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

Eric Dumazet
2012-08-29T08:43:05Z

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@redhat.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.