netdev - Re: [net-next PATCH v4 3/3] net: reserve ports for applications using fixed port numbers

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Date:	Tue, 16 Feb 2010 16:25:22 +0200
From:	Octavian Purdila <opurdila@...acom.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Linux Kernel Developers <linux-kernel@...r.kernel.org>,
	Neil Horman <nhorman@...driver.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [net-next PATCH v4 3/3] net: reserve ports for applications using fixed port numbers

On Tuesday 16 February 2010 15:06:26 you wrote:
> Octavian Purdila wrote:
> > On Tuesday 16 February 2010 11:37:04 you wrote:
> >>>  	BUILD_BUG_ON(sizeof(struct inet_skb_parm) > sizeof(dummy_skb->cb));
> >>>
> >>> +	sysctl_local_reserved_ports = kzalloc(65536 / 8, GFP_KERNEL);
> >>> +	if (!sysctl_local_reserved_ports)
> >>> +		goto out;
> >>> +
> >>
> >> I think we should also consider the ports in ip_local_port_range,
> >> since we can only reserve the ports in that range.
> >
> > That is subject to changes at runtime, which means we will have to
> > readjust the bitmap at runtime which introduces the need for additional
> > synchronization operations which I would rather avoid.
> 
> Why? As long as the bitmap is global, this will not be hard.
> 

For the more important point see bellow, but with regard to reallocation, this 
means we need to at least use rcu_read_lock() in the fast path to avoid races 
between freeing the old bitmap and doing a read in progress. 

Granted, that is a light operation, but would it makes things so much more 
complicated just so that we save one memory page (assuming the range is the 
default [32000 64000] one).

> Consider that if one user writes a port number which is beyond
> the ip_local_port_range into ip_local_reserved_ports, we should
> not accept this, because it doesn't make any sense. But with your
> patch, we do.
> 

I think it should be allowed. I see ip_local_reserved_ports and ip_local_range 
as independent settings that can be change at any time.

That way I can flag port 8080 even if the current range is [32000, 64000] and 
then later I can expand the range to [1024, 64000] without loosing the 8080 
reservation.
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