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

From: Cong Wang <[email-masked]>
Date: Tue, 16 Feb 2010 21:06:26 +0800

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.

> 
>>> +	{
>>> +		.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 :)
> 

Oh, I see.

Thanks.