Subject: Rate Limitation of Router Reachability Probing for possible dead routers (is Re: ipv6: Do not depend on rt->n in rt6_probe().)

From: YOSHIFUJI Hideaki <[email-masked]>
Date: Tue, 22 Jan 2013 04:28:36 +0900

(2013年01月22日 03:41), YOSHIFUJI Hideaki wrote:
> (2013年01月22日 03:28), Dan Carpenter wrote:
>> Hello YOSHIFUJI Hideaki / 吉藤英明,
>>
>> This is a semi-automatic email about new static checker warnings.
>>
>> The patch 2152caea7196: "ipv6: Do not depend on rt->n in 
>> rt6_probe()." from Jan 17, 2013, leads to the following Smatch 
>> complaint:
>>
>> net/ipv6/route.c:495 rt6_probe()
>> 	 error: we previously assumed 'neigh' could be null (see line 490)
>>
>> net/ipv6/route.c
>>    489	
>>    490		if (!neigh ||
>>                      ^^^^^
>> New test.
>>
>>    491		    time_after(jiffies, neigh->updated + rt->rt6i_idev->cnf.rtr_probe_interval)) {
>>    492			struct in6_addr mcaddr;
>>    493			struct in6_addr *target;
>>    494	
>>    495			neigh->updated = jiffies;
>>                         ^^^^^^^^^^^^^^
>> Old dereference.
>>
>>    496	
>>    497			if (neigh)
>>                             ^^^^^
>> Another new test.
> 
> Oh, right, I'll fix.  Thanks!

Ok, fix is easy, but in fact, we have broken router reachability
probing.

Here rt->n was neighbour entry for (unreachable) router.
The specification says, we SHOUDLD probe such router, but we
should  have some rate limit (once per minute, or so).

We used "rt->n->updated" for this purpose, but now, if NS failed,
we may immediately removes neighbour entry for it.  So,
we might continue sending NS to dead router every 1 second.

Any ideas?

--yoshfuji