bpo-44698: Fix undefined behaviour in complex exponentiation. (GH-27278) (GH-27367)

(cherry picked from commit 1d582bb) Co-authored-by: T. Wouters <thomas@python.org>

Author: miss-islington

Lib/test/test_complex.py

@@ -1,4 +1,5 @@
 import unittest
+import sys
 from test import support
 from test.test_grammar import (VALID_UNDERSCORE_LITERALS,
  INVALID_UNDERSCORE_LITERALS)
@@ -206,6 +207,26 @@ def test_pow(self):
  b = 5.1+2.3j
  self.assertRaises(ValueError, pow, a, b, 0)
 
+ # Check some boundary conditions; some of these used to invoke
+ # undefined behaviour (https://bugs.python.org/issue44698). We're
+ # not actually checking the results of these operations, just making
+ # sure they don't crash (for example when using clang's
+ # UndefinedBehaviourSanitizer).
+ values = (sys.maxsize, sys.maxsize+1, sys.maxsize-1,
+ -sys.maxsize, -sys.maxsize+1, -sys.maxsize+1)
+ for real in values:
+ for imag in values:
+ with self.subTest(real=real, imag=imag):
+ c = complex(real, imag)
+ try:
+ c ** real
+ except OverflowError:
+ pass
+ try:
+ c ** c
+ except OverflowError:
+ pass
+
  def test_boolcontext(self):
  for i in range(100):
  self.assertTrue(complex(random() + 1e-6, random() + 1e-6))

Misc/NEWS.d/next/Core and Builtins/2021-07-21-15-26-56.bpo-44698.DA4_0o.rst

@@ -0,0 +1 @@
+Fix undefined behaviour in complex object exponentiation.

Objects/complexobject.c

@@ -529,8 +529,6 @@ static PyObject *
 complex_pow(PyObject *v, PyObject *w, PyObject *z)
 {
  Py_complex p;
- Py_complex exponent;
- long int_exponent;
  Py_complex a, b;
  TO_COMPLEX(v, a);
  TO_COMPLEX(w, b);
@@ -540,12 +538,21 @@ complex_pow(PyObject *v, PyObject *w, PyObject *z)
  return NULL;
  }
  errno = 0;
- exponent = b;
- int_exponent = (long)exponent.real;
- if (exponent.imag == 0. && exponent.real == int_exponent)
- p = c_powi(a, int_exponent);
- else
- p = _Py_c_pow(a, exponent);
+ // Check if w is an integer value that fits inside a C long, so we can
+ // use a faster algorithm. TO_COMPLEX(w, b), above, already handled the
+ // conversion from larger longs, as well as other types.
+ if (PyLong_Check(w)) {
+ int overflow = 0;
+ long int_exponent = PyLong_AsLongAndOverflow(w, &overflow);
+ if (int_exponent == -1 && PyErr_Occurred())
+ return NULL;
+ if (overflow == 0)
+ p = c_powi(a, int_exponent);
+ else
+ p = _Py_c_pow(a, b);
+ } else {
+ p = _Py_c_pow(a, b);
+ }
 
  Py_ADJUST_ERANGE2(p.real, p.imag);
  if (errno == EDOM) {