netdev - Re: [PATCH] net: dev_forward_skb(): Scrub packet's per-netns info only when crossing netns

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Date:   Tue, 20 Mar 2018 13:35:00 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Ben Greear <greearb@...delatech.com>
Cc:     Liran Alon <LIRAN.ALON@...CLE.COM>, shmulik.ladkani@...il.com,
        netdev@...r.kernel.org, mrv@...atatu.com, daniel@...earbox.net,
        davem@...emloft.net, linux-kernel@...r.kernel.org,
        yuval.shaia@...CLE.COM, idan.brown@...CLE.COM
Subject: Re: [PATCH] net: dev_forward_skb(): Scrub packet's per-netns info only when crossing netns

Ben Greear <greearb@...delatech.com> writes:

> On 03/20/2018 09:44 AM, Liran Alon wrote:
>>
>>
>> On 20/03/18 18:24, ebiederm@...ssion.com wrote:
>>>
>>> I don't believe the current behavior is a bug.
>>>
>>> I looked through the history.  Basically skb_scrub_packet
>>> started out as the scrubbing needed for crossing network
>>> namespaces.
>>>
>>> Then tunnels which needed 90% of the functionality started
>>> calling it, with the xnet flag added.  Because the tunnels
>>> needed to preserve their historic behavior.
>>>
>>> Then dev_forward_skb started calling skb_scrub_packet.
>>>
>>> A veth pair is supposed to give the same behavior as a cross-over
>>> cable plugged into two local nics.  A cross over cable won't
>>> preserve things like the skb mark.  So I don't see why anyone would
>>> expect a veth pair to preserve the mark.
>>
>> I disagree with this argument.
>>
>> I think that a skb crossing netns is what simulates a real packet
>> crossing physical computers. Following your argument, why would
>> skb->mark should be preserved when crossing netdevs on same netns via
>> routing? But this does today preserve skb->mark.
>>
>> Therefore, I do think that skb->mark should conceptually only be
>> scrubbed when crossing netns. Regardless of the netdev used to cross
>> it.
>
> It should be scrubbed in VETH as well.  That is one way to make virtual routers.  Possibly
> the newer VRF features will give another better way to do it, but you should not break
> things that used to work.
>
> Now, if you want to add a new feature that allows one to configure the kernel (or VETH) for
> a new behavior, then that might be something to consider.
>
>>> Right now I don't see the point of handling packets that don't cross
>>> network namespace boundaries specially, other than to preserve backwards
>>> compatibility.
>
> Well, backwards compat is a big deal all by itself!

Absolutely agreed.

Eric