linux-kernel - Re: [RFC 2/2] kvm: eventfd: detect integer overflow using check_*

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Date:	Wed, 26 Nov 2014 10:23:45 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	Andrey Ryabinin <ryabinin.a.a@...il.com>, mingo <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/2] kvm: eventfd: detect integer overflow using check_*_overflow

On Wed, Nov 26, 2014 at 9:55 AM, Sasha Levin <sasha.levin@...cle.com> wrote:
>
> Good point. Do you think there's an advantage in using GCC's overflow
> checker in this case?

No. However, if your coccinelle script can be changed to verify that
the type of the expression is unsigned, _that_ would be useful.

And the "multiplication overflow" may actually be a way to generate
better code. Possibly. I'm not entirely sure exactly what gcc actually
does. How many multiplication overflow tests do we actually have,
though?

For plain unsigned additions, "a + b < a" is already optimal (ie gcc
realizes it's an overflow check and generates a test against the carry
flag, at least when it doesn't screw up)

                     Linus
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