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

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Date:	Sun, 6 Oct 2013 18:19:10 +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 04:36:56PM +0100, Oussama Ghorbel wrote:
> On Sun, Oct 6, 2013 at 4:13 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
> > 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.
> >
> In that case the function will make a return and hlen will be zero.
> Subtracting hlen in  ip6gre_tunnel_change_mtu has no effect ...

Oh yes, somehow I missed that.

We depend on t->hlen when pushing the gre header onto the skb. t->hlen == 0
should never be the case because we assume t->hlen > sizeof(struct ipv6_hdr).

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