Subject: Buglet in PyLong_AsLong
From: "Tim Peters" <tim_one@email.msn.com>
To: "Guido van Rossum" <guido@CNRI.Reston.VA.US>
Date: Sat, 23 May 1998 21:45:53 -0400
Guido, the overflow checking in PyLong_AsLong is off a little:
1) If the C in use sign-extends right shifts on signed longs, there's a
spurious overflow error when converting the most-negative int:
Python 1.5.1 (#0, Apr 13 1998, 20:22:04) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> x = -1L << 31
>>> x
-2147483648L
>>> int(x)
Traceback (innermost last):
File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>
2) If C does not sign-extend, some genuine overflows won't be caught.
The attached should repair both, and, because I installed a new disk and a C
compiler today, it's even been compiled this time <wink>.
Python 1.5.1 (#0, May 23 1998, 20:24:58) [MSC 32 bit (Intel)] on win32
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> x = -1L << 31
>>> x
-2147483648L
>>> int(x)
-2147483648
>>> int(-x)
Traceback (innermost last):
File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>> int(-x-1)
2147483647
>>> int(x-1)
Traceback (innermost last):
File "<stdin>", line 1, in ?
OverflowError: long int too long to convert
>>>
end-casing-ly y'rs - tim
diff --git a/Objects/longobject.c b/Objects/longobject.c
index becf1bc..9657c08 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -175,10 +175,11 @@
PyLong_AsLong(vv)
PyObject *vv;
{
+ /* This version by Tim Peters */
register PyLongObject *v;
- long x, prev;
+ unsigned long x, prev;
int i, sign;
-
+
if (vv == NULL || !PyLong_Check(vv)) {
PyErr_BadInternalCall();
return -1;
@@ -194,13 +195,22 @@
while (--i >= 0) {
prev = x;
x = (x << SHIFT) + v->ob_digit[i];
- if ((x >> SHIFT) != prev) {
- PyErr_SetString(PyExc_OverflowError,
- "long int too long to convert");
- return -1;
- }
+ if ((x >> SHIFT) != prev)
+ goto overflow;
}
- return x * sign;
+ /* Haven't lost any bits, but if the sign bit is set we're in
+ * trouble *unless* this is the min negative number. So,
+ * trouble iff sign bit set && (positive || some bit set other
+ * than the sign bit).
+ */
+ if ((long)x < 0 && (sign > 0 || (x << 1) != 0))
+ goto overflow;
+ return (long)x * sign;
+
+ overflow:
+ PyErr_SetString(PyExc_OverflowError,
+ "long int too long to convert");
+ return -1;
}
/* Get a C long int from a long int object.