linux-kernel - Re: [PATCH jiffies] Avoid undefined behavior from signed overflow

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Date:	Sun, 28 Jul 2013 11:46:59 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, john.stultz@...aro.org,
	davem@...emloft.net, arnd@...db.de, mingo@...nel.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH jiffies] Avoid undefined behavior from signed overflow

On Sat, 2013-07-27 at 15:58 -0700, Paul E. McKenney wrote:
> According to the C standard 3.4.3p3, overflow of a signed integer results
> in undefined behavior.  This commit therefore changes the definitions
> of time_after() and time_after_eq() to avoid this undefined behavior.
> The trick is that the subtraction is done using unsigned arithmetic,
> which according to 6.2.5p9 cannot overflow because it is defined as
> modulo arithmetic.  This has the added (though admittedly quite small)
> benefit of shortening two lines of code by four characters each.
> 
> Note that the C standard considers the cast from signed to
> unsigned to be implementation-defined, see 6.3.1.3p3.  However, on a
> two-complement system, an implementation that defines anything other
> than a reinterpretation of the bits is free come to me, and I will be
> happy to act as a witness for its being committed to an insane asylum.
> (Although I have nothing against saturating arithmetic or signals in
> some cases, these things really should not be the default.)
> 
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: John Stultz <john.stultz@...aro.org>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> 
> diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
> index 97ba4e7..97967ba 100644
> --- a/include/linux/jiffies.h
> +++ b/include/linux/jiffies.h
> @@ -101,13 +101,13 @@ static inline u64 get_jiffies_64(void)
>  #define time_after(a,b)		\
>  	(typecheck(unsigned long, a) && \
>  	 typecheck(unsigned long, b) && \
> -	 ((long)(b) - (long)(a) < 0))
> +	 ((long)((b) - (a)) < 0))
>  #define time_before(a,b)	time_after(b,a)
>  
>  #define time_after_eq(a,b)	\
>  	(typecheck(unsigned long, a) && \
>  	 typecheck(unsigned long, b) && \
> -	 ((long)(a) - (long)(b) >= 0))
> +	 ((long)((a) - (b)) >= 0))
>  #define time_before_eq(a,b)	time_after_eq(b,a)
>  



time_after64() & time_after_eq64() probably need the same.




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