Tim Peters <t...@python.org> added the comment:
This doesn't actually matter - the code can never trigger. It would be fine to
replace it with an assert to that effect (see below for a specific suggestion).
The reason: The indices in this code are into vectors of PyObject*. These
vectors can't contain more than
floor(PY_SSIZE_T_MAX / sizeof(PyObject*))
pointers (see listobject.c & Python's heap allocation routines). So the
largest legit index this code can ever see is 1 less than that. Since pointers
are at least 4 bytes on all machines Python runs on, that implies (with room to
spare) that
assert(ofs <= (PY_SSIZE_T_MAX - 1) / 2);
can't fail. Which in turn implies that, mathematically,
2*ofs + 1 <= PY_SSIZE_T_MAX
So
if (ofs <= 0) /* int overflow */
can't happen, regardless of how the platform C treats signed overflow (signed
overflow can't happen to begin with). The existing `while (ofs < maxofs)`
check already ensures that `ofs` is a legit index, and _any_ legit index into a
PyObject* vector can be doubled and incremented without overflowing Py_ssize_t.
In fact, that would remain so even if listobject.c allowed its PyObject*
vectors to contain twice as many pointers as they actually can contain now.
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue35091>
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