netdev - Re: [PATCH net-next] net/vxlan: Avoid unaligned access in vxlan_build

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Date:   Fri, 23 Sep 2016 19:41:34 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     David Miller <davem@...emloft.net>, Jiri Benc <jbenc@...hat.com>,
        Netdev <netdev@...r.kernel.org>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Alexander Duyck <aduyck@...antis.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net-next] net/vxlan: Avoid unaligned access in
 vxlan_build_skb()

On (09/23/16 10:38), Alexander Duyck wrote:
> 
> So basically what I was thinking is we do something like reserving
> NET_IP_ALIGN and continue writing headers to skb->data, but we force
> the tracking for the inner headers into frag[0] so that we can keep
> the inner headers aligned without messing up the alignment for outer
> headers.  In theory the inner offset and all that would still be
> functional but might need a few tweaks.  You could probably even use
> the skb->encapsulation bit to indicate you are doing this.  You could
> almost think of it as us doing something like the inverse of
> pskb_pull_tail.  The general idea here is we want to actually leave
> the data in skb->data, but just reference it from frag[0] so that we
> don't accidentally pull in the 2 byte padding for alignment when
> transmitting the frame.

yes, I think something along this line could do the trick.. I tried
hacking it a bit today for vxlan, and it could be extended for all
these encaps protocols. Let me fix/test this more next week, maybe
we can discuss in Tokyo.

--Sowmini