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Subject: Buglet in PyLong_AsLong

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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
This commit is contained in:
Guido van Rossum 1998-05-26 14:33:37 +00:00
parent 2028dd0423
commit f753181272
1 changed files with 18 additions and 8 deletions
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26
Objects/longobject.c
View file

@ -175,10 +175,11 @@ long
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 @@ PyLong_AsLong(vv)
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.