netdev - Re: [RFC] Kernel unaligned access at __skb_flow

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Date:	Fri, 29 Jan 2016 15:31:54 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Michael Dalton <mwdalton@...gle.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, Jan 29, 2016 at 3:04 PM, Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
> On (01/29/16 15:00), Tom Herbert wrote:
>
>> The sparc documentation is pretty clear
>> http://docs.oracle.com/cd/E19253-01/816-4854/hwovr-2/index.html, seems
>> like unaligned accesses are not allowed in the architecture.
>
> yes, but looks like you  can paper over some of this with
> memcpy (as was happening with the saddr ref in skb_flow_dissect
> that puzzled me and Eric because it did not generate any traps).
>
Well, I am more worried about what happens when an unaligned
encapsulated TPC/IP packet gets into the stack. It seems unlikely we'd
want to replace a bunch of address, seq numbers, etc. with memcpy all
over the stack. ;-)

But even within flow dissector, to be completely correct, we need to
replace all 32-bit accesses with the mempy (flow_label, mpls label,
keyid) and be vigilant about new ones coming in. For that matter, if
we really want to be pedantic, nothing prevents a driver from giving
us a packet with 1 byte alignment in which case we need to consider
access to 16-bit values also! Considering that this is a very narrow
use case (requires encapsulated Ethernet and architecture that is
alignment sensitive), I wonder if we should just punt on flow
dissection when we see non 4-byte alignment and
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is not set.

Tom