linux-kernel - 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 15:42:27 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	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>,
	Vijay Pandurangan <vijayp@...ayp.ca>,
	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 1:59 PM, Ben Greear <greearb@...delatech.com> wrote:
>
> On 04/30/2016 12:54 PM, Tom Herbert wrote:
>>
>> We've put considerable effort into cleaning up the checksum interface
>> to make it as unambiguous as possible, please be very careful to
>> follow it. Broken checksum processing is really hard to detect and
>> debug.
>>
>> CHECKSUM_UNNECESSARY means that some number of _specific_ checksums
>> (indicated by csum_level) have been verified to be correct in a
>> packet. Blindly promoting CHECKSUM_NONE to CHECKSUM_UNNECESSARY is
>> never right. If CHECKSUM_UNNECESSARY is set in such a manner but the
>> checksum it would refer to has not been verified and is incorrect this
>> is a major bug.
>
>
> Suppose I know that the packet received on a packet-socket has
> already been verified by a NIC that supports hardware checksumming.
>
> Then, I want to transmit it on a veth interface using a second
> packet socket.  I do not want veth to recalculate the checksum on
> transmit, nor to validate it on the peer veth on receive, because I do
> not want to waste the CPU cycles.  I am assuming that my app is not
> accidentally corrupting frames, so the checksum can never be bad.
>
> How should the checksumming be configured for the packets going into
> the packet-socket from user-space?
>
Checksum unnecessary conversion to checksum complete should be done
and then the computed value can be sent to user space. If you want to
take advantage of the value on transmit then we would to change the
interface. But I'm not sure why recalculating the the checksum in the
host is even a problem. With all the advances in checksum offload,
LCO, RCO it seems like that shouldn't be a common case anyway.

> Also, I might want to send raw frames that do have
> broken checksums (lets assume a real NIC, not veth), and I want them
> to hit the wire with those bad checksums.
>
> How do I configure the checksumming in this case?

Just send raw packets with bad checksums (no checksum offload).

Tom

>
>
> Thanks,
> Ben
>
>
>
>>
>> Tom
>>
>> On Sat, Apr 30, 2016 at 12:40 PM, Ben Greear <greearb@...delatech.com>
>> wrote:
>>>
>>>
>>>
>>> On 04/30/2016 11:33 AM, Ben Hutchings wrote:
>>>>
>>>>
>>>> On Thu, 2016-04-28 at 12:29 +0200, Sabrina Dubroca wrote:
>>>>>
>>>>>
>>>>> Hello,
>>>
>>>
>>>
>>>>>>
>>>>>>
>>>>>> http://dmz2.candelatech.com/?p=linux-4.4.dev.y/.git;a=commitdiff;h=8153e983c0e5eba1aafe1fc296248ed2a553f1ac;hp=454b07405d694dad52e7f41af5816eed0190da8a
>>>>>
>>>>>
>>>>> Actually, no, this is not really a regression.
>>>>
>>>>
>>>> [...]
>>>>
>>>> It really is.  Even though the old behaviour was a bug (raw packets
>>>> should not be changed), if there are real applications that depend on
>>>> that then we have to keep those applications working somehow.
>>>
>>>
>>>
>>> To be honest, I fail to see why the old behaviour is a bug when sending
>>> raw packets from user-space.  If raw packets should not be changed, then
>>> we need some way to specify what the checksum setting is to begin with,
>>> otherwise, user-space has not enough control.
>>>
>>> A socket option for new programs, and sysctl configurable defaults for
>>> raw
>>> sockets
>>> for old binary programs would be sufficient I think.
>>>
>>>
>>> Thanks,
>>> Ben
>>>
>>> --
>>> Ben Greear <greearb@...delatech.com>
>>> Candela Technologies Inc  http://www.candelatech.com
>>
>>
>
> --
> Ben Greear <greearb@...delatech.com>
> Candela Technologies Inc  http://www.candelatech.com