netdev - Re: net/arp: ARP cache aging failed.

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Date:   Wed, 23 Nov 2016 10:33:13 +0200 (EET)
From:   Julian Anastasov <ja@....bg>
To:     yuehaibing <yuehaibing@...wei.com>
cc:     davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: net/arp: ARP cache aging failed.


	Hello,

On Wed, 23 Nov 2016, yuehaibing wrote:

> 	As to my topo,HOST1 and HOST3 share one route on HOST2, tcp connection between HOST2 and HOST3 may call tcp_ack to set dst->pending_confirm.
> 	
> So dst_neigh_output may wrongly freshed  n->confirmed which stands for HOST1,however HOST1'MAC had been changed.
> 
> 	The possibility of this occurred Significantly increases ,when ping and TCP transaction are set the same processor affinity on the HOST2.
> 
> 	It seems that the issue is brought in commit 5110effee8fde2edfacac9cd12a9960ab2dc39ea ("net: Do delayed neigh confirmation.").

	Bad news. Problem is not in delayed confirmation but
in the mechanism to use same dst for different neighbours on
LAN. We don't have a dst->neighbour reference anymore.

	For IPv4 this is related to rt->rt_uses_gateway but
also to DST_NOCACHE. In the other cases we can not call
dst_confirm, may be we should lookup the neigh entry instead.
But we need a way to reduce such lookups on every packet,
for example, by remembering in struct sock and checking if
some bits of jiffies (at least 4-5) are changed from
previous lookup.

Regards

--
Julian Anastasov <ja@....bg>