netdev - Re: IPv6 xfrm GSO fragmentation bug

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Message-ID: <20150831071916.GG25499@secunet.com>
Date:	Mon, 31 Aug 2015 09:19:16 +0200
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	<netdev@...r.kernel.org>
Subject: Re: IPv6 xfrm GSO fragmentation bug

On Sun, Aug 30, 2015 at 04:24:32PM +0800, Herbert Xu wrote:
> Hi Steffen:
> 
> I received a bug report regarding poor IPComp performance over
> IPv6:
> 
> 	https://bugzilla.redhat.com/show_bug.cgi?id=1257952
> 
> It appears to have been caused by
> 
> commit dd767856a36e00b631d65ebc4bb81b19915532d6
> Author: Steffen Klassert <steffen.klassert@...unet.com>
> Date:   Tue Oct 11 01:44:30 2011 +0000
> 
>     xfrm6: Don't call icmpv6_send on local error
> 
> which addded an MTU check without a GSO override.
> 
> Fixing it obviously isn't difficult, but I am wondering why I
> can't find a corresponding patch for IPv4.  Do we need that check
> to be present in xfrm6_output at all? If we do why isn't it needed
> for IPv4 as well?

As far as I remember, this was to catch local message size
errors before __xfrm6_output calls ip6_fragment which would
use icmpv6_send for the error notification. IPv4 does not do
fragmentation in the xfrm4_output functions, so this mtu
check was not needed there.

I think just adding the gso checks should be fine.

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