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

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Message-ID: <CAKgT0Uenb+D44D4g2SieNDf3+P_wnv2WYSBZ+NU_87e011PRjA@mail.gmail.com>
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