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

miss-islington · skirpichev · web-flow · commit f41d8d89e79d · 2024-12-02T15:42:02.000+01:00
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 f7bb658) Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>
diff --git a/Lib/test/test_complex.py b/Lib/test/test_complex.py
@@ -301,6 +301,11 @@ def test_pow(self):
                     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
@@ -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
@@ -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);

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,2 @@`
	`1`	+Fix possible undefined behavior division by zero in :class:`complex`'s
	`2`	+:c:func:`_Py_c_pow`.
Original file line number	Diff line number	Diff line change
`@@ -147,7 +147,7 @@ _Py_c_pow(Py_complex a, Py_complex b)`
`147`	`147`	`at = atan2(a.imag, a.real);`
`148`	`148`	`phase = at*b.real;`
`149`	`149`	`if (b.imag != 0.0) {`
`150`		`- len /= exp(at*b.imag);`
	`150`	`+ len = exp(-atb.imag);`
`151`	`151`	`phase += b.imag*log(vabs);`
`152`	`152`	`}`
`153`	`153`	`r.real = len*cos(phase);`