gh-101037: Fix potential memory underallocation for zeros of int subt…

…ypes (#101038)

This PR fixes object allocation in long_subtype_new to ensure that there's at least one digit in all cases, and makes sure that the value of that digit is copied over from the source long.

Needs backport to 3.11, but not any further: the change to require at least one digit was only introduced for Python 3.11.

Fixes #101037.

Include/cpython/longintrepr.h

@@ -71,6 +71,9 @@ typedef long stwodigits; /* signed variant of twodigits */
         0 <= ob_digit[i] <= MASK.
    The allocation function takes care of allocating extra memory
    so that ob_digit[0] ... ob_digit[abs(ob_size)-1] are actually available.
+   We always allocate memory for at least one digit, so accessing ob_digit[0]
+   is always safe. However, in the case ob_size == 0, the contents of
+   ob_digit[0] may be undefined.
 
    CAUTION:  Generic code manipulating subtypes of PyVarObject has to
    aware that ints abuse  ob_size's sign bit.

Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst

@@ -0,0 +1,2 @@
+Fix potential memory underallocation issue for instances of :class:`int`
+subclasses with value zero.

Objects/longobject.c

@@ -5638,6 +5638,11 @@ long_subtype_new(PyTypeObject *type, PyObject *x, PyObject *obase)
     n = Py_SIZE(tmp);
     if (n < 0)
         n = -n;
+    /* Fast operations for single digit integers (including zero)
+     * assume that there is always at least one digit present. */
+    if (n == 0) {
+        n = 1;
+    }
     newobj = (PyLongObject *)type->tp_alloc(type, n);
     if (newobj == NULL) {
         Py_DECREF(tmp);