Messages in this thread | | | Date | Sat, 5 Oct 2013 16:06:36 +0200 | From | Hannes Frederic Sowa <> | Subject | Re: [PATCH] Fix the upper MTU limit in ipv6 GRE tunnel |
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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@gmail.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
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