8000 gh-101037: Fix potential memory underallocation for zeros of int subtypes by mdickinson · Pull Request #101038 · python/cpython · GitHub
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gh-101037: Fix potential memory underallocation for zeros of int subtypes #101038

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Merged
merged 2 commits into from
Jan 21, 2023
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gh-101037: Fix potential memory underallocation for zeros of int subtypes #101038

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gh-101037: Fix memory allocation for zeros of int subtypes
mdickinson Jan 14, 2023
877067c
Add note to longintrepr.h to clarify that ob_digit[0] is always safe …
mdickinson Jan 14, 2023
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3 changes: 3 additions & 0 deletions Include/cpython/longintrepr.h
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Expand Up @@ -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.
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core and Builtins/2023-01-14-17-03-08.gh-issue-101037.9ATNuf.rst
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@@ -0,0 +1,2 @@
Fix potential memory underallocation issue for instances of :class:`int`
subclasses with value zero.
5 changes: 5 additions & 0 deletions Objects/longobject.c
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Expand Up @@ -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);
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