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Date: Sun, 6 Oct 2013 15:42:15 +0100 From: Oussama Ghorbel <ou.ghorbel@...il.com> To: Oussama Ghorbel <ou.ghorbel@...il.com>, "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 Sat, Oct 5, 2013 at 3:06 PM, Hannes Frederic Sowa <hannes@...essinduktion.org> wrote: > 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. > This hard_header_len initialization that you have shown above is taken from ip6gre_tunnel_setup, however this same variable seems to be reinitialized in ip6gre_tnl_link_config() which are called from ip6gre_newlink() 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 .. > 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 ... > Greetings, > > Hannes > Thanks, Oussama -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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