Subject: Re: [PATCH V2 2/2] ipvs: Extend MTU check to account for IPv6 NAT defrag changes

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 29 Aug 2012 09:02:39 +0200

On Tue, 2012-08-28 at 07:49 -0700, Eric Dumazet wrote:
> On Tue, 2012-08-28 at 16:23 +0200, Jesper Dangaard Brouer wrote:
> > This patch is necessary, to make IPVS work, after Patrick McHardys
> > IPv6 NAT defragmentation changes.
> > 
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> > In V2: the tunnel mode is no longer a special case.
> > 
> >  net/netfilter/ipvs/ip_vs_xmit.c |    9 ++++++++-
> >  1 files changed, 8 insertions(+), 1 deletions(-)
> > 
> > diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> > index 67a3978..56f6d5d 100644
> > --- a/net/netfilter/ipvs/ip_vs_xmit.c
> > +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> > @@ -88,7 +88,14 @@ __ip_vs_dst_check(struct ip_vs_dest *dest, u32 rtos)
> >  static inline bool
> >  __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
> >  {
> > -	if (skb->len > mtu && !skb_is_gso(skb)) {
> > +	if (IP6CB(skb)->frag_max_size) {
> > +		/* frag_max_size tell us that, this packet have been
> > +		 * defragmented by netfilter IPv6 conntrack module.
> > +		 */
> > +		if (IP6CB(skb)->frag_max_size > mtu)
> > +			return true; /* largest fragment violate MTU */
Implicit:         else
     			return false

(if it makes it more clear, not sure)
> > +	}
> > +	else if (skb->len > mtu && !skb_is_gso(skb)) {
> >  		return true; /* Packet size violate MTU size */
> >  	}
> 
> Couldnt you use a single test ?
> 
> if (IP6CB(skb)->frag_max_size > mtu)
> 	return true;
> 
> if (skb->len > mtu && !skb_is_gso(skb))
> 	return true;
> 

Nope, this will not work.

If (IP6CB(skb)->frag_max_size > 0) then we have a defragmented packet,
this means that skb->len cannot be used for MTU checking, because
skb->len is now the total length of all the fragments (which your
solution will fall-through to)

The unreadable version of the function is:

static inline bool
__mtu_check_toobig_v6_unreadable(const struct sk_buff *skb, u32 mtu)
{
       return !!((!IP6CB(skb)->frag_max_size &&
                  (skb->len > mtu && !skb_is_gso(skb)))
                 || IP6CB(skb)->frag_max_size > mtu);
}

Perhaps you like this version better (you seem to use constructions like
this), but I really dislike it, because I (personally) find it
harder/slower to read this kind of code, and I also believe its more
error prone when someone needs to extend this.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer