Missing error checks in `termios` module

[sobolevn]

Bug report

There are multiple cases where PyList_SetItem does not check for -1 in termios module. We need to fix this. There are two options to fix this:

• < 0 check
• PyList_SET_ITEM usage

There are two possible errors that PyList_SetItem can produce:

    if (!PyList_Check(op)) {
        Py_XDECREF(newitem);
        PyErr_BadInternalCall();
        return -1;
    }
    if (!valid_index(i, Py_SIZE(op))) {
        Py_XDECREF(newitem);
        PyErr_SetString(PyExc_IndexError,
                        "list assignment index out of range");
        return -1;
    }

Each case needs an explanation.

cpython/Modules/termios.c

Lines 116 to 124 in 8c07137

	PyObject *v;
	int i;
	for (i = 0; i < NCCS; i++) {
	char ch = (char)mode.c_cc[i];
	v = PyBytes_FromStringAndSize(&ch, 1);
	if (v == NULL)
	goto err;
	PyList_SetItem(cc, i, v);
	}

Here cc is always a list and i goes from 0 to some other int value by i++, it is safe to use PyList_SET_ITEM.

cpython/Modules/termios.c

Lines 129 to 137 in 8c07137

	if ((mode.c_lflag & ICANON) == 0) {
	v = PyLong_FromLong((long)mode.c_cc[VMIN]);
	if (v == NULL)
	goto err;
	PyList_SetItem(cc, VMIN, v);
	v = PyLong_FromLong((long)mode.c_cc[VTIME]);
	if (v == NULL)
	goto err;
	PyList_SetItem(cc, VTIME, v);

Here cc is a list, but VMIN and VTIME are 3rd party constants from termios.h. I guess it is safer to use explicit error check.

cpython/Modules/termios.c

Lines 140 to 153 in 8c07137

	if (!(v = PyList_New(7)))
	goto err;
	PyList_SetItem(v, 0, PyLong_FromLong((long)mode.c_iflag));
	PyList_SetItem(v, 1, PyLong_FromLong((long)mode.c_oflag));
	PyList_SetItem(v, 2, PyLong_FromLong((long)mode.c_cflag));
	PyList_SetItem(v, 3, PyLong_FromLong((long)mode.c_lflag));
	PyList_SetItem(v, 4, PyLong_FromLong((long)ispeed));
	PyList_SetItem(v, 5, PyLong_FromLong((long)ospeed));
	if (PyErr_Occurred()) {
	Py_DECREF(v);
	goto err;
	}
	PyList_SetItem(v, 6, cc);

Here we need to check for possibel PyLong_FromLong error, but we can use PyList_SET_ITEM.

Linked PRs

• gh-110260: Check for PyList_SetItem errors in termios module #110261