Subject: Re: [PATCH] negative dev use in /proc/net/rose_neigh

From: pidoux <[email-masked]>
Date: Tue, 21 Oct 2008 16:14:27 +0200

David,

I tried the patch you proposed.
I agree that it makes sense, however it does not prevent rose->neighbour->use to become negative as displayed in
/proc/net/rose_neigh use field value.

I already looked at all of the pieces of code that do "rose->neighbour->use--;"
The only place that caused use to underflow (negative) is actually inside rose_kill_by_neigh().
This is why I had put a test and a warning there.

I think that inside of sk_for_each() loop in rose_kill_by_neigh() when rose->neighbour->use-- becomes = 0 then 
rose->neighbour should be NULLed and in that case only.
However it seems that rose->neighbour is not actually NULLed for in that case the comparison would not be true anymore 
and the decrement would not occur.

I will soon report the printk output of rose->neighbour->use-- inside of the loop to illustrate what happens here.

Bernard Pidoux

David Miller wrote:
> From: Bernard Pidoux <[email-masked]>
> Date: Tue, 30 Sep 2008 23:44:55 +0200
> 
>> I suspect that the bug was unravelled when we added more than one
>> neighbour per route. The protocole accepts three, but this was not
>> much used during the early days since the density of radio stations
>> on the network was not big (only one node station per destination
>> address usually). The network is now denser with Internet links.
>>
>> However, I don't understand why the test
>>
>> if (rose->neighbour == neigh)
>>
>> does not work, for
>> rose->neighbour = NULL;
>> should prevent next comparison to be valid and thus instruction
>> rose->neighbour->use--; not executed.
>>
>> I have seen that a problem with sk_for_each() macro was identified a
>> while ago into ax25 code. The problem here could be similar ?
> 
> I took a look at this some more.
> 
> That neighbour case loop you mention does set ->neighbour to NULL.
> 
> But other paths do not.  Just look for all of the pieces of code
> that do "rose->neighbour->use--;" and you'll find a few that do not
> NULL it out.
> 
> One such example is rose_kill_by_device().
> 
> That would cause a problem because, while rose_disconnect() marks
> the socket DEAD, it doesn't actually remove it from "rose_list".
> That happens later when the user actually closes the socket or
> some other similar event occurs.
> 
> Therefore if rose_kill_by_neigh() happens next, the ->neighbour could
> match and we'll decrement again.
> 
> But I have no idea how safe it is to NULL out this ->neighbour
> unconditionally.  Lots of code seems to deref the thing unconditionally.
> For example the ROSE_STATE_2 handling in rose_release().
> 
> I guess since rose_disconnect() sets sk->sk_state to TCP_CLOSE, we'll
> be OK here.
> 
> Can you try this patch?
> 
> diff --git a/net/rose/af_rose.c b/net/rose/af_rose.c
> index a7f1ce1..41dd630 100644
> --- a/net/rose/af_rose.c
> +++ b/net/rose/af_rose.c
> @@ -197,6 +197,7 @@ static void rose_kill_by_device(struct net_device *dev)
>  		if (rose->device == dev) {
>  			rose_disconnect(s, ENETUNREACH, ROSE_OUT_OF_ORDER, 0);
>  			rose->neighbour->use--;
> +			rose->neighbour = NULL;
>  			rose->device = NULL;
>  		}
>  	}
> @@ -625,6 +626,7 @@ static int rose_release(struct socket *sock)
>  
>  	case ROSE_STATE_2:
>  		rose->neighbour->use--;
> +		rose->neighbour = NULL;
>  		release_sock(sk);
>  		rose_disconnect(sk, 0, -1, -1);
>  		lock_sock(sk);
> 
>