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

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Date:	Mon, 8 Oct 2012 12:38:12 +0200
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Eric Dumazet <eric.dumazet@...il.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, Oct 08, 2012 at 11:55:44AM +0200, Eric Dumazet wrote:
> 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.

Sure, we can :)

> 
> 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;
> }

I'll go with the version above and send an updated patch.

Thanks.

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