netdev - Re: Misalignment, MIPS, and ip

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Message-Id: <20161208.181453.1681964885150767592.davem@davemloft.net>
Date:   Thu, 08 Dec 2016 18:14:53 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     Jason@...c4.com
Cc:     dave.taht@...il.com, netdev@...r.kernel.org,
        linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
        wireguard@...ts.zx2c4.com
Subject: Re: Misalignment, MIPS, and ip_hdr(skb)->version

From: "Jason A. Donenfeld" <Jason@...c4.com>
Date: Thu, 8 Dec 2016 23:20:04 +0100

> Hi David,
> 
> On Thu, Dec 8, 2016 at 1:37 AM, David Miller <davem@...emloft.net> wrote:
>> You really have to land the IP header on a proper 4 byte boundary.
>>
>> I would suggest pushing 3 dummy garbage bytes of padding at the front
>> or the end of your header.
> 
> Are you sure 3 bytes to get 4 byte alignment is really the best? I was
> thinking that adding 1 byte to get 2 byte alignment might be better,
> since it would then ensure that the subsequent TCP header winds up
> being 4 byte aligned. Or is this in fact not the desired trade off,
> and so I should stick with the 3 bytes you suggested?

If the IP header is 4 byte aligned, the TCP header will be as well.