netdev - Re: Can we rely on ethernet header padding?

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Message-ID: <20130319142553.3e08e7b9@nehalam.linuxnetplumber.net>
Date:	Tue, 19 Mar 2013 14:25:53 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Michal Kubecek <mkubecek@...e.cz>
Cc:	Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: Can we rely on ethernet header padding?

On Tue, 19 Mar 2013 16:48:06 +0100
Michal Kubecek <mkubecek@...e.cz> wrote:

> On Tue, Mar 19, 2013 at 08:21:35AM -0700, Eric Dumazet wrote:
> > 
> > Normally a driver has NET_SKB_PAD bytes of headroom before the ethernet
> > header, so the bridge code is safe only if all drivers use this
> > NET_SKB_PAD padding on receive side. And they really should for
> > performance reasons.
> > 
> > Better not touch bridge code to catch offending drivers
> 
> That makes sense. Thank you for your reply.
> 

My view is that the bridge code must check before assuming headroom.
But because of that, it means a packet copy would be necessary for cases
where packets arrive without enough headroom.
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