bpo-30245: possible overflow when organize struct.pack_into error message

Lib/test/test_struct.py

@@ -599,6 +599,17 @@ def test_boundary_error_message_with_negative_offset(self):
                 'offset -11 out of range for 10-byte buffer'):
             struct.pack_into('<B', byte_list, -11, 123)
 
+    def test_boundary_error_message_with_large_offset(self):
+        # Test overflows cause by large offset and value size (issue 30245)
+        regex = (
+            "pack_into requires a buffer of at least " + str(sys.maxsize + 4) +
+            " bytes for packing 4 bytes at offset " + str(sys.maxsize) +
+            " \(actual buffer size is 10\)"
+        )
+
+        with self.assertRaisesRegex(struct.error, regex):
+            struct.pack_into('<I', bytearray(10), sys.maxsize, 1)
+
     def test_issue29802(self):
         # When the second argument of struct.unpack() was of wrong type
         # the Struct object was decrefed twice and the reference to

Modules/_struct.c

@@ -1929,11 +1929,15 @@ s_pack_into(PyObject *self, PyObject **args, Py_ssize_t nargs, PyObject *kwnames
 
     /* Check boundaries */
     if ((buffer.len - offset) < soself->s_size) {
+
+        assert(offset >= 0);
+        assert(soself->s_size >= 0);
+
         PyErr_Format(StructError,
-                     "pack_into requires a buffer of at least %zd bytes for "
+                     "pack_into requires a buffer of at least %zu bytes for "
                      "packing %zd bytes at offset %zd "
                      "(actual buffer size is %zd)",
-                     soself->s_size + offset,
+                     (size_t)soself->s_size + (size_t)offset,
                      soself->s_size,
                      offset,
                      buffer.len);