Subject: Re: [PATCH 3/14][TUN]: Introduce the tun_net structure.

From: Pavel Emelyanov <[email-masked]>
Date: Fri, 11 Apr 2008 11:55:59 +0400

Paul E. McKenney wrote:
> On Thu, Apr 10, 2008 at 07:06:24PM +0400, Pavel Emelyanov wrote:
>> This is the first step in making tuntap devices work in net 
>> namespaces. The structure mentioned is pointed by generic
>> net pointer with tun_net_id id, and tun driver fills one on 
>> its load. It will contain only the tun devices list.
>>
>> So declare this structure and introduce net init and exit hooks.
> 
> OK, I have to ask...  What prevents someone else from invoking
> net_generic() concurrently with a call to tun_exit_net(), potentially
> obtaining a pointer to the structure that tun_exit_net() is about
> to kfree()?

It's the same as if the tun_net was directly pointed by the struct 
net. Nobody can grant, that the pointer got by you from the struct
net is not going to become free, unless you provide this security
by yourself.

But if you call net_generic to get some pointer other than tun_net,
then you're fine (due to RCU), providing you play the same rules with
the pointer you're getting.

Maybe I'm missing something in your question, can you provide some
testcase, that you suspect may cause an OOPS?

> 						Thanx, Paul
> 
>> Signed-off-by: Pavel Emelyanov <[email-masked]>
>>
>> ---
>>  drivers/net/tun.c |   53 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
>>  1 files changed, 52 insertions(+), 1 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 7b816a0..9bfba02 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -63,6 +63,7 @@
>>  #include <linux/if_tun.h>
>>  #include <linux/crc32.h>
>>  #include <net/net_namespace.h>
>> +#include <net/netns/generic.h>
>>
>>  #include <asm/system.h>
>>  #include <asm/uaccess.h>
>> @@ -73,6 +74,11 @@ static int debug;
>>
>>  /* Network device part of the driver */
>>
>> +static unsigned int tun_net_id;
>> +struct tun_net {
>> +	struct list_head dev_list;
>> +};
>> +
>>  static LIST_HEAD(tun_dev_list);
>>  static const struct ethtool_ops tun_ethtool_ops;
>>
>> @@ -873,6 +879,37 @@ static const struct ethtool_ops tun_ethtool_ops = {
>>  	.set_rx_csum	= tun_set_rx_csum
>>  };
>>
>> +static int tun_init_net(struct net *net)
>> +{
>> +	struct tun_net *tn;
>> +
>> +	tn = kmalloc(sizeof(*tn), GFP_KERNEL);
>> +	if (tn == NULL)
>> +		return -ENOMEM;
>> +
>> +	INIT_LIST_HEAD(&tn->dev_list);
>> +
>> +	if (net_assign_generic(net, tun_net_id, tn)) {
>> +		kfree(tn);
>> +		return -ENOMEM;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +static void tun_exit_net(struct net *net)
>> +{
>> +	struct tun_net *tn;
>> +
>> +	tn = net_generic(net, tun_net_id);
>> +	kfree(tn);
>> +}
>> +
>> +static struct pernet_operations tun_net_ops = {
>> +	.init = tun_init_net,
>> +	.exit = tun_exit_net,
>> +};
>> +
>>  static int __init tun_init(void)
>>  {
>>  	int ret = 0;
>> @@ -880,9 +917,22 @@ static int __init tun_init(void)
>>  	printk(KERN_INFO "tun: %s, %s\n", DRV_DESCRIPTION, DRV_VERSION);
>>  	printk(KERN_INFO "tun: %s\n", DRV_COPYRIGHT);
>>
>> +	ret = register_pernet_gen_device(&tun_net_id, &tun_net_ops);
>> +	if (ret) {
>> +		printk(KERN_ERR "tun: Can't register pernet ops\n");
>> +		goto err_pernet;
>> +	}
>> +
>>  	ret = misc_register(&tun_miscdev);
>> -	if (ret)
>> +	if (ret) {
>>  		printk(KERN_ERR "tun: Can't register misc device %d\n", TUN_MINOR);
>> +		goto err_misc;
>> +	}
>> +	return 0;
>> +
>> +err_misc:
>> +	unregister_pernet_gen_device(tun_net_id, &tun_net_ops);
>> +err_pernet:
>>  	return ret;
>>  }
>>
>> @@ -899,6 +949,7 @@ static void tun_cleanup(void)
>>  	}
>>  	rtnl_unlock();
>>
>> +	unregister_pernet_gen_device(tun_net_id, &tun_net_ops);
>>  }
>>
>>  module_init(tun_init);
>> -- 
>> 1.5.3.4
>>
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