From: Miss Islington (bot) <31488909+miss-islington@users.noreply.github.com>
Date: Mon, 2 Dec 2024 14:42:02 +0000 (+0100)
Subject: [3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211... 
X-Git-Tag: v3.13.1~10
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=f41d8d89e79d634895868656f50a0e16e339f9d6;p=thirdparty%2FPython%2Fcpython.git

[3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211) (#127216)

Note, that transformed expression is not an equivalent for original one (1/exp(-x) != exp(x) in general for floating-point numbers). Though, the difference seems to be ~1ULP for good libm implementations.

It's more interesting why division was used from beginning. Closest algorithm I've found (no error checks, of course;)) - it's Algorithm 190 from ACM: https://dl.acm.org/doi/10.1145/366663.366679. It uses subtraction in the exponent.

(cherry picked from commit f7bb658124aba74be4c13f498bf46cfded710ef9)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
---

diff --git a/Lib/test/test_complex.py b/Lib/test/test_complex.py
index d5e58e3c6bc5..a991040d811a 100644
--- a/Lib/test/test_complex.py
+++ b/Lib/test/test_complex.py
@@ -301,6 +301,11 @@ class ComplexTest(ComplexesAreIdenticalMixin, unittest.TestCase):
                     except OverflowError:
                         pass
 
+        # gh-113841: possible undefined division by 0 in _Py_c_pow()
+        x, y = 9j, 33j**3
+        with self.assertRaises(OverflowError):
+            x**y
+
     def test_pow_with_small_integer_exponents(self):
         # Check that small integer exponents are handled identically
         # regardless of their type.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-11-24-07-01-28.gh-issue-113841.WFg-Bu.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-11-24-07-01-28.gh-issue-113841.WFg-Bu.rst
new file mode 100644
index 000000000000..2b07fdfcc6b5
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2024-11-24-07-01-28.gh-issue-113841.WFg-Bu.rst
@@ -0,0 +1,2 @@
+Fix possible undefined behavior division by zero in :class:`complex`'s
+:c:func:`_Py_c_pow`.
diff --git a/Objects/complexobject.c b/Objects/complexobject.c
index 1aa3960a1c63..502b4a9a9611 100644
--- a/Objects/complexobject.c
+++ b/Objects/complexobject.c
@@ -147,7 +147,7 @@ _Py_c_pow(Py_complex a, Py_complex b)
         at = atan2(a.imag, a.real);
         phase = at*b.real;
         if (b.imag != 0.0) {
-            len /= exp(at*b.imag);
+            len *= exp(-at*b.imag);
             phase += b.imag*log(vabs);
         }
         r.real = len*cos(phase);