netdev - Re: [PATCH next 3/3] net: Use l3_dev instead of skb->dev for L3 processing

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Date:	Thu, 3 Mar 2016 13:43:50 -0800
From:	Mahesh Bandewar <maheshb@...gle.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Mahesh Bandewar <mahesh@...dewar.net>,
	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	netdev <netdev@...r.kernel.org>, Tim Hockin <thockin@...gle.com>,
	Alex Pollitt <alex.pollitt@...aswitch.com>,
	Matthew Dupre <matthew.dupre@...aswitch.com>
Subject: Re: [PATCH next 3/3] net: Use l3_dev instead of skb->dev for L3 processing

On Wed, Mar 2, 2016 at 8:45 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Mon, Feb 29, 2016 at 2:08 PM, Mahesh Bandewar <mahesh@...dewar.net> wrote:
> > From: Mahesh Bandewar <maheshb@...gle.com>
> >
> > netif_receive_skb_core() dispatcher uses skb->dev device to send it
> > to the packet-handlers (e.g. ip_rcv, ipv6_rcv etc). These packet
> > handlers intern use the device passed to determine the net-ns to
> > further process these packets.  Now with the nomination logic, the
> > dispatcher will call netif_get_l3_dev() helper to select the device
> > to be used for this processing. Since l3_dev is initialized to self,
> > normal packet processing should not change.
> >
>
> So, if I understand your patches correctly, _logically_ the skb is still
> passed into the slave's netns via dev_forward_skb() but now goes over
> the iptable rules from the default netns by only changing the netns
> parameter to these hooks?
>
We are using different dev pointer for L3 processing than skb->dev. All
netns, routing etc, associated with this dev (l3_dev) should be used for L3.

> That is ugly... Logically, you should still need to continue to pass
> the skb upper to the stack in default netns until ip_local_deliver_finish().
>
>
> So, how about adding an iptable hook in ipvlan so that skb will
> continue traverse in the original stack and then moved into slave's
> netns? This might be harder since logically we need an L3 entrance
> to the stack.
>
> Thoughts?

As you mentioned logically we should be able to pass the skb in master's ns
until L3 processing is completed. This patch series attempts to do that by
disassociating this logic from skb->dev and adding it to l3_dev. This should
include not just IPT but all that is done in L3 phase (IPT, routing etc.)
Also since dev->l3_dev is same as dev, this should not break any existing logic.

That's the generic implementation as far as the stack is concerned and IPvlan
uses it to make the IPT hooks symmetric.

Another IPT hook may be good enough (however I haven't
given much thought to it) for IPvlan, but this generic approach will be for
whole of L3. Also currently this I have implemented for the ingress path
but that does not mean the same cannot be extended for the egress path
(in fact I'm thinking about that)