netdev - Re: [net PATCH] flow_dissector: Fix unaligned access in __skb_flow_dissector when used by eth_get

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Message-ID: <1454182011.7627.141.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sat, 30 Jan 2016 11:26:51 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	Alexander Duyck <aduyck@...antis.com>,
	Netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Tom Herbert <tom@...bertland.com>
Subject: Re: [net PATCH] flow_dissector: Fix unaligned access in
 __skb_flow_dissector when used by eth_get_headlen

On Sat, 2016-01-30 at 10:36 -0800, Alexander Duyck wrote:

> As far as the NET_IP_ALIGN on the page offset I think it is a horrible
> idea.  Basically it means we have to allocate at least 1K more space
> than we need since page sizes are powers of 2, and buffer sizes in
> hardware are measured in 1K increments.

I thought Ethernet frames were something like ~1500 bytes.

Can't you program the NIC to not receive frames bigger than MTU + 18, or
at least 2046 ?

Of course, if the NIC is able to push 2048 bytes (instead of 2046 max),
using an offset of 2 is not going to work.