netdev - Re: [PATCH net] bridge: clean the nf_bridge status when forwarding the skb

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Date:	Fri, 18 Oct 2013 13:35:55 +0200
From:	Antonio Quartulli <antonio@...hcoding.com>
To:	Pablo Neira Ayuso <pablo@...filter.org>
Cc:	Antonio Quartulli <antonio@...n-mesh.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net] bridge: clean the nf_bridge status when forwarding
 the skb

On Fri, Oct 18, 2013 at 01:10:41PM +0200, Pablo Neira Ayuso wrote:
> On Thu, Oct 17, 2013 at 01:37:35PM +0200, Antonio Quartulli wrote:
> > On Thu, Oct 17, 2013 at 04:28:57AM -0700, Pablo Neira Ayuso wrote:
> > > Hi,
> > > > +
> > > > +/**
> > > > + * br_netfilter_skb_free - clean the NF bridge data in an skb
> > > > + * @skb: the skb which the data to free belongs to
> > > > + */
> > > > +void br_netfilter_skb_free(struct sk_buff *skb)
> > > > +{
> > > > +	nf_bridge_put(skb->nf_bridge);
> > > > +	skb->nf_bridge = NULL;
> > > > +}
> > > 
> > > This should be nf_reset.
> > 
> > You think I should directly use nf_reset instead of this function?
> > 
> > I see that nf_reset() cleans up the conntrack part too: does it also become
> > useless once the packet exits the bridge interface?
> 
> The conntrack should not attached if it's forwarded to another netif,
> see dev_forward_skb.
> 
> But I'm not sure what scenario you're trying to handle with this
> change, if you could please elaborate.


This is a sample scenario (nf bridge is on):

[eth0] ---> [br0] ---> [bat0] ---> [br1]

where the relation '[a] ---> [b]' means 'a is enslaved in b' (bat0 is a
batman-adv virtual interface..in this situation it should not matter: it
just removs an header from an incoming skb and delivers it).

The problem I was having was due to an skb entering br0 first and br1 later.
When reaching br1 skb->nf_bridge was != NULL because of the previous processing
in br0.

To clarify, the packet arriving on eth0 is 'delivered' to br0. It is not
forwarded to another port of the bridge. Therefore I am not sure that we should
clean the conntrack part too.

> 
> Perhaps your fix is more conservative to avoid breaking strange setups
> that have been relying on this behaviour. I know of people deploying
> strange configurations using netfilter bridge.
> 

could be.

Cheers,

-- 
Antonio Quartulli

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