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

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Message-Id: <201002161306.29708.opurdila@ixiacom.com>
Date:	Tue, 16 Feb 2010 13:06:29 +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 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. 

> > +	{
> > +		.procname	= "ip_local_reserved_ports",
> > +		.data		= NULL, /* initialized in sysctl_ipv4_init */
> > +		.maxlen		= 65536,
> > +		.mode		= 0644,
> > +		.proc_handler	= proc_dobitmap,
> > +	},
> 
> Isn't there an off-by-one here?
> 
> In patch 2/3, you use 0 to set the fist bit, then how about 65535 which
> writes 65536th bit? This is beyond the range of port number.
> 

This seems fine to me, 65535 is the value used by both the port checking 
function and the proc read/write function. And it translates indeed to  
65536th bit, but that is also bit 65535 if you start counting bits from 0 
instead of 1. The usual computing/natural arithmetic confusion for the meaning 
of first :)

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