Hi Florian, Hannes,
On Tue, 12 Jul 2016 08:56:56 +0300 Shmulik Ladkani
<[email protected]> wrote:
> On Sat, 9 Jul 2016 15:22:30 +0200 Florian Westphal <[email protected]> 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.
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.
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));
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.
This fine-tuining leaves standard l2 bridging case (e.g 2x taps bridged)
of a gso skb unaffected, as sk would be NULL.
Many thanks,
Shmulik
