netdev - Re: [RFC net-next] net: xfrm: support setting an output mark.

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Date:   Thu, 10 Aug 2017 02:13:15 +0900
From:   Lorenzo Colitti <lorenzo@...gle.com>
To:     Steffen Klassert <steffen.klassert@...unet.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Nathan Harold <nharold@...gle.com>,
        Jonathan Basseri <misterikkit@...gle.com>
Subject: Re: [RFC net-next] net: xfrm: support setting an output mark.

On Tue, Aug 8, 2017 at 4:51 PM, Steffen Klassert
<steffen.klassert@...unet.com> wrote:
> I thought you can just split the 32 bit mark into two 16 bit marks
> by setting an appropriate mask at the xfrm and the routing mark.
> But this has the drawback that the socket needs to know how possibly
> tunneled packets should be routed.

Right. And if those bits are already used for something else (e.g.,
Android uses something like 20 bits for marks) then that's not
possible.

Also - the other approach of using the SA mark for routing the
tunneled packet, that has backwards compatibility issues. If someone
is using mark-based routing, and has configured an SA with a mark,
then making the mark influence the routing lookup would change how
those tunnels are routed and possibly break them.

> So we transform the packet and may 'transform' the mark on the packet
> too. This could make sense, but we have to point out the differences
> between the xfrm_mark and the output_mark on the SA very explicit.

Ack. Where should this be pointed out? I've sent out a non-RFC version
to netdev, mostly unchanged but including a fair bit more rationale in
the commit message:

https://patchwork.ozlabs.org/patch/799891/

Or did you mean it should be be documented this in the ip-xfrm man page, or...?