linux-kernel - Re: [PATCH 4.4 92/97] ip6_vti: adjust vti mtu according to mtu of lower device

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Date:   Fri, 6 Apr 2018 09:47:14 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     Ben Hutchings <ben.hutchings@...ethink.co.uk>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Petr Vorel <pvorel@...e.cz>,
        Alexey Kodanev <alexey.kodanev@...cle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sasha Levin <alexander.levin@...rosoft.com>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH 4.4 92/97] ip6_vti: adjust vti mtu according to mtu of
 lower device

On Thu, Apr 05, 2018 at 05:36:36PM +0200, Stefano Brivio wrote:
> On Wed, 04 Apr 2018 01:09:16 +0100
> Ben Hutchings <ben.hutchings@...ethink.co.uk> wrote:
> 
> > On Fri, 2018-03-23 at 10:55 +0100, Greg Kroah-Hartman wrote:
> > > 4.4-stable review patch.  If anyone has any objections, please let me know.
> > > 
> > > ------------------
> > > 
> > > From: Alexey Kodanev <alexey.kodanev@...cle.com>
> > > 
> > > 
> > > [ Upstream commit 53c81e95df1793933f87748d36070a721f6cb287 ]  
> > [...]
> > 
> > There are a couple of follow-ups to this:
> > 
> > c6741fbed6dc vti6: Properly adjust vti6 MTU from MTU of lower device
> > 7a67e69a339a vti6: Keep set MTU on link creation or change, validate it
> > 
> > The second of those will fail to build on branches older than 4.10
> > though.  It might be better to revert this one instead.
> 
> Thanks Ben for spotting this.
> 
> Actually,
>   53c81e95df17 ("ip6_vti: adjust vti mtu according to mtu of lower device")
> alone improves things already, despite being "fixed" by
>   c6741fbed6dc ("vti6: Properly adjust vti6 MTU from MTU of lower device")
> 
> With just 53c81e95df17 the MTU of a vti6 interface will be somewhat
> linked to the MTU of the lower layer, but will be underestimated.
> 
> With c6741fbed6dc the calculation of MTU from lower layer will be
> accurate instead.
> 
> However, without
>   7a67e69a339a ("vti6: Keep set MTU on link creation or change, validate it")
> but with
>   53c81e95df17 ("ip6_vti: adjust vti mtu according to mtu of lower device")
> assignment of MTU on link change is discarded, so this would actually
> introduce a bug.
> 
> Fixing
>   7a67e69a339a ("vti6: Keep set MTU on link creation or change, validate it")
> for 4.4 up to 4.9 is trivial, we simply need to adjust for the lack of
>   b96f9afee4eb ("ipv4/6: use core net MTU range checking")
> and reflect the change introduced by
>   f8a554b4aa96 ("vti6: Fix dev->max_mtu setting").
> 
> So, Greg, here comes the backport of
>   7a67e69a339a ("vti6: Keep set MTU on link creation or change, validate it")
> based on latest linux-4.4.y branch, in case you want to keep the existing
> change and add the follow-ups on top. Please let me know if I should submit
> it formally.

Ick, that's a mess.  How about I just revert this patch from the stable
trees, and then someone sends me either a list of git commits to apply,
or patches, for the different trees if it's really needed?

thanks,

greg k-h