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

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Date:   Wed, 28 Sep 2016 15:56:44 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net/vxlan: Avoid unaligned access in
 vxlan_build_skb()

On (09/28/16 11:08), Alexander Duyck wrote:
> > - then udp_gro_receive -> vxlan_gro_receive pulls up vxlan header
> >   into linear part, and then..
> 
> This is the point where we need to stop, drop the existing headers,
> call skb_reserve(NET_IP_ALIGN), and then pick back up where we left
> off.  We just have to make sure the skbuff isn't shared.
  :
> In general the interface behind the tunnel shouldn't need to know
> about any of the data in front of the tunnel, so you should be able to
> drop the original header offsets and reset things if you want so you
> could overwrite the old headers.

Except that some of these encapsulations may be doing things like
[inner-mac, outer-ip] fdb learning, so we may not be able to lose
the outer headers at this early point in all cases.

> If I am not mistaken I think the multi-buffer approach is the approach
> taken by other OSes, although for us it is more difficult since we

yes, I think that's approximately what gets done with mblk/mbufs
(with pullups as needed). But as you point out, this type of
->frag_list chaining is a non-trivial change.

> I'm sure we are all going to be talking about this in great detail
> next week at netdev/netconf.. :-)

Agree.

--Sowmini