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

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Message-ID: <CALCETrXs4fdKDA1C4ts3qzgbB6PrmCZZyK3R4cBLaO9RbBp0YA@mail.gmail.com>
Date:	Fri, 18 Jul 2014 12:57:32 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
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 Fri, Jul 18, 2014 at 12:16 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 07/17/2014 10:00 PM, Stephen Rothwell wrote:
>> Hi all,
>>
>> After merging the tip tree, today's linux-next build (x86_64
>> allmodconfig) produced these warnings:
>>
>> In file included from arch/x86/vdso/vdso2c.c:161:0:
>> arch/x86/vdso/vdso2c.c: In function 'main':
>> arch/x86/vdso/vdso2c.h:118:6: warning: assuming signed overflow
>> does not occur when assuming that (X + c) < X is always false
>> [-Wstrict-overflow] In file included from
>> arch/x86/vdso/vdso2c.c:165:0: arch/x86/vdso/vdso2c.h:118:6:
>> warning: assuming signed overflow does not occur when assuming that
>> (X + c) < X is always false [-Wstrict-overflow]
>>
>> Probably introduced by commit e6577a7ce99a ("x86, vdso: Move the
>> vvar area before the vdso text").
>>
>
> This seems toxic.
>
> I always wonder if we shouldn't use -fwrapv for the kernel...

This particular warning is IMO in a particularly dumb category: GCC
optimizes some code and then warns about a construct that wasn't there
in the original code.  In this case, I think it unrolled a loop and
discovered that one iteration contained a test that was always true.
Big deal.

(OTOH, the code in question was buggy, but not all for the reason that
GCC thought it was.)

--Andy

>
>         -hpa
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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