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

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Date:	Fri, 18 Jul 2014 13:15:17 -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 01:08 PM, Andy Lutomirski wrote:
> 
> i isn't an index in to the syms array at all.  This code is completely
> wrong.  See the patch I sent in reply to Stephen's original email.
> 
> But, to your earlier point, presumably this could warn:
> 
> for (int i = 0; i < 10; i++)
>   if (array[i] > array[5] + 1)
>     fail();
> 
> I think that's absurd.  There's nothing wrong with that code.  A given
> test should have to be always true or always false on *all* loop
> iterations to be flagged, I think.
> 

No, the issue is that gcc is telling you that the code will do the wrong
thing in this case.  Yes, only for one iteration, but still.

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.

	-hpa

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