linux-kernel - Re: linux-next: build warning after merge of the tip tree

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Message-ID: <53C9889F.3040107@zytor.com>
Date:	Fri, 18 Jul 2014 13:50:39 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Stephen Rothwell <sfr@...b.auug.org.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: linux-next: build warning after merge of the tip tree

On 07/18/2014 01:20 PM, Andy Lutomirski wrote:
>>
>> The reason this is a concern is that: (x > x + n) and its variants is
>> often used to mean (x > INT_MAX - n) without the type knowledge, but
>> that is actually invalid standard C because signed types are not
>> guaranteed to wrap.
> 
> Right, but the constant in this case is *much* less than INT_MAX.
> Anyway, this is moot.

It isn't about the constant (n) at all, it is about the value of x.

> I do wonder whether the kind of people who build hardened kernels
> should enable -fwrapv, though.

-fwrapv in gcc makes signed arithmetic strict 2's-complement, which is
what I think we want in the kernel.  Someone would just have to make
sure there isn't some key codepath in the kernel which gets pessimized.

	-hpa


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