linux-kernel - 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 14:20:25 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	Octavian Purdila <opurdila@...acom.com>,
	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>
Subject: Re: [net-next PATCH v4 3/3] net: reserve ports for applications
 using fixed port numbers

Le mardi 16 février 2010 à 21:06 +0800, Cong Wang a écrit :
> 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.
> 
> 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 disagree with you. This is perfectly OK.

A port not being flagged in ip_local_reserved_ports doesnt mean it can
be used for allocation.

If you want to really block ports from being used at boot, you could for
example :

# temporarly reduce the ip_local_port_range
echo "61000 61001" >/proc/sys/net/ipv4/ip_local_port_range
# Build our bitmap (could be slow, if a remote database is read)
for port in $LIST_RESERVED_PORT
do
  echo $port >/proc/sys/net/ipv4/ip_local_reserved_ports
done
echo "10000 61000" >/proc/sys/net/ipv4/ip_local_port_range


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