Subject: Re: [net-next PATCH V2] net: codel: Avoid undefined behavior from signed overflow

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 1 Nov 2013 01:50:43 -0700

On Thu, Oct 31, 2013 at 10:10:55PM +0100, Jesper Dangaard Brouer wrote:
> From: Jesper Dangaard Brouer <[email-masked]>
> 
> As described in commit 5a581b367 (jiffies: Avoid undefined
> behavior from signed overflow), according to the C standard
> 3.4.3p3, overflow of a signed integer results in undefined
> behavior.
> 
> To fix this, do as the above commit, and do an unsigned
> subtraction, and interpreting the result as a signed
> two's-complement number.  This is based on the theory from
> RFC 1982 and is nicely described in wikipedia here:
>  https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
> 
> A side-note, I have seen practical issues with the previous logic
> when dealing with 16-bit, on a 64-bit machine (gcc version
> 4.4.5). This were 32-bit, which I have not observed issues with.
> 
> Cc: Paul E. McKenney <[email-masked]>
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>

Looks good to me!

Acked-by: Paul E. McKenney <[email-masked]>

> ---
> 
>  include/net/codel.h |   19 +++++++++++++++----
>  1 files changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/include/net/codel.h b/include/net/codel.h
> index 389cf62..3b04ff5 100644
> --- a/include/net/codel.h
> +++ b/include/net/codel.h
> @@ -72,10 +72,21 @@ static inline codel_time_t codel_get_time(void)
>  	return ns >> CODEL_SHIFT;
>  }
> 
> -#define codel_time_after(a, b)		((s32)(a) - (s32)(b) > 0)
> -#define codel_time_after_eq(a, b)	((s32)(a) - (s32)(b) >= 0)
> -#define codel_time_before(a, b)		((s32)(a) - (s32)(b) < 0)
> -#define codel_time_before_eq(a, b)	((s32)(a) - (s32)(b) <= 0)
> +/* Dealing with timer wrapping, according to RFC 1982, as desc in wikipedia:
> + *  https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
> + * codel_time_after(a,b) returns true if the time a is after time b.
> + */
> +#define codel_time_after(a, b)						\
> +	(typecheck(codel_time_t, a) &&					\
> +	 typecheck(codel_time_t, b) &&					\
> +	 ((s32)((a) - (b)) > 0))
> +#define codel_time_before(a, b) 	codel_time_after(b, a)
> +
> +#define codel_time_after_eq(a, b)					\
> +	(typecheck(codel_time_t, a) &&					\
> +	 typecheck(codel_time_t, b) &&					\
> +	 ((s32)((a) - (b)) >= 0))
> +#define codel_time_before_eq(a, b)	codel_time_after_eq(b, a)
> 
>  /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
>  struct codel_skb_cb {
>