Subject: Re: LRO/GSO interaction when packets are forwarded

From: Stephen Hemminger <[email-masked]>
Date: Tue, 22 Apr 2008 16:01:30 -0700

On Tue, 22 Apr 2008 22:15:13 +0100
Ben Hutchings <[email-masked]> wrote:

> Kieran Mansley wrote:
> > On Fri, 2008-03-07 at 08:25 -0800, Stephen Hemminger wrote:
> > > On Fri, 07 Mar 2008 14:09:57 +0000
> > > Kieran Mansley <[email-masked]> wrote:
> > > 
> > > > We've seen a couple of problems when using a bridge or IP forwarding
> > > > combined with LRO packets generated by a network device driver.  As you
> > > > know, LRO packets can be either be page based (and passed up with
> > > > lro_receive_page()) or use the skb frag_list (and passed up with
> > > > lro_receive_skb()).  In both cases it is likely that the device driver
> > > > will have set CHECKSUM_UNNECESSARY to indicate that the packet has been
> > > > checksummed by the device, and gso_size to mark it as an LRO packet and
> > > > indicate the actual received MSS.
> > > 
> > > First off, no hardware should ever do LRO on non-local packets. If the
> > > hardware isn't smart enough to do this, I guess the bridge code to have
> > > an API to turn it off. IP should also turn it off if ip_forwarding
> > > is enabled on that device.
> > 
> > If the only way to deal with this is to prevent LRO in those cases,
> > having an API to turn if off would clearly be helpful: working out in
> > the hardware or driver which packets can be forwarded or not is hard,
> > and would probably be a layering violation in itself.  
> <snip>
> 
> Here's a first try at disabling LRO where it should not be used.  I've
> given it a little testing and would appreciate comments on whether this
> is a reasonable approach.
> 
> There's one piece clearly missing: since the disabling of LRO can race
> with packet reception, and since LRO can potentially be reenabled with
> ethtool, the IP forwarding and bridging code needs to detect and drop
> LRO'd packets.
> 
> Ben.
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 7c1d446..d7e8f1f 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -866,6 +866,7 @@ extern struct net_device	*__dev_get_by_name(struct net *net, const char *name);
>  extern int		dev_alloc_name(struct net_device *dev, const char *name);
>  extern int		dev_open(struct net_device *dev);
>  extern int		dev_close(struct net_device *dev);
> +extern void		dev_disable_lro(struct net_device *dev);
>  extern int		dev_queue_xmit(struct sk_buff *skb);
>  extern int		register_netdevice(struct net_device *dev);
>  extern void		unregister_netdevice(struct net_device *dev);
> diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
> index 298e0f4..1bd6631 100644
> --- a/net/bridge/br_if.c
> +++ b/net/bridge/br_if.c
> @@ -387,6 +387,7 @@ int br_add_if(struct net_bridge *br, struct net_device *dev)
>  		goto err2;
>  
>  	rcu_assign_pointer(dev->br_port, p);
> +	dev_disable_lro(dev);
>  	dev_set_promiscuity(dev, 1);
>  
>  	list_add_rcu(&p->list, &br->port_list);
> diff --git a/net/core/dev.c b/net/core/dev.c
> index e1df1ab..62cca32 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -90,6 +90,7 @@
>  #include <linux/if_ether.h>
>  #include <linux/netdevice.h>
>  #include <linux/etherdevice.h>
> +#include <linux/ethtool.h>
>  #include <linux/notifier.h>
>  #include <linux/skbuff.h>
>  #include <net/net_namespace.h>
> @@ -1108,6 +1109,29 @@ int dev_close(struct net_device *dev)
>  }
>  
>  
> +/**
> + *	dev_disable_lro - disable Large Receive Offload on a device
> + *	@dev: device
> + *
> + *	Disable Large Receive Offload (LRO) on a net device.  Must be
> + *	called under RTNL.  This is needed if received packets may be
> + *	forwarded to another interface.
> + */
> +void dev_disable_lro(struct net_device *dev)
> +{
> +	if (dev->ethtool_ops && dev->ethtool_ops->get_flags &&
> +	    dev->ethtool_ops->set_flags) {
> +		u32 flags = dev->ethtool_ops->get_flags(dev);
> +		if (flags & ETH_FLAG_LRO) {
> +			flags &= ~ETH_FLAG_LRO;
> +			dev->ethtool_ops->set_flags(dev, flags);
> +		}
> +	}
> +	WARN_ON(dev->features & NETIF_F_LRO);
> +}
> +EXPORT_SYMBOL(dev_disable_lro);

Maybe add ASSERT_RTNL() instead of comment.

> +
> +
>  static int dev_boot_phase = 1;
>  
>  /*
> diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
> index 6848e47..f88d395 100644
> --- a/net/ipv4/devinet.c
> +++ b/net/ipv4/devinet.c
> @@ -171,6 +171,8 @@ static struct in_device *inetdev_init(struct net_device *dev)
>  	in_dev->dev = dev;
>  	if ((in_dev->arp_parms = neigh_parms_alloc(dev, &arp_tbl)) == NULL)
>  		goto out_kfree;
> +	if (IPV4_DEVCONF(in_dev->cnf, FORWARDING))
> +		dev_disable_lro(dev);
>  	/* Reference in_dev->dev */
>  	dev_hold(dev);
>  	/* Account for reference dev->ip_ptr (below) */
> @@ -1250,6 +1252,8 @@ static void inet_forward_change(struct net *net)
>  	read_lock(&dev_base_lock);
>  	for_each_netdev(net, dev) {
>  		struct in_device *in_dev;
> +		if (on)
> +			dev_disable_lro(dev);
>  		rcu_read_lock();
>  		in_dev = __in_dev_get_rcu(dev);
>  		if (in_dev)
> @@ -1257,8 +1261,6 @@ static void inet_forward_change(struct net *net)
>  		rcu_read_unlock();
>  	}
>  	read_unlock(&dev_base_lock);
> -
> -	rt_cache_flush(0);
>  }

Why did you delete the route cache flush here?