linux-kernel - Re: [PATCH] IPv6: Allow the MTU of ipip6 tunnel to be set below 1280

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Message-ID: <CABfLueFvkvVsdLGwS7U5LqM5xDX=-XRt+jt8gY8Lm+=ETPtaGw@mail.gmail.com>
Date:	Thu, 3 Oct 2013 13:37:34 +0100
From:	Oussama Ghorbel <ou.ghorbel@...il.com>
To:	Oussama Ghorbel <oghorbell@...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,
	Oussama Ghorbel <ou.ghorbel@...il.com>
Subject: Re: [PATCH] IPv6: Allow the MTU of ipip6 tunnel to be set below 1280

I will send a new patch, the diff will be:

diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 46ba243..4b51b03 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -1429,9 +1429,17 @@ ip6_tnl_ioctl(struct net_device *dev, struct
ifreq *ifr, int cmd)
 static int
 ip6_tnl_change_mtu(struct net_device *dev, int new_mtu)
 {
-       if (new_mtu < IPV6_MIN_MTU) {
-               return -EINVAL;
+       struct ip6_tnl *tnl = netdev_priv(dev);
+
+       if (tnl->parms.proto == IPPROTO_IPIP) {
+               if (new_mtu < 68)
+                       return -EINVAL;
+       } else {
+               if (new_mtu < IPV6_MIN_MTU)
+                       return -EINVAL;
        }
+       if (new_mtu > 0xFFF8 - dev->hard_header_len)
+               return -EINVAL;
        dev->mtu = new_mtu;
        return 0;
 }


On Sun, Sep 29, 2013 at 5:33 PM, Oussama Ghorbel <ou.ghorbel@...il.com> wrote:
> On Sun, Sep 29, 2013 at 4:45 PM, Hannes Frederic Sowa
> <hannes@...essinduktion.org> wrote:
>> On Sun, Sep 29, 2013 at 10:40:11AM +0100, Oussama Ghorbel wrote:
>>> On Fri, Sep 27, 2013 at 6:03 PM, Hannes Frederic Sowa
>>> <hannes@...essinduktion.org> wrote:
>>> > Ok, let's go with one function per protocol type. Seems easier.
>>> >
>>> > It seems to get more hairy, because it depends on the tunnel driver if the
>>> > prepended ip header is accounted in hard_header_len. :/
>>> >
>>> > I don't know if it works out cleanly. Otherwise I would be ok if the checks
>>> > just get repeated in ip6_tunnel and leave the rest as-is.
>>> >
>>> Yes, It will be the clean way to do it.
>>
>> Fine. :)
>>
>>> >
>>> > Linux currently cannot create "jumbograms" (only the receiving side
>>> > is supported).
>>> >
>>> I understand, but what are the benefit from this limit or the harm
>>> from not specifying it?
>>> Please check this comment from eth.c
>>>
>>> /**
>>>  * eth_change_mtu - set new MTU size
>>>  * @dev: network device
>>>  * @new_mtu: new Maximum Transfer Unit
>>>  *
>>>  * Allow changing MTU size. Needs to be overridden for devices
>>>  * supporting jumbo frames.
>>>  */
>>> int eth_change_mtu(struct net_device *dev, int new_mtu)
>>
>> Hmm, I cannot judge without the full patch. Will it be applicable
>> to all net_devices or just ethernet ones? The name could be a bit
>> misleading. Remindes me a lot of dev_set_mtu based on the signature, btw.
>
> Normally to all net_devices, otherwise it could get complicated to
> check for every dev separately ...
> But, never mind, the comment below solve the issue
>
>>
>>> So wouldn't be a good idea to let our function open for jumbo frames...?
>>
>> Hm, we can document the fact that the function would needed to be updated in
>> that case. But we should not allow to set a mtu which would require jumbograms
>> currently.
>
> OK, sounds a good. I will check the mtu against the limit
> IPV6_MAXPLEN, and document the jumbo restriction ...
>
>>
>> Greetings,
>>
>>   Hannes
>>
>
> Regards,
> Oussama
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