netdev - Re: [PATCH] ipv6: sit: update mtu check to take care of gso packets

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Date:	Tue, 17 Dec 2013 13:34:39 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Michal Schmidt <mschmidt@...hat.com>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv6: sit: update mtu check to take care of gso packets

On Tue, 2013-12-17 at 21:19 +0100, Hannes Frederic Sowa wrote:
> Hi Eric!
> 
> On Mon, Dec 16, 2013 at 06:31:23AM -0800, Eric Dumazet wrote:
> > In the future, we probably want to extend the MTU check to do the
> > right thing for GSO packets...
> 
> How could that look like?

I think I already posted a draft for this.

Idea would be to have a helper to compute the mtu of each segment :

static inline unsigned int gso_size_with_headers(const struct sk_buff *skb)
{
       unsigned int hdrlen = skb_transport_header(skb) - skb_network_header(skb);

       if (skb_shinfo(skb)->gso_type & (SKB_GSO_TCPV4 | SKB_GSO_TCPV6))
               hdrlen += tcp_hdrlen(skb);
       else
               hdrlen += 8; // sizeof(struct udphdr)

       return skb_shinfo(skb)->gso_size + hdrlen;
}

...

       len = skb_is_gso(skb) ? gso_size_with_headers(skb) : skb->len;

       if (unlikely(len > dst_mtu(&rt->dst) && ...



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