Re: 3.3-rc snmp6 panic

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 15 mars 2012 à 01:25 -0400, Dave Jones a écrit :
> I've been seeing an occasional panic when I shut down my router
> since I put 3.3 on there. It happens about once a week, always during
> shutdown.  It wedges before I can get a good capture of the trace.
> This is the best I've captured so far.. https://twitpic.com/8wh5l5
> (apologies in advance for blurriness)
> 
> From comparing the Code: line, and the objdump output, the code it's
> choking on in mld_sendpack seems to be a skb_dst macro in the NF_HOOK..
> 
> 
>         err = NF_HOOK(NFPROTO_IPV6, NF_INET_LOCAL_OUT, skb, NULL, skb->dev,
>     2dd9:       4c 8b 43 20             mov    0x20(%rbx),%r8
>     2ddd:       e9 00 00 00 00          jmpq   2de2 <mld_sendpack+0x1b2>
> static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
> {
>         /* If refdst was not refcounted, check we still are in a 
>          * rcu_read_lock section
>          */
>         WARN_ON((skb->_skb_refdst & SKB_DST_NOREF) &&
>     2de2:       48 8b 43 58             mov    0x58(%rbx),%rax
>     2de6:       a8 01                   test   $0x1,%al
>     2de8:       0f 85 d2 01 00 00       jne    2fc0 <mld_sendpack+0x390>
>                 !rcu_read_lock_held() &&
>                 !rcu_read_lock_bh_held());
>         return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
>     2dee:       48 83 e0 fe             and    $0xfffffffffffffffe,%rax
>     2df2:       48 89 df                mov    %rbx,%rdi
>     2df5:       ff 50 58                callq  *0x58(%rax)           <-----  BOOM
> 
> 
> This machine is running an snmpd, for my mrtg setup, so the teardown of that
> service is probably what's triggering it. But I can start/stop it in a loop
> as much as I want without it happening, so maybe the kernel needs to accumulate
> some state from it for a while first ?
> 
> Anyone have any ideas what's happening here ?
> 

Hmm, please try :

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 8c2e3ab..22b7664 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -1077,7 +1077,7 @@ struct dst_entry *icmp6_dst_alloc(struct net_device *dev,
 	struct net *net = dev_net(dev);
 
 	if (unlikely(!idev))
-		return NULL;
+		return ERR_PTR(-ENODEV);
 
 	rt = ip6_dst_alloc(&net->ipv6.ip6_dst_ops, dev, 0);
 	if (unlikely(!rt)) {