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_struct.c: Fix UB from integer overflow in prepare_s #145158

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Merged
gpshead merged 2 commits into from
Feb 24, 2026
Merged

_struct.c: Fix UB from integer overflow in prepare_s #145158

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Avoid possible undefined behaviour from signed overflow in `struct` m…
StanFromIreland Feb 23, 2026
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3 changes: 3 additions & 0 deletions Lib/test/test_struct.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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)

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10 changes: 9 additions & 1 deletion Modules/_struct.c
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Expand Up @@ -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;
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