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Date: Sat, 5 Oct 2013 16:06:36 +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 Fri, Oct 04, 2013 at 10:52:13AM +0100, Oussama Ghorbel wrote: > Unlike ipv4, the struct member hlen holds the length of the GRE and ipv6 > headers. This length is also counted in dev->hard_header_len. > Perhaps, it's more clean to modify the hlen to count only the GRE header > without ipv6 header as the variable name suggest, but the simple way to fix > this without regression risk is simply modify the calculation of the limit > in ip6gre_tunnel_change_mtu function. > Verified in kernel version v3.11. > > Signed-off-by: Oussama Ghorbel <ou.ghorbel@...il.com> > --- > net/ipv6/ip6_gre.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c > index 90747f1..41487ab 100644 > --- a/net/ipv6/ip6_gre.c > +++ b/net/ipv6/ip6_gre.c > @@ -1175,9 +1175,8 @@ done: > > static int ip6gre_tunnel_change_mtu(struct net_device *dev, int new_mtu) > { > - struct ip6_tnl *tunnel = netdev_priv(dev); > if (new_mtu < 68 || > - new_mtu > 0xFFF8 - dev->hard_header_len - tunnel->hlen) > + new_mtu > 0xFFF8 - dev->hard_header_len) > return -EINVAL; > dev->mtu = new_mtu; > return 0; Hmmm... dev->hard_header_len is initialized to LL_MAX_HEADER + sizeof(struct ipv6hdr) + 4 but won't include the additional head space needed for GRE_SEQ, GRE_KEY etc. if at time of tunnel creation the routing table did not had a good guess for the outgoing device. 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? ;) Greetings, Hannes -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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