netdev - Re: The check of upper MTU limit when changing it in ip6 gre tunnel seems incorrect.

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Message-ID: <CABfLueHGnjQZ7V+waNgxh0o=3nfcANvkKGragO2ZAGxDKNuyAg@mail.gmail.com>
Date:	Fri, 4 Oct 2013 10:58:00 +0100
From:	Oussama Ghorbel <ou.ghorbel@...il.com>
To:	netdev@...r.kernel.org
Subject: Re: The check of upper MTU limit when changing it in ip6 gre tunnel
 seems incorrect.

I have sent this patch to address the issue.

[PATCH] Fix the upper MTU limit in ipv6 GRE tunnel

On Wed, Oct 2, 2013 at 5:44 PM, Oussama Ghorbel <ou.ghorbel@...il.com> wrote:
> The check of upper MTU limit when changing it in ip6 gre tunnel seems incorrect.
> The function in question is:
>
> 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)
>         return -EINVAL;
>     dev->mtu = new_mtu;
>     return 0;
> }
>
> However the dev->hard_header_len and tunnel->hlen are initialized in
> the following way in ip6gre_tnl_link_config():
>
> int addend = sizeof(struct ipv6hdr) + 4;
> ...
> dev->hard_header_len = rt->dst.dev->hard_header_len + addend;
> ...
> t->hlen = addend; // t is ip6_tnl pointer
>
> As you see the information t->hlen is already included in
> dev->hard_header_len, so why calculate it twice?
>
> Thanks
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