netdev - Re: ip_tunnel mtu calculation

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Date:	Sat, 29 Jun 2013 21:36:39 -0700
From:	Pravin Shelar <pshelar@...ira.com>
To:	Timo Teras <timo.teras@....fi>
Cc:	netdev@...r.kernel.org
Subject: Re: ip_tunnel mtu calculation

On Sat, Jun 29, 2013 at 7:57 AM, Timo Teras <timo.teras@....fi> wrote:
> Hi,
>
> I'm reviewing changes since 3.9 to net-next and observed that, the
> tunnel refactoring had the following change in ip_gre xmit path.
>
> In ip_tunnel_xmit() mtu is now calculated as:
>         if (df)
>                 mtu = dst_mtu(&rt->dst) - dev->hard_header_len
>                                         - sizeof(struct iphdr);
>         else
>                 mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
>
> And it used to be in ip_gre.c: ipgre_tunnel_xmit():
>         if (df)
>                 mtu = dst_mtu(&rt->dst) - dev->hard_header_len - tunnel->hlen;
>         else
>                 mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
>
> I notice that tunnel->hlen is replaced with sizeof(struct iphdr), but
> in case of GRE those are not the same thing. And the refactored
> ip_gre.c does not set hard_header_len either. So it would like the mtu
> is now miscalculated (planning to give a full test-spin for net-next
> next week).
>
> It seems the tunnel->hlen used to be the full length, including
> sizeof(struct iphdr).
>
> But the new, refactored code seems exclude sizeof(struct iphdr) from
> the tunnel->hlen. So would the following be appropriate?
>
This is ip-tunnel layer, skb has gre header pushed. so mtu does not
need to account gre header when compared to skb->len.
But I missed one comparison for mtu check where iph->tot-len is used
rather that skb-len, which is correct length.

gre module is using iph->tot_len for pmtu check which is wrong for
gre-tap device. This bug is there even before restructuring.
I will send out patch for ip-tunnels code for now.

Thanks.
> diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
> index 394cebc..ac3a9a1 100644
> --- a/net/ipv4/ip_tunnel.c
> +++ b/net/ipv4/ip_tunnel.c
> @@ -564,7 +564,7 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
>
>         if (df)
>                 mtu = dst_mtu(&rt->dst) - dev->hard_header_len
> -                                       - sizeof(struct iphdr);
> +                       - tunnel->hlen - sizeof(struct iphdr);
>         else
>                 mtu = skb_dst(skb) ? dst_mtu(skb_dst(skb)) : dev->mtu;
>
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