netdev - Re: [PATCH] net: skb_set_dev do not unconditionally drop ref to dst

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Date:	Mon, 30 Apr 2012 08:53:22 +0200
From:	Frank Blaschka <blaschka@...ux.vnet.ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	arnd@...db.de, davem@...emloft.net, netdev@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH] net: skb_set_dev do not unconditionally drop ref to dst

On Mon, Apr 30, 2012 at 07:51:33AM +0200, Eric Dumazet wrote:
> On Mon, 2012-04-30 at 07:38 +0200, Frank Blaschka wrote:
> > From: Frank Blaschka <frank.blaschka@...ibm.com>
> > 
> > commit 8a83a00b0735190384a348156837918271034144 unconditionally
> > drops dst reference when skb->dev is set. This causes a regression
> > with VLAN and the qeth_l3 network driver. qeth_l3 can not get gw
> > information from the skb coming from the vlan driver. It is only
> > valid to drop the dst in case of different name spaces.
> > 
> > Signed-off-by: Frank Blaschka <frank.blaschka@...ibm.com>
> > ---
> >  net/core/dev.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -1881,8 +1881,8 @@ EXPORT_SYMBOL(netif_device_attach);
> >  #ifdef CONFIG_NET_NS
> >  void skb_set_dev(struct sk_buff *skb, struct net_device *dev)
> >  {
> > -	skb_dst_drop(skb);
> >  	if (skb->dev && !net_eq(dev_net(skb->dev), dev_net(dev))) {
> > +		skb_dst_drop(skb);
> >  		secpath_reset(skb);
> >  		nf_reset(skb);
> >  		skb_init_secmark(skb);
> > 
> 
> You forgot CC Arnd Bergmann <arnd@...db.de> ?
> 
> But we do want to do the skb_dst_drop() in dev_forward_skb()
> 
> Your patch breaks dev_forward_skb() then.
> 
> But apparently this forward path was alredy broken in Arnd patch...
>
Hi Eric,

do you have and suggestions how to fix this regression?
The qeth_l3 driver needs the gw information to address the frame.
The way to look at dst works well for the non vlan case and was
also working for vlan prior Arnds changes. 
> 
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