Subject: Re: [PATCH v2] neigh: set nud_state to NUD_INCOMPLETE when probing router reachability

From: Duan Jiong <[email-masked]>
Date: Mon, 12 May 2014 11:26:10 +0800

于 2014年05月12日 06:04, Hannes Frederic Sowa 写道:
> On Thu, May 8, 2014, at 22:16, Duan Jiong wrote:
>>
>> Since commit 7e98056964("ipv6: router reachability probing"), a router falls
>> into NUD_FAILED will be probed.
>>
>> Now if function rt6_select() selects a router which neighbour state is NUD_FAILED,
>> and at the same time function rt6_probe() changes the neighbour state to NUD_PROBE,
>> then function dst_neigh_output() can directly send packets, but actually the
>> neighbour still is unreachable. If we set nud_state to NUD_INCOMPLETE instead
>> NUD_PROBE, packets will not be sent out until the neihbour is reachable.
>>
>> In addition, because the route should be probes with a single NS, so we must
>> set neigh->probes to neigh_max_probes(), then the neigh timer timeout and function
>> neigh_timer_handler() will not send other NS Messages.
>>
>> Signed-off-by: Duan Jiong <[email-masked]>
>> ---
>> Changes from v1:
>> 	*modify changelog to explain in detail why use neigh_max_probes().
>>
>>  net/core/neighbour.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/core/neighbour.c b/net/core/neighbour.c
>> index 8f8a96e..32d872e 100644
>> --- a/net/core/neighbour.c
>> +++ b/net/core/neighbour.c
>> @@ -1248,8 +1248,8 @@ void __neigh_set_probe_once(struct neighbour *neigh)
>>  	neigh->updated = jiffies;
>>  	if (!(neigh->nud_state & NUD_FAILED))
>>  		return;
>> -	neigh->nud_state = NUD_PROBE;
>> -	atomic_set(&neigh->probes, NEIGH_VAR(neigh->parms, UCAST_PROBES));
>> +	neigh->nud_state = NUD_INCOMPLETE;
>> +	atomic_set(&neigh->probes, neigh_max_probes(neigh));
> 
> Wouldn't it be better if we neigh_suspect the neighbour and leaving the state in NUD_PROBE? We call down to ->output in case neighbour is in NUD_PROBE state, so we must just disable connected 'fast-path'.
> 

You can look into neigh_event_send() called in neigh_resolve_output(), and if neigh->nud_state
still is NUD_PROBE, the neigh_event_send() will return 0, so the packet will still be sent out
without probe.

So, using neigh_suspect is not a good idea.

Thanks,
  Duan
  
> Greetings,
>   Hannes
> .