Issue #27581: Don’t rely on overflow wrapping in PySequence_Tuple()

Patch by Xiang Zhang.

Misc/NEWS

@@ -25,6 +25,9 @@ Core and Builtins
 - Issue #27507: Add integer overflow check in bytearray.extend().  Patch by
   Xiang Zhang.
 
+- Issue #27581: Don't rely on wrapping for overflow check in
+  PySequence_Tuple().  Patch by Xiang Zhang.
+
 - Issue #27443: __length_hint__() of bytearray iterators no longer return a
   negative integer for a resized bytearray.
 

Objects/abstract.c

@@ -1724,21 +1724,22 @@ PySequence_Tuple(PyObject *v)
             break;
         }
         if (j >= n) {
-            Py_ssize_t oldn = n;
+            size_t newn = (size_t)n;
             /* The over-allocation strategy can grow a bit faster
                than for lists because unlike lists the
                over-allocation isn't permanent -- we reclaim
                the excess before the end of this routine.
                So, grow by ten and then add 25%.
             */
-            n += 10;
-            n += n >> 2;
-            if (n < oldn) {
+            newn += 10u;
+            newn += newn >> 2;
+            if (newn > PY_SSIZE_T_MAX) {
                 /* Check for overflow */
                 PyErr_NoMemory();
                 Py_DECREF(item);
                 goto Fail;
             }
+            n = (Py_ssize_t)newn;
             if (_PyTuple_Resize(&result, n) != 0) {
                 Py_DECREF(item);
                 goto Fail;