netdev - Re: [PATCH 3/3] ipv4: Don't report stale pmtu values to userspace

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Date:	Mon, 08 Oct 2012 11:55:44 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Steffen Klassert <steffen.klassert@...unet.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/3] ipv4: Don't report stale pmtu values to userspace

On Mon, 2012-10-08 at 10:48 +0200, Steffen Klassert wrote:
> We report cached pmtu values even if they are already expired.
> Change this to not report these values after they are expired.
> 
> Signed-off-by: Steffen Klassert <steffen.klassert@...unet.com>
> ---
>  net/ipv4/route.c |   17 +++++++++--------
>  1 files changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 741df67..24b52dd 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -2187,8 +2187,16 @@ static int rt_fill_info(struct net *net,  __be32 dst, __be32 src,
>  	    nla_put_be32(skb, RTA_GATEWAY, rt->rt_gateway))
>  		goto nla_put_failure;
>  
> +	expires = rt->dst.expires;
> +	if (expires) {
> +		if (time_before(jiffies, expires))
> +			expires -= jiffies;

Seeing this racy code, we could fix it as well.

jiffies is volatile and can change between the test and subtraction.

So we could in theory get strange result.

I suggest using :

if (expires) {
	unsigned long now = jiffies;

	if (time_before(now, expires))
		expires -= now;
	else
		expires = 0;
}

or 

if (expires)
	expires = max_t(signed long, 0L, expires - jiffies);




> +		else
> +			expires = 0;
> +	}
> +


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