Subject: Re: inet_hash_connect: source port allocation

From: Stephen Hemminger <[email-masked]>
Date: Mon, 29 Nov 2010 11:38:58 -0800

On Mon, 29 Nov 2010 20:07:35 +0100
Eric Dumazet <[email-masked]> wrote:

> Le lundi 29 novembre 2010 à 19:46 +0100, Eric Dumazet a écrit :
> > Le lundi 29 novembre 2010 à 18:29 +0000, John Haxby a écrit :
> > 
> > > Sorry,  I think I phrased my question badly.
> > > 
> > > inet_csk_get_port() starts its search for a free port with
> > > 
> > >      smallest_rover = rover = net_random() % remaining + low;
> > > 
> > > whereas __inet_hash_connect() basically misses out that call to 
> > > net_random() so you get a predictable port number.
> > > 
> > > Is there any good reason why that is the case?
> > > 
> > 
> > It seems random select was done at bind() time only in commit
> > 6df716340da3a6f ([TCP/DCCP]: Randomize port selection)
> > 
> > It probably should be done in autobind too.
> > 
> > 
> 
> I'll test following patch :
> 
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 1b344f3..65c3702 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -466,20 +466,18 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
>  	int twrefcnt = 1;
>  
>  	if (!snum) {
> -		int i, remaining, low, high, port;
> -		static u32 hint;
> -		u32 offset = hint + port_offset;
> +		int remaining, low, high, port;
>  		struct hlist_node *node;
>  		struct inet_timewait_sock *tw = NULL;
>  
>  		inet_get_local_port_range(&low, &high);
>  		remaining = (high - low) + 1;
> +		port = net_random() % remaining + low;
>  
>  		local_bh_disable();
> -		for (i = 1; i <= remaining; i++) {
> -			port = low + (i + offset) % remaining;
> +		do {
>  			if (inet_is_reserved_local_port(port))
> -				continue;
> +				goto next_nolock;
>  			head = &hinfo->bhash[inet_bhashfn(net, port,
>  					hinfo->bhash_size)];
>  			spin_lock(&head->lock);
> @@ -510,16 +508,17 @@ int __inet_hash_connect(struct inet_timewait_death_row *death_row,
>  			tb->fastreuse = -1;
>  			goto ok;
>  
> -		next_port:
> +next_port:
>  			spin_unlock(&head->lock);
> -		}
> +next_nolock:
> +			if (++port > high)
> +				port = low;
> +		} while (--remaining > 0);
>  		local_bh_enable();
>  
>  		return -EADDRNOTAVAIL;
>  
>  ok:
> -		hint += i;
> -
>  		/* Head lock still held and bh's disabled */
>  		inet_bind_hash(sk, tb, port);
>  		if (sk_unhashed(sk)) {
> 

The original algorithm works better than uses if the port space is small
and being reused rapidly. Because the hint in the old algorithm is sequential ports
get used up sequentially.

You should look a the port randomization RFC. The earlier versions of the
RFC were better before the BSD guys started putting in their non-scalable
algorithms :-)

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