From a25e1f9b398540c23ad29be7ce9990b6d7019567 Mon Sep 17 00:00:00 2001
From: Mark Shannon <mark@hotpy.org>
Date: Wed, 13 Aug 2025 18:08:41 +0100
Subject: [PATCH] Add test to check that the margin used for overflow
 protection is larger than the stack space used by the interpreter

---
 Lib/test/test_call.py       | 16 ++++++++++++++++
 Modules/_testinternalcapi.c | 14 ++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/Lib/test/test_call.py b/Lib/test/test_call.py
index 1c73aaafb71fd5..be6b10fde63b5f 100644
--- a/Lib/test/test_call.py
+++ b/Lib/test/test_call.py
@@ -12,6 +12,10 @@
     import _testlimitedcapi
 except ImportError:
     _testlimitedcapi = None
+try:
+    import _testinternalcapi
+except ImportError:
+    _testinternalcapi = None
 import struct
 import collections
 import itertools
@@ -1037,6 +1041,18 @@ def test_unexpected_keyword_suggestion_via_getargs(self):
 @cpython_only
 class TestRecursion(unittest.TestCase):
 
+    def test_margin_is_sufficient(self):
+
+        def get_sp():
+            return _testinternalcapi.get_stack_pointer()
+
+        this_sp = _testinternalcapi.get_stack_pointer()
+        lower_sp = _testcapi.pyobject_vectorcall(get_sp, (), ())
+        self.assertLess(lower_sp, this_sp)
+        # Add an (arbitrary) extra 25% for safety
+        safe_margin = (this_sp - lower_sp) * 5 / 4
+        self.assertLess(safe_margin, _testinternalcapi.get_stack_margin())
+
     @skip_on_s390x
     @unittest.skipIf(is_wasi and Py_DEBUG, "requires deep stack")
     @skip_if_sanitizer("requires deep stack", thread=True)
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 243c7346576fc6..e6f79e68a6efda 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -125,6 +125,18 @@ get_c_recursion_remaining(PyObject *self, PyObject *Py_UNUSED(args))
     return PyLong_FromLong(remaining);
 }
 
+static PyObject*
+get_stack_pointer(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    uintptr_t here_addr = _Py_get_machine_stack_pointer();
+    return PyLong_FromSize_t(here_addr);
+}
+
+static PyObject*
+get_stack_margin(PyObject *self, PyObject *Py_UNUSED(args))
+{
+    return PyLong_FromSize_t(_PyOS_STACK_MARGIN_BYTES);
+}
 
 static PyObject*
 test_bswap(PyObject *self, PyObject *Py_UNUSED(args))
@@ -2380,6 +2392,8 @@ static PyMethodDef module_functions[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
     {"get_c_recursion_remaining", get_c_recursion_remaining, METH_NOARGS},
+    {"get_stack_pointer", get_stack_pointer, METH_NOARGS},
+    {"get_stack_margin", get_stack_margin, METH_NOARGS},
     {"test_bswap", test_bswap, METH_NOARGS},
     {"test_popcount", test_popcount, METH_NOARGS},
     {"test_bit_length", test_bit_length, METH_NOARGS},