netdev - Re: [PATCH] xfrm: don't segment UFO packets

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Message-ID: <20160317110131.GH3347@gauss.secunet.com>
Date:	Thu, 17 Mar 2016 12:01:31 +0100
From:	Steffen Klassert <steffen.klassert@...unet.com>
To:	Jiri Bohac <jbohac@...e.cz>
CC:	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>
Subject: Re: [PATCH] xfrm: don't segment UFO packets

On Thu, Mar 17, 2016 at 11:49:53AM +0100, Jiri Bohac wrote:
> On Thu, Mar 17, 2016 at 11:24:59AM +0100, Steffen Klassert wrote:
> > > > On Wed, Mar 16, 2016 at 05:00:26PM +0100, Jiri Bohac wrote:
> > > Fixes my broken case. 
> > 
> > Is this IPv4 or IPv6? IPv4 should not create a GSO skb
> > if IPsec is done. It checks for rt->dst.header_len
> > in __ip_append_data() and does a fallback to the
> > standard case if rt->dst.header_len is non zero.
> 
> It's IPv6.
> 
> > In IPv6 this check is missing, so this could be the
> > problem if this is IPv6.
> 
> Doesn't the check do exactly the opposite of what the RFC says?
> The RFC wants ESP to be performed first and fragmentation after
> that. UDPv4 currently seems to be doing the opposite. 

No, __ip_append_data() only prepares the packets for fragmentation
and enqueues them. Then __ip_make_skb() dequeues and builds
one skb with a fraglist. Then the xfrm layer is called, so
esp linearizes (unfortunately) the skb and applies the
transformation. Fragmentation happens after that.