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

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Message-ID: <1346229785.2522.8.camel@edumazet-glaptop>
Date:	Wed, 29 Aug 2012 01:43:05 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	netdev@...r.kernel.org, Patrick McHardy <kaber@...sh.net>,
	lvs-devel@...r.kernel.org, Julian Anastasov <ja@....bg>,
	Simon Horman <horms@...ge.net.au>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Hans Schillstrom <hans@...illstrom.com>,
	Wensong Zhang <wensong@...ux-vs.org>,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH V2 2/2] ipvs: Extend MTU check to account for IPv6 NAT
 defrag changes

On Wed, 2012-08-29 at 09:02 +0200, Jesper Dangaard Brouer wrote:
> 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 <brouer@...hat.com>
> > > ---
> > > 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)
> 

If the packet was not fragmented, its was a single frame.

But if this frame length is above mtu, packet is not too big ?

Sorry if its a stupid question.



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