Hello Shmulik,
On Wed, Jul 13, 2016, at 16:00, Shmulik Ladkani wrote:
> Hi Florian, Hannes,
>
> On Tue, 12 Jul 2016 08:56:56 +0300 Shmulik Ladkani
> <shmulik.ladk...@ravellosystems.com> wrote:
> > On Sat, 9 Jul 2016 15:22:30 +0200 Florian Westphal <f...@strlen.de> wrote:
> > > >
> > > > > What about setting IPCB FORWARD flag in iptunnel_xmit if
> > > > > skb->skb_iif != 0... instead?
> >
> > I've came up with a suggestion that does not abuse IPSKB_FORWARDED,
> > while properly addressing the use case (and similar ones), without
> > introducing the cost of entering 'skb_gso_validate_mtu' in the local
> > case.
> >
> > How about:
> >
> > @@ -220,12 +220,15 @@ static int ip_finish_output_gso(struct net *net,
> > struct sock *sk,
> > struct sk_buff *skb, unsigned int mtu)
> > {
> > netdev_features_t features;
> > + int local_trusted_gso;
> > struct sk_buff *segs;
> > int ret = 0;
> >
> > - /* common case: locally created skb or seglen is <= mtu */
> > - if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) ||
> > - skb_gso_validate_mtu(skb, mtu))
> > + local_trusted_gso = (IPCB(skb)->flags & IPSKB_FORWARDED) == 0 &&
> > + !(skb_shinfo(skb)->gso_type & SKB_GSO_DODGY);
> > + /* common case: locally created skb from a trusted gso source or
> > + * seglen is <= mtu */
> > + if (local_trusted_gso || skb_gso_validate_mtu(skb, mtu))
> > return ip_finish_output2(net, sk, skb);
> >
> > /* Slowpath - GSO segment length is exceeding the dst MTU.
> >
> > This well addresses the usecase where we have gso-skb arriving from an
> > untrusted source, thus its gso_size is out of our control (e.g. tun/tap,
> > macvtap, af_packet, xen-netfront...).
> >
> > Locally "gso trusted" skbs (the common case) will NOT suffer the
> > additional (possibly costy) call to 'skb_gso_validate_mtu'.
> >
> > Also, if IPSKB_FORWARDED is true, behavior stays exactly the same.
Sorry for the late reply, I am right now travelling and can't review
that closely.
> Any commnets regarding the latest suggestion above?
> I'd like to post it as v2 - if it is in the right direction.
>
> It handles the problem of gso_size values which are not in host's
> control, it addresses the usecase described, and has a benefit of not
> overloading IPSKB_FORWARDED with a new semantic that might be hard to
> maintain.
I liked the fact that setting IPSKB_FORWARDED was only contained in
vxlan and as such wouldn't have as much impact. It was more logically
easy to review for me actually.
> PS:
> Also, if we'd like to pinpoint it even further, we can:
>
> local_trusted_gso = (IPCB(skb)->flags & IPSKB_FORWARDED) == 0 &&
> (!sk || !(skb_shinfo(skb)->gso_type & SKB_GSO_DODGY));
This also looks valid but too random. It seems to be a mix of random
conditions to make it work. ;)
>
> Which ensures only the following conditions go to the expensive
> skb_gso_validate_mtu:
>
> 1. IPSKB_FORWARDED is on
> 2. IPSKB_FORWARDED is off, but sk exists and gso_size is untrusted.
> Meaning: we have a packet arriving from higher layers (sk is set)
> with a gso_size out of host's control.
When can this really happen? In general we don't want to refragment gso
skb's and I think we can only make an exception for vxlan or udp.
> This fine-tuining leaves standard l2 bridging case (e.g 2x taps bridged)
> of a gso skb unaffected, as sk would be NULL.
Bridging does not in general orphan the socket, no?
Bye,
Hannes
