netdev - Re: [PATCH ipsec-next] xfrm: interface: support collect metadata mode

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Message-ID: <20201202093747.GA85961@gauss3.secunet.de>
Date:   Wed, 2 Dec 2020 10:37:47 +0100
From:   Steffen Klassert <steffen.klassert@...unet.com>
To:     Eyal Birger <eyal.birger@...il.com>
CC:     <herbert@...dor.apana.org.au>, David Miller <davem@...emloft.net>,
        "Jakub Kicinski" <kuba@...nel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH ipsec-next] xfrm: interface: support collect metadata mode

On Fri, Nov 27, 2020 at 02:32:44PM +0200, Eyal Birger wrote:
> Hi Steffen,
> 
> On Fri, Nov 27, 2020 at 11:44 AM Steffen Klassert
> <steffen.klassert@...unet.com> wrote:
> >
> > On Sat, Nov 21, 2020 at 04:28:23PM +0200, Eyal Birger wrote:
> > > This commit adds support for 'collect_md' mode on xfrm interfaces.
> > >
> > > Each net can have one collect_md device, created by providing the
> > > IFLA_XFRM_COLLECT_METADATA flag at creation. This device cannot be
> > > altered and has no if_id or link device attributes.
> > >
> > > On transmit to this device, the if_id is fetched from the attached dst
> > > metadata on the skb. The dst metadata type used is METADATA_IP_TUNNEL
> > > since the only needed property is the if_id stored in the tun_id member
> > > of the ip_tunnel_info->key.
> >
> > Can we please have a separate metadata type for xfrm interfaces?
> >
> > Sharing such structures turned already out to be a bad idea
> > on vti interfaces, let's try to avoid that misstake with
> > xfrm interfaces.
> 
> My initial thought was to do that, but it looks like most of the constructs
> surrounding this facility - tc, nft, ovs, ebpf, ip routing - are built around
> struct ip_tunnel_info and don't regard other possible metadata types.

That is likely because most objects that have a collect_md mode are
tunnels. We have already a second metadata type, and I don't see
why we can't have a third one. Maybe we can create something more
generic so that it can have other users too.

> For xfrm interfaces, the only metadata used is the if_id, which is stored
> in the metadata tun_id, so I think other than naming consideration, the use
> of struct ip_tunnel_info does not imply tunneling and does not limit the
> use of xfrmi to a specific mode of operation.

I agree that this can work, but it is a first step into a wrong direction.
Using a __be64 field of a completely unrelated structure as an u32 if_id
is bad style IMO.

> On the other hand, adding a new metadata type would require changing all
> other places to regard the new metadata type, with a large number of
> userspace visible changes.

I admit that this might have some disadvantages too, but I'm not convinced
that this justifies the 'ip_tunnel_info' hack.