[issue38913] Py_BuildValue("(s#O)", ...) segfaults if entered with exception raised

[danielen]

danielen <daniele at grinta.net> added the comment:

The problem arises from this code in do_mktuple(), staring at line 394 in modsupport.c:

            if (**p_format == '#') {
                ++*p_format;
                if (flags & FLAG_SIZE_T)
                    n = va_arg(*p_va, Py_ssize_t);
                else {
                    if (PyErr_WarnEx(PyExc_DeprecationWarning,
                                "PY_SSIZE_T_CLEAN will be required for '#' formats", 1)) {
                        return NULL;
                    }
                    n = va_arg(*p_va, int);
                }
            }

If this is entered with an exception raised, PyErr_WarnEx() return NULL, thus this function return NULL without consuming the argument relative to the string length for the "s#" specifier. This argument is then consumed at the next iteration for the "O" specifier, resulting in a segmentation fault when the string length is interpreted as an object pointer.

I don't know what is the best solution: either ignoring the return value of PyErr_WarnEx or swapping the lines from 

                    if (PyErr_WarnEx(PyExc_DeprecationWarning,
                                "PY_SSIZE_T_CLEAN will be required for '#' formats", 1)) {
                        return NULL;
                    }
                    n = va_arg(*p_va, int);


to 

                    n = va_arg(*p_va, int);
                    if (PyErr_WarnEx(PyExc_DeprecationWarning,
                                "PY_SSIZE_T_CLEAN will be required for '#' formats", 1)) {
                        return NULL;
                    }

The handling of the "y#" just below suffers from the same problem.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38913>
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