netdev - Re: [PATCH v3] IPv4: Tunnel: Fix effective path mtu calculation

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Date:   Tue, 30 Jun 2020 10:33:35 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:     Oliver Herms <oliver.peter.herms@...il.com>,
        netdev@...r.kernel.org, davem@...emloft.net, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH v3] IPv4: Tunnel: Fix effective path mtu calculation

On Tue, 30 Jun 2020 17:51:41 +0200 Nicolas Dichtel wrote:
> Le 30/06/2020 à 08:22, Jakub Kicinski a écrit :
> [snip]
> > My understanding is that for a while now tunnels are not supposed to use
> > dev->hard_header_len to reserve skb space, and use dev->needed_headroom, 
> > instead. sit uses hard_header_len and doesn't even copy needed_headroom
> > of the lower device.  
> 
> I missed this. I was wondering why IPv6 tunnels uses hard_header_len, if there
> was a "good" reason:
> 
> $ git grep "hard_header_len.*=" net/ipv6/
> net/ipv6/ip6_tunnel.c:                  dev->hard_header_len =
> tdev->hard_header_len + t_hlen;
> net/ipv6/ip6_tunnel.c:  dev->hard_header_len = LL_MAX_HEADER + t_hlen;
> net/ipv6/sit.c:         dev->hard_header_len = tdev->hard_header_len +
> sizeof(struct iphdr);
> net/ipv6/sit.c: dev->hard_header_len    = LL_MAX_HEADER + t_hlen;
> 
> A cleanup would be nice ;-)

I did some archaeological investigatin' yesterday, and I saw
c95b819ad75b ("gre: Use needed_headroom") which converted GRE.
Then I think Pravin used GRE as a base for better ip_tunnel infra 
and the no-hard_header_len-abuse gospel has spread to other IPv4
tunnels. AFAICT IPv6 tunnels were not as lucky, and SIT just got
missed in the IPV4 conversion..