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

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Message-ID: <86c71cc0-462c-2365-00ea-7f9e79c204b7@6wind.com>
Date:   Wed, 1 Jul 2020 00:27:34 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Jakub Kicinski <kuba@...nel.org>
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

Le 30/06/2020 à 19:33, Jakub Kicinski a écrit :
> 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.
Thanks for the pointer.

> 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..
Yep, I agree with you, it's probably "historical".