netdev - Re: [PATCH 3.2 085/115] veth: don’t modify ip_summed; doing so treats packets with bad checksums as good.

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Date:	Sat, 30 Apr 2016 18:01:29 -0400
From:	Vijay Pandurangan <vijayp@...ayp.ca>
To:	Ben Greear <greearb@...delatech.com>
Cc:	Tom Herbert <tom@...bertland.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Sabrina Dubroca <sd@...asysnail.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	LKML <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
	akpm@...ux-foundation.org, "David S. Miller" <davem@...emloft.net>,
	Cong Wang <cwang@...pensource.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Evan Jones <ej@...njones.ca>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Phil Sutter <phil@....cc>,
	Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
	Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [PATCH 3.2 085/115] veth: don’t modify ip_summed; doing so treats packets with bad checksums as good.

On Sat, Apr 30, 2016 at 5:52 PM, Ben Greear <greearb@...delatech.com> wrote:
>>
>> Good point, so if you had:
>>
>> eth0 <-> raw <-> user space-bridge <-> raw <-> vethA <-> veth B <->
>> userspace-stub <->eth1
>>
>> and user-space hub enabled this elide flag, things would work, right?
>> Then, it seems like what we need is a way to tell the kernel
>> router/bridge logic to follow elide signals in packets coming from
>> veth. I'm not sure what the best way to do this is because I'm less
>> familiar with conventions in that part of the kernel, but assuming
>> there's a way to do this, would it be acceptable?
>
>
> You cannot receive on one veth without transmitting on the other, so
> I think the elide csum logic can go on the raw-socket, and apply to packets
> in the transmit-from-user-space direction.  Just allowing the socket to make
> the veth behave like it used to before this patch in question should be good
> enough, since that worked for us for years.  So, just an option to modify
> the
> ip_summed for pkts sent on a socket is probably sufficient.

I don't think this is right. Consider:

- App A  sends out corrupt packets 50% of the time and discards inbound data.
- App B doesn't care about corrupt packets and is happy to receive
them and has some way of dealing with them (special case)
- App C is a regular app, say nc or something.

In your world, where A decides what happens to data it transmits,
then
A<--veth-->B and A<---wire-->B will have the same behaviour

but

A<-- veth --> C and A<-- wire --> C will have _different_ behaviour: C
will behave incorrectly if it's connected over veth but correctly if
connected with a wire. That is a bug.

Since A cannot know what the app it's talking to will desire, I argue
that both sides of a message must be opted in to this optimization.






>
>>> There may be no sockets on the vethB port.  And reader/writer is not
>>> a good way to look at it since I am implementing a bi-directional bridge
>>> in
>>> user-space and each packet-socket is for both rx and tx.
>>
>>
>> Sure, but we could model a bidrectional connection as two
>> unidirectional sockets for our discussions here, right?
>
>
> Best not to I think, you want to make sure that one socket can
> correctly handle tx and rx.  As long as that works, then using
> uni-directional sockets should work too.
>
>
> Thanks,
> Ben
>
> --
> Ben Greear <greearb@...delatech.com>
> Candela Technologies Inc  http://www.candelatech.com