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

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Date:	Fri, 29 Jan 2016 13:16:02 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, Jan 29, 2016 at 12:01 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2016-01-29 at 14:44 -0500, Sowmini Varadhan wrote:
>> On (01/29/16 11:37), Eric Dumazet wrote:
>> >
>> > I have no idea why reading iph->saddr or iph->daddr would not hit the
>> > problem, but accessing the 32bit ipv6 flow label would be an issue.
>> >
>> > Something is fishy.
>>
>> I was wondering about this myself. Even on sparc, I only first
>> ran into the errors for ipv6. I dont know if the fact that the
>> saddr is memcpy'ed masks the error (even though the problem
>> is still there).
>
> Oh right, recent work in flow dissector added all these memcpy()
>
> I was still looking at linux-4.3 ;)

It has to be something recent.  I know back when I wrote the code I
tested it on a few different architectures and had to add a few bits
in __skb_get_poff so that it would read doff as a u8 instead of
bitfield in a u32.

Looking at the code it seems like this should be an easy fix.  Just
swap the two lines that acquire and test the flow label with the check
for dissector_uses_key(... _KEY_FLOW_LABEL).  Then ixgbe will stop
trying to grab flow labels.

- Alex