+            return 0;
+        }
+        int res = 0;
+        Py_ssize_t n = PyTuple_GET_SIZE(exc);
+        for (Py_ssize_t i = 0; i < n; i++) {
             /* Test recursively */
-             if (PyErr_GivenExceptionMatches(
-                 err, PyTuple_GET_ITEM(exc, i)))
-             {
-                 return 1;
-             }
+            if (PyErr_GivenExceptionMatches(err, PyTuple_GET_ITEM(exc, i))) {
+                res = 1;
+                break;
+            }
         }
-        return 0;
+        return res;
     }
     /* err might be an instance, so check its class. */
     if (PyExceptionInstance_Check(err))