address review: implement a different solution with a mutex flag

Now modification of the Struct() while packing trigger a RuntimeError
serhiy-storchaka · Jan 11, 2026 · Jan 3, 2026 · Jan 10, 2026 · Jan 10, 2026 · Jan 10, 2026
commit e611953812a84c985ee25bbd8b4d871430f60462
diff --git a/Lib/test/test_struct.py b/Lib/test/test_struct.py
@@ -579,7 +579,7 @@ def test_Struct_reinitialization(self):
 
     def check_sizeof(self, format_str, number_of_codes):
         # The size of 'PyStructObject'
-        totalsize = support.calcobjsize('2n3P')
+        totalsize = support.calcobjsize('2n3PB')
         # The size taken up by the 'formatcode' dynamic array
         totalsize += struct.calcsize('P3n0P') * (number_of_codes + 1)
         support.check_sizeof(self, struct.Struct(format_str), totalsize)
@@ -818,14 +818,16 @@ def test_endian_table_init_subinterpreters(self):
 
     def test_Struct_object_mutation_via_dunders(self):
         S = struct.Struct('?I')
+        buf = array.array('b', b' '*100)
 
         class Evil():
             def __bool__(self):
                 # This rebuilds format codes during S.pack().
                 S.__init__('I')
                 return True
 
-        self.assertEqual(S.pack(Evil(), 1), struct.Struct('?I').pack(True, 1))
+        self.assertRaises(RuntimeError, S.pack, Evil(), 1)
+        self.assertRaises(RuntimeError, S.pack_into, buf, 0, Evil(), 1)
 
 
 class UnpackIteratorTest(unittest.TestCase):

diff --git a/Misc/NEWS.d/next/Library/2026-01-03-10-15-32.gh-issue-143379.iz-hU7.rst b/Misc/NEWS.d/next/Library/2026-01-03-10-15-32.gh-issue-143379.iz-hU7.rst
@@ -1,4 +1,4 @@
 Fix use-after-free in :meth:`struct.Struct.pack` when the
 :class:`struct.Struct` object is mutated by dunder methods (like
-:meth:`object.__bool__`) during packing of arguments.  Patch by Sergey B
-Kirpichev.
+:meth:`object.__bool__`) during packing of arguments.  Now this trigger
+:exc:`RuntimeError`.  Patch by Sergey B Kirpichev.
diff --git a/Modules/_struct.c b/Modules/_struct.c
@@ -70,6 +70,7 @@ typedef struct {
     formatcode *s_codes;
     PyObject *s_format;
     PyObject *weakreflist; /* List of weak references */
+    PyMutex mutex; /* to prevent mutation during packing */
 } PyStructObject;
 
 #define PyStructObject_CAST(op)     ((PyStructObject *)(op))
@@ -1773,6 +1774,7 @@ s_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
         s->s_codes = NULL;
         s->s_size = -1;
         s->s_len = -1;
+        s->mutex = (PyMutex){0};
     }
     return self;
 }
@@ -1816,6 +1818,11 @@ Struct___init___impl(PyStructObject *self, PyObject *format)
 
     Py_SETREF(self->s_format, format);
 
+    if (PyMutex_IsLocked(&self->mutex)) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "Call Struct.__init__() in struct.pack()");
+        return -1;
+    }
     ret = prepare_s(self);
     return ret;
 }
@@ -2146,24 +2153,14 @@ static int
 s_pack_internal(PyStructObject *soself, PyObject *const *args, int offset,
                 char* buf, _structmodulestate *state)
 {
-    formatcode *code, *frozen_codes;
+    formatcode *code;
     /* XXX(nnorwitz): why does i need to be a local?  can we use
        the offset parameter or do we need the wider width? */
-    Py_ssize_t i, frozen_size = 1;
+    Py_ssize_t i;
 
-    /* Take copy of soself->s_codes, as dunder methods of arguments (e.g.
-     __bool__ of __float__) could modify the struct object during packing. */
-    for (code = soself->s_codes; code->fmtdef != NULL; code++) {
-        frozen_size++;
-    }
-    frozen_codes = PyMem_Malloc(frozen_size * sizeof(formatcode));
-    if (!frozen_codes) {
-        goto err;
-    }
-    memcpy(frozen_codes, soself->s_codes, frozen_size * sizeof(formatcode));
     memset(buf, '\0', soself->s_size);
     i = offset;
-    for (code = frozen_codes; code->fmtdef != NULL; code++) {
+    for (code = soself->s_codes; code->fmtdef != NULL; code++) {
         const formatdef *e = code->fmtdef;
         char *res = buf + code->offset;
         Py_ssize_t j = code->repeat;
@@ -2177,7 +2174,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args, int offset,
                 if (!isstring && !PyByteArray_Check(v)) {
                     PyErr_SetString(state->StructError,
                                     "argument for 's' must be a bytes object");
-                    goto err;
+                    return -1;
                 }
                 if (isstring) {
                     n = PyBytes_GET_SIZE(v);
@@ -2199,7 +2196,7 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args, int offset,
                 if (!isstring && !PyByteArray_Check(v)) {
                     PyErr_SetString(state->StructError,
                                     "argument for 'p' must be a bytes object");
-                    goto err;
+                    return -1;
                 }
                 if (isstring) {
                     n = PyBytes_GET_SIZE(v);
@@ -2225,20 +2222,15 @@ s_pack_internal(PyStructObject *soself, PyObject *const *args, int offset,
                     if (PyLong_Check(v) && PyErr_ExceptionMatches(PyExc_OverflowError))
                         PyErr_SetString(state->StructError,
                                         "int too large to convert");
-                    goto err;
+                    return -1;
                 }
             }
             res += code->size;
         }
     }
 
     /* Success */
-    PyMem_Free(frozen_codes);
     return 0;
-err:
-    /* Failure */
-    PyMem_Free(frozen_codes);
-    return -1;
 }
 
 
@@ -2272,17 +2264,22 @@ s_pack(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
         return NULL;
     }
     char *buf = PyBytesWriter_GetData(writer);
-    /* Take copy of soself->s_size, as dunder methods of arguments (e.g.
-     __bool__ of __float__) could modify the struct object during packing. */
-    Py_ssize_t frozen_size = soself->s_size;
 
     /* Call the guts */
+    if (PyMutex_IsLocked(&soself->mutex)) {
+        PyErr_SetString(PyExc_RuntimeError, "XXX");
+        PyBytesWriter_Discard(writer);
+        return NULL;
+    }
+    PyMutex_Lock(&soself->mutex);
     if ( s_pack_internal(soself, args, 0, buf, state) != 0 ) {
+        PyMutex_Unlock(&soself->mutex);
         PyBytesWriter_Discard(writer);
         return NULL;
     }
+    PyMutex_Unlock(&soself->mutex);
 
-    return PyBytesWriter_FinishWithSize(writer, frozen_size);
+    return PyBytesWriter_FinishWithSize(writer, soself->s_size);
 }
 
 PyDoc_STRVAR(s_pack_into__doc__,
@@ -2378,11 +2375,13 @@ s_pack_into(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
     }
 
     /* Call the guts */
+    PyMutex_Lock(&soself->mutex);
     if (s_pack_internal(soself, args, 2, (char*)buffer.buf + offset, state) != 0) {
+        PyMutex_Unlock(&soself->mutex);
         PyBuffer_Release(&buffer);
         return NULL;
     }
-
+    PyMutex_Unlock(&soself->mutex);
     PyBuffer_Release(&buffer);
     Py_RETURN_NONE;
 }