netdev - Re: [PATCH] Fix the upper MTU limit in ipv6 GRE tunnel

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Message-ID: <20131006151351.GB9295@order.stressinduktion.org>
Date:	Sun, 6 Oct 2013 17:13:51 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Oussama Ghorbel <ou.ghorbel@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix the upper MTU limit in ipv6 GRE tunnel

On Sun, Oct 06, 2013 at 03:42:15PM +0100, Oussama Ghorbel wrote:
> The initialization  in ip6gre_tnl_link_config is done as the following:
> static void ip6gre_tnl_link_config(struct ip6_tnl *t, int set_mtu)
> {
> ...
> int addend = sizeof(struct ipv6hdr) + 4;
> ...
> /* Precalculate GRE options length */
> if (t->parms.o_flags&(GRE_CSUM|GRE_KEY|GRE_SEQ)) {
> if (t->parms.o_flags&GRE_CSUM)
> addend += 4;
> if (t->parms.o_flags&GRE_KEY)
> addend += 4;
> if (t->parms.o_flags&GRE_SEQ)
> addend += 4;
> }
> ...
> dev->hard_header_len = rt->dst.dev->hard_header_len + addend;
> ...
> t->hlen = addend;
> ..
> }
> 
> Unless they are other reasons, the hard_header_len is taken into
> account the GRE_KEY, GRE_SEQ ..

But only if a new route is found. The hard_header_len reinitialization is
guarded by a (rt == NULL). We may have not found one on boot up.

> > To make this correct we would have to refactor the usage of the variables a
> > bit as is done in ipv4/ip_tunnel.c. The safest thing would be to leave this
> > check as-is currently although we exclude some allowed mtus.
> >
> > Perhaps you want to take a look how to achieve that? ;)
> >
> Why not, consistency is good ...

Thanks,

  Hannes

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