Subject: Re: [PATCH]Fix BUG of ip_rt_send_redirect()

From: Stephen Hemminger <[email-masked]>
Date: Thu, 16 Nov 2006 19:51:22 -0800

On Thu, 16 Nov 2006 23:57:50 -0500
Li Yewang <[email-masked]> wrote:

> [1]Summary of the problem:
> On IA32 system, If jiffies - b > 0x7fffffff, router can not send
> redirect packet.unsigned long b = rt->u.dst.rate_last
> +(ip_rt_redirect_load << rt->u.dst.rate_tokens)
> 
> [2]Full description of the problem:
> In linux kernel, if time_after(jiffies, (rt->u.dst.rate_last
> +(ip_rt_redirect_load << rt->u.dst.rate_tokens)) == false,
> router will not send redirect packet. Here define b = rt-
> >u.dst.rate_last +(ip_rt_redirect_load << rt->u.dst.rate_tokens):
> 
> 1. If (jiffies - b <= 0x7fffffff), time_after(jiffies, b) == true,
> router will send redirect packet. 
> 
> 2. If (jiffies - b > 0x7fffffff), time_after(jiffies, b) == false,
> router will not send redirect packet. For example: when I add a router
> after system boot, jiffies = (unsigned long)(-300000), 
> rt->u.dst.rate_last = 0, rt->u.dst.rate_tokens = 0, b = 20, 
> time_after((unsigned long)(-300000), 20) == false, send redirect packet
> can not be send even if router is used in the first time.
> 
> When router send a  redirect packet in time b, and before jiffies
> increased to 0x7fffffff + b, router can send redirect packet.
> But if a redirect packet must be send in 0x80000000+ b, 
> time_after(jiffies, b) == false, redirect packet will not be send also.
> So between time (0x80000000+ b) to time b, router do not send redirect
> packet. That is to say, in a circle of jiffies, router has 24.9 days can
> not send redirect packet (0x80000000/1000/60/60/24=24.9).
> 
>     jiffies                          b
> 
> ------|------------------------------|----------------------|---->time
>  0x80000000+b                        b                0x7fffffff+b
>       |<--jiffies - b < 0x7fffffff-->|
> 
> Following is my patch.
> 
> Signed-off-by: Li Yewang <[email-masked]>
> 
> --- linux-2.6.9/net/ipv4/route.c.org	2006-11-16 08:49:48.000000000 +0800
> +++ linux-2.6.9/net/ipv4/route.c	2006-11-16 08:51:30.000000000 +0800
> @@ -1196,7 +1196,8 @@ void ip_rt_send_redirect(struct sk_buff 
>  	/* No redirected packets during ip_rt_redirect_silence;
>  	 * reset the algorithm.
>  	 */
> -	if (time_after(jiffies, rt->u.dst.rate_last + ip_rt_redirect_silence)
> +	if (time_after(jiffies, rt->u.dst.rate_last + ip_rt_redirect_silence) ||
> +	    time_after(rt->u.dst.rate_last, jiffies))
>  		rt->u.dst.rate_tokens = 0;
>  
>  	/* Too many ignored redirects; do not send anything
> @@ -1212,7 +1213,8 @@ void ip_rt_send_redirect(struct sk_buff 
>  	 */
>  	if (time_after(jiffies,
>  		       (rt->u.dst.rate_last +
> -			(ip_rt_redirect_load << rt->u.dst.rate_tokens)))) {
> +			(ip_rt_redirect_load << rt->u.dst.rate_tokens))) ||
> +	    time_after(rt->u.dst.rate_last, jiffies)) {
>  		icmp_send(skb, ICMP_REDIRECT, ICMP_REDIR_HOST, rt->rt_gateway);
>  		rt->u.dst.rate_last = jiffies;
>  		++rt->u.dst.rate_tokens;
> 
>

Wouldn't making ip_rt_redirect_load and other unsigned long instead of int fix the problem?

Remember time_after() works correctly for values that wraparound.