_struct.c: Fix UB from integer overflow in prepare_s (GH-145158)

Avoid possible undefined behaviour from signed overflow in `struct` module

As discovered via oss-fuzz.

Author: StanFromIreland

Lib/test/test_struct.py

@@ -552,6 +552,9 @@ def test_count_overflow(self):
         hugecount2 = '{}b{}H'.format(sys.maxsize//2, sys.maxsize//2)
         self.assertRaises(struct.error, struct.calcsize, hugecount2)
 
+        hugecount3 = '{}i{}q'.format(sys.maxsize // 4, sys.maxsize // 8)
+        self.assertRaises(struct.error, struct.calcsize, hugecount3)
+
     def test_trailing_counter(self):
         store = array.array('b', b' '*100)
 

Misc/NEWS.d/next/Library/2026-02-23-20-52-55.gh-issue-145158.vWJtxI.rst

@@ -0,0 +1,2 @@
+Avoid undefined behaviour from signed integer overflow when parsing format
+strings in the :mod:`struct` module.

Modules/_struct.c

@@ -1678,7 +1678,15 @@ prepare_s(PyStructObject *self)
             case 's': _Py_FALLTHROUGH;
             case 'p': len++; ncodes++; break;
             case 'x': break;
-            default: len += num; if (num) ncodes++; break;
+            default:
+                if (num > PY_SSIZE_T_MAX - len) {
+                    goto overflow;
+                }
+                len += num;
+                if (num) {
+                    ncodes++;
+                }
+                break;
         }
 
         itemsize = e->size;