linux-kernel - Re: [PATCH] Make sure the value in abs() does not get truncated if it is greater than 2^32

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Date:	Thu, 10 Sep 2009 13:39:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rolf Eike Beer <eike-kernel@...tec.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Make sure the value in abs() does not get truncated if
 it is greater than 2^32

On Thu, 3 Sep 2009 23:12:01 +0200
Rolf Eike Beer <eike-kernel@...tec.de> wrote:

> I was just digging a bit around in linux/kernel.h and stumbled over the abs() 
> makro. For me it looks as it would return wrong results on 64 bit platforms 
> if the input value is greater than 2^32.
> 

Yes, it will truncate.

> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index d6320a3..1e6eb66 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -145,7 +145,7 @@ extern int _cond_resched(void);
>  #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
>  
>  #define abs(x) ({				\
> -		int __x = (x);			\
> +		long __x = (x);			\
>  		(__x < 0) ? -__x : __x;		\
>  	})

I wonder if that ends up producing worse code in the normal case.

We could use typeof to address that but what about

	unsigned foo = -1;
	signed bar = abs(foo);

?

That'll currently return 1 but if we use typeof it'll return
0xffffffffU which gets turned into -1.

hrm.
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