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Date: Fri, 23 Sep 2016 17:43:41 -0700 From: Alexander Duyck <alexander.duyck@...il.com> To: Sowmini Varadhan <sowmini.varadhan@...cle.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 Fri, Sep 23, 2016 at 4:41 PM, Sowmini Varadhan <sowmini.varadhan@...cle.com> wrote: > 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. Agreed. Keep in mind we only really need it for the architectures that need to set NET_IP_ALIGN so we may want to end up wrapping the code in ifndef checks for HAVE_EFFICIENT_UNALIGNED_ACCESS. - Alex
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