Subject: Re: [PATCH] conntrack: Reduce conntrack count in nf_conntrack_free()

From: Patrick McHardy <[email-masked]>
Date: Wed, 25 Mar 2009 14:39:16 +0100

Eric Dumazet wrote:
> I have a litle problem on __nf_conntrack_find() being exported.
> 
> Problem is that with SLAB_DESTROY_BY_RCU we must take a reference on object
> to recheck it. So ideally  only nf_conntrack_find_get() should be used,
> or callers of __nf_conntrack_find() should lock nf_conntrack_lock
> (as properly done for example in net/netfilter/nf_conntrack_netlink.c, line 1292)
> 
> Here is preliminary patch for review (not tested at all, its 4h50 am here :) )
> 
> Could you help me, by checking __nf_conntrack_find() use in net/netfilter/xt_connlimit.c ?
> and line 1246 of net/netfilter/nf_conntrack_netlink.c
> 
> This part is a litle bit gray for me. :)

In case of xt_connlimit, it seems fine to just take a reference.
In case of ctnetlink, keeping the unreferenced lookup under the
lock seems fine. We unfortunately have to export some internals
like nf_conntrack lock for ctnetlink anyways, so I don't think
it would be worth to change it to take references and unexport
the lookup function.

> +/*
> + * Warning :
> + * - Caller must take a reference on returned object
> + *   and recheck nf_ct_tuple_equal(tuple, &h->tuple)
> + * OR
> + * - Caller must lock nf_conntrack_lock before calling this function
> + */
>  struct nf_conntrack_tuple_hash *
>  __nf_conntrack_find(struct net *net, const struct nf_conntrack_tuple *tuple)
>  {
>  	struct nf_conntrack_tuple_hash *h;
> -	struct hlist_node *n;
> +	struct hlist_nulls_node *n;
>  	unsigned int hash = hash_conntrack(tuple);
>  
>  	/* Disable BHs the entire time since we normally need to disable them
>  	 * at least once for the stats anyway.
>  	 */
>  	local_bh_disable();
> -	hlist_for_each_entry_rcu(h, n, &net->ct.hash[hash], hnode) {
> +begin:
> +	hlist_nulls_for_each_entry_rcu(h, n, &net->ct.hash[hash], hnnode) {
>  		if (nf_ct_tuple_equal(tuple, &h->tuple)) {
>  			NF_CT_STAT_INC(net, found);
>  			local_bh_enable();
> @@ -261,6 +270,13 @@ __nf_conntrack_find(struct net *net, const struct nf_conntrack_tuple *tuple)
>  		}
>  		NF_CT_STAT_INC(net, searched);
>  	}
> +	/*
> +	 * if the nulls value we got at the end of this lookup is
> +	 * not the expected one, we must restart lookup.
> +	 * We probably met an item that was moved to another chain.
> +	 */
> +	if (get_nulls_value(n) != hash)
> +		goto begin;

Are you sure this is enough? An entry might have been reused and added
to the same chain I think, so I think we need to recheck the tuple.

>  	local_bh_enable();
>  
>  	return NULL;
> @@ -275,11 +291,18 @@ nf_conntrack_find_get(struct net *net, const struct nf_conntrack_tuple *tuple)
>  	struct nf_conn *ct;
>  
>  	rcu_read_lock();
> +begin:
>  	h = __nf_conntrack_find(net, tuple);
>  	if (h) {
>  		ct = nf_ct_tuplehash_to_ctrack(h);
>  		if (unlikely(!atomic_inc_not_zero(&ct->ct_general.use)))
>  			h = NULL;
> +		else {
> +			if (unlikely(!nf_ct_tuple_equal(tuple, &h->tuple))) {
> +				nf_ct_put(ct);
> +				goto begin;

Ah I see, the hash comparison above is only an optimization?

> +			}
> +		}
>  	}
>  	rcu_read_unlock();