Subject: Re: [PATCH] net: ip_finish_output_gso: If skb_gso_network_seglen exceeds MTU, do segmentation even for non IPSKB_FORWARDED skbs

From: Shmulik Ladkani <[email-masked]>
Date: Tue, 12 Jul 2016 08:56:56 +0300

On Sat, 9 Jul 2016 15:22:30 +0200 Florian Westphal <[email-masked]> wrote:
> Shmulik Ladkani <[email-masked]> wrote:
> > I'd appreciate any suggestion how to determine traffic is local OTHER
> > THAN testing IPSKB_FORWARDED; If we have such a way, there wouldn't be an
> > impact on local traffic.
> >   
> > > 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.

Regards,
Shmulik