gh-144475: Fix a heap buffer overflow in partial_repr

Lib/test/test_functools.py

@@ -514,6 +514,70 @@ def test_partial_genericalias(self):
         self.assertEqual(alias.__args__, (int,))
         self.assertEqual(alias.__parameters__, ())
 
+    # Issue 144475
+    def test_repr_for_segfault(self):
+        g_partial = None
+
+        class Function:
+            def __init__(self, name):
+                self.name = name
+
+            def __call__(self):
+                return None
+
+            def __repr__(self):
+                return f"Function({self.name})"
+
+        class EvilObject:
+            def __init__(self, name, is_trigger=False):
+                self.name = name
+                self.is_trigger = is_trigger
+                self.triggered = False
+
+            def __repr__(self):
+                if self.is_trigger and not self.triggered and g_partial is not None:
+                    self.triggered = True
+                    new_args_tuple = (None,)
+                    new_keywords_dict = {"keyword": None}
+                    new_tuple_state = (Function("new_function"), new_args_tuple, new_keywords_dict, None)
+                    g_partial.__setstate__(new_tuple_state)
+                    gc.collect()
+                return f"EvilObject({self.name})"
+
+        trigger = EvilObject("trigger", is_trigger=True)
+        victim = EvilObject("victim")
+
+        p = functools.partial(Function("old_function"), victim, victim=trigger)
+        g_partial = p
+        self.assertEqual(repr(g_partial),"functools.partial(Function(old_function), EvilObject(victim), victim=EvilObject(trigger))")
+
+        trigger.triggered = False
+        g_partial = None
+        p = functools.partial(Function("old_function"), trigger, victim=victim)
+        g_partial = p
+        self.assertEqual(repr(g_partial),"functools.partial(Function(old_function), EvilObject(trigger), victim=EvilObject(victim))")
+
+
+        trigger.triggered = False
+        p = functools.partial(Function("old_function"), trigger, victim)
+        g_partial = p
+
+        trigger.triggered = False
+        p = functools.partial(Function("old_function"), trigger=trigger, victim=victim)
+        g_partial = p
+        self.assertEqual(repr(g_partial),"functools.partial(Function(old_function), trigger=EvilObject(trigger), victim=EvilObject(victim))")
+
+        trigger.triggered = False
+        victim1 = EvilObject("victim")
+        victim2 = EvilObject("victim")
+        victim3 = EvilObject("victim")
+        victim4 = EvilObject("victim")
+        victim5 = EvilObject("victim")
+        p = functools.partial(Function("old_function"), trigger, victim1, victim2, victim3, victim4, victim=victim5)
+        g_partial = p
+        self.assertEqual(repr(g_partial),"functools.partial(Function(old_function), EvilObject(trigger), EvilObject(victim), EvilObject(victim), EvilObject(victim), EvilObject(victim), victim=EvilObject(victim))")
+
+
 
 @unittest.skipUnless(c_functools, 'requires the C _functools module')
 class TestPartialC(TestPartial, unittest.TestCase):

Misc/NEWS.d/next/C_API/2026-02-07-16-37-42.gh-issue-144475.8tFEXw.rst

@@ -0,0 +1,6 @@
+Fixed a bug in :func:`functools.partial` when calling :func:`repr` on a partial
+object that could occur when the ``fn``, ``args``, or ``kw`` arguments are modified
+during a call to :func:`repr`. Now, calls to :func:`repr` will use the original
+arguments when generating the string representation of the partial object.
+Subsequent calls will use the updated arguments instead.
+

Modules/_functoolsmodule.c

@@ -689,6 +689,9 @@ partial_repr(PyObject *self)
     partialobject *pto = partialobject_CAST(self);
     PyObject *result = NULL;
     PyObject *arglist;
+    PyObject *fn;
+    PyObject *args;
+    PyObject *kw;
     PyObject *mod;
     PyObject *name;
     Py_ssize_t i, n;
@@ -701,52 +704,55 @@ partial_repr(PyObject *self)
             return NULL;
         return PyUnicode_FromString("...");
     }
+    /* Reference arguments in case they change */
+    fn = Py_NewRef(pto->fn);
+    args = Py_NewRef(pto->args);
+    kw = Py_NewRef(pto->kw);
+    assert(PyTuple_Check(args));
+    assert(PyDict_Check(kw));
 
     arglist = Py_GetConstant(Py_CONSTANT_EMPTY_STR);
     if (arglist == NULL)
-        goto done;
+        goto arglist_error;
     /* Pack positional arguments */
-    assert(PyTuple_Check(pto->args));
-    n = PyTuple_GET_SIZE(pto->args);
+    n = PyTuple_GET_SIZE(args);
     for (i = 0; i < n; i++) {
         Py_SETREF(arglist, PyUnicode_FromFormat("%U, %R", arglist,
-                                        PyTuple_GET_ITEM(pto->args, i)));
+                                        PyTuple_GET_ITEM(args, i)));
         if (arglist == NULL)
-            goto done;
+            goto arglist_error;
     }
     /* Pack keyword arguments */
-    assert (PyDict_Check(pto->kw));
-    for (i = 0; PyDict_Next(pto->kw, &i, &key, &value);) {
+    for (i = 0; PyDict_Next(kw, &i, &key, &value);) {
         /* Prevent key.__str__ from deleting the value. */
         Py_INCREF(value);
         Py_SETREF(arglist, PyUnicode_FromFormat("%U, %S=%R", arglist,
                                                 key, value));
         Py_DECREF(value);
         if (arglist == NULL)
-            goto done;
+            goto arglist_error;
     }
 
     mod = PyType_GetModuleName(Py_TYPE(pto));
-    if (mod == NULL) {
-        goto error;
-    }
+    if (mod == NULL)
+        goto mod_error;
+
     name = PyType_GetQualName(Py_TYPE(pto));
-    if (name == NULL) {
-        Py_DECREF(mod);
-        goto error;
-    }
-    result = PyUnicode_FromFormat("%S.%S(%R%U)", mod, name, pto->fn, arglist);
-    Py_DECREF(mod);
+    if (name == NULL)
+        goto name_error;
+
+    result = PyUnicode_FromFormat("%S.%S(%R%U)", mod, name, fn, arglist);
     Py_DECREF(name);
+ name_error:
+    Py_DECREF(mod);
+ mod_error:
     Py_DECREF(arglist);
-
- done:
+ arglist_error:
+    Py_DECREF(fn);
+    Py_DECREF(args);
+    Py_DECREF(kw);
     Py_ReprLeave(self);
     return result;
- error:
-    Py_DECREF(arglist);
-    Py_ReprLeave(self);
-    return NULL;
 }
 
 /* Pickle strategy: