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Date: Fri, 21 Aug 2015 12:22:07 +0200
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Subject: RE: [PHP-DEV] Overflow checks and integral vars comparison
From: anatol.php@belski.net ("Anatol Belski")



> -----Original Message-----
> From: Sherif Ramadan [mailto:theanomaly.is@gmail.com]
> Sent: Friday, August 21, 2015 12:00 PM
> To: Anatol Belski <anatol.php@belski.net>
> Cc: Dmitry Stogov <dmitry@php.net>; Xinchen Hui <xinchen.h@zend.com>;
> Nikita Popov <nikita.ppv@gmail.com>; Pierre Joye =
<pierre.php@gmail.com>;
> Bob Weinand <bobwei9@hotmail.com>; Jakub Zelenka <bukka@php.net>; Matt
> Wilmas <php_lists@realplain.com>; PHP Internals =
<internals@lists.php.net>
> Subject: Re: [PHP-DEV] Overflow checks and integral vars comparison
>=20
> I think you're a little optimistic about how effective these macros =
would be for
> overflow checks. Also, if we're talking ANSI C or C99, then size_t is =
always
> unsigned, and as far as I know GCC 2.4 always treats it as such.
> If we're trying to stick to C here anyway.
>=20
> As far as architecture specific stuff I would much rather rely on =
using the built-in
> GCC overflow checks here https://gcc.gnu.org/onlinedocs/gcc/Integer-
> Overflow-Builtins.html
>=20
Yes, this is a good idea as a further extension of such mechanics. =
However you're talking about a different topic that I've proposed now.
> ... as they are much safer and likely going to be far more performant =
than doing
> all these casts everywhere. Not to mention the fact that you can =
actually catch
> the overflow at the actual arithmetic level, where it's safe, and =
hopefully be able
> to rely on the ISA's overflow or carry bits. If we're trying to detect =
overflows or
> wraps after the fact, you don't add much in the way of security. For =
example,
> I'm not at all sure how (zlong) < (zend_long)INT_MIN will ever detect =
an
> overflow.
>=20
What I'm talking about is detecting whether a variable of zend_long or =
size_t is in the safe range to be passed to a signature requiring int. =
This is quite a minimalistic start in this direction.

Also please remember that there is not only GCC in the world. It is =
quite another topic to implement overflow checks portable ways, some =
intrinsics can be here of some help, too. But rather than hitting quite =
a global thing, I'd rather start on what is simple and is for sure an =
issue at least with some dependency libs.

Regards

Anatol