netdev - Re: [PATCH 2/2] ipv4: Change rt->rt

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Message-Id: <20120723.174312.898352203879290533.davem@davemloft.net>
Date:	Mon, 23 Jul 2012 17:43:12 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ja@....bg
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] ipv4: Change rt->rt_iif encoding.

From: Julian Anastasov <ja@....bg>
Date: Tue, 24 Jul 2012 03:24:38 +0300 (EEST)

> On Mon, 23 Jul 2012, David Miller wrote:
> 
>> Hmmm, the problem is that when we decapsulate VLAN devices, we're left
>> with the parent device's index in skb->skb_iif.
> 
> 	Not sure if it is a problem with VLANs, can be also
> with some virtual devices but may be they use dev_forward_skb()
> where skb_iif is zeroed.

dev_forward_skb() gives the packet to netif_rx() which will thus send
the packet down to __netif_receive_skb() which will set the
skb->skb_iif to the new device's ifindex.  It will not stay at zero
:-)

> 	I was also worrying about ip_mc_output looping packets with
> skb_iif because skb_clone copies the field but may be such loops
> happen only for locally originated traffic where skb_iif starts with
> 0.

Loopback of multicast packets is done in ip_mc_output(), via
the clone that you mention, via dev_loopback_xmit().

dev_loopback_xmit() gives the packet to netif_rx_ni() which again
lands it back at __netif_receive_skb(), which (with my changes)
will adjust the skb->skb_iif to match whatever sits in skb->dev
at the time.

It should be alright.
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