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

[Oussama Ghorbel]

On Mon, Oct 7, 2013 at 3:02 AM, Hannes Frederic Sowa
<hannes@xxxxxxxxxxxxxxxxxxx> wrote:
> On Sun, Oct 06, 2013 at 08:18:15PM +0100, Oussama Ghorbel wrote:
>> Yes, to summarize, the idea of this patch was to fix the incoherence
>> in the condition of ip6gre_tunnel_change_mtu function
>>
>>   if (new_mtu < 68 ||
>>    new_mtu > 0xFFF8 - dev->hard_header_len - tunnel->hlen)
>>
>> From the ip6gre_tnl_link_config function we can see that:
>> The variable addend is equal the ipv6 header + gre header (including
>> the gre options)
>> On the other hand hard_header_len equal to the header of the lower
>> layer + addend.
>> So the quantity - (dev->hard_header_len + tunnel->hlen) equals - (eth
>> header + ipv6 header + gre header + ipv6 header + gre header) which by
>> no means this would represent anything!  (I've just taken ipv6 over
>> ethernet as example)
>>
>> As we have seen there is another approach to fix this issue is to
>> re-factor the hlen to hold only the length of gre as it's done for
>> ipv4 gre, however the solution provided in the patch seems to be
>> regression risk-less.
>
> I agree, it actually does not worsen the situation:
>
> Acked-by: Hannes Frederic Sowa <hannes@xxxxxxxxxxxxxxxxxxx>
>
>> Although the value hold by hlen is not coherent with the variable name
>> nor with ipv4, I think there is an advantage of the current approach
>> of ipv6 hlen over ipv4 hlen, because we save the calculation of ipv6
>> header each time. Ex:
>> In ipv4 gre and in the function ipgre_header:
>> iph = (struct iphdr *)skb_push(skb, t->hlen + sizeof(*iph));
>> In ipv6 and in the function ip6gre_header
>> ipv6h = (struct ipv6hdr *)skb_push(skb, t->hlen);
>
> I see your point. But we should take care that t->hlen is always initialized,
> regardless if we got a route and outgoing device or not.
>

OK, I'll investigate on this and I'll open a dedicated thread mail/send patch...

> Greetings,
>
>   Hannes
>
Thanks,
Oussama
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