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Avoid undefined behaviour due to overflow in i_divmod (Objects/intobject.c).

This commit is contained in:
Mark Dickinson 2009-12-04 11:24:38 +00:00
parent 820b0ea36c
commit 1691025ff2
1 changed files with 10 additions and 1 deletions
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11
Objects/intobject.c
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@ -580,7 +580,16 @@ i_divmod(register long x, register long y,
if (y == -1 && UNARY_NEG_WOULD_OVERFLOW(x))
return DIVMOD_OVERFLOW;
xdivy = x / y;
xmody = x - xdivy * y;
/* xdiv*y can overflow on platforms where x/y gives floor(x/y)
* for x and y with differing signs. (This is unusual
* behaviour, and C99 prohibits it, but it's allowed by C89;
* for an example of overflow, take x = LONG_MIN, y = 5 or x =
* LONG_MAX, y = -5.) However, x - xdivy*y is always
* representable as a long, since it lies strictly between
* -abs(y) and abs(y). We add casts to avoid intermediate
* overflow.
*/
xmody = (long)(x - (unsigned long)xdivy * y);
/* If the signs of x and y differ, and the remainder is non-0,
* C89 doesn't define whether xdivy is now the floor or the
* ceiling of the infinitely precise quotient. We want the floor,