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Date: Mon, 24 Aug 2015 16:15:09 +0200
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Subject: RE: [PHP-DEV] Overflow checks and integral vars comparison
From: anatol.php@belski.net ("Anatol Belski")

Hi Rowan,

> -----Original Message-----
> From: Rowan Collins [mailto:rowan.collins@gmail.com]
> Sent: Monday, August 24, 2015 12:29 AM
> To: internals@lists.php.net
> Subject: Re: [PHP-DEV] Overflow checks and integral vars comparison
>=20
> On 22/08/2015 02:38, Sherif Ramadan wrote:
> > I see. So you're not actually doing overflow checks then? Because at
> > the point you'd be checking this zend_long or size_t it could have
> > already overflowed or wrapped. The subject may have misled me to
> > understand differently.
>=20
> I think I understand the confusion: you are thinking of overflow as =
something
> which happens *within* a type based on some operation (addition,
> multiplication, etc).
>=20
> Anatol is talking about overflows which occur when casting *between*
> types: a value of 2^33 can safely be passed around as a 64-bit =
integer, no
> overflow has occurred; but attempting to cast it to a 32-bit integer =
will
> immediately overflow the 32-bit integer.
>=20
> Since many PHP extensions are wrappers around libraries which may only =
deal in
> 32-bit types, this cast is common, necessitating range checks like the =
ones
> proposed.
>=20
Yep, that's the exact point. Not touching arithmetic operations as it is =
a complex thing. Even on 64-bit fe there are intrinsics for doing =
128-bit math which could be used. ASM could be helpful ofc, however it =
needs much care, fe even within GCC versions. So it is big and should be =
another story :)=20

Overflows in the cast can happen when passing args, comparing variables =
of different type or signess, or even with the numeric constants. Hence =
that cast (zend_long)INT_MAX for example. At the end - it's still an =
overflow/underflow issue. I'm going for an RFC with this to target 7.1.

Thanks.

anatol