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

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 11 Apr 2008 08:04:40 -0700

On Fri, Apr 11, 2008 at 11:55:59AM +0400, Pavel Emelyanov wrote:
> 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.

So tun_net acquires some lock before calling net_generic(), and that
same lock is held when calling tun_exit_net()?  Or is there but a
single tun_net task, so that it will never call tun_net_exit()
at the same time that it calls net_generic() for the tun_net pointer?

> 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.

Agreed, RCU protects the net_generic structure, but not the structures
pointed to by that structure.

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

Just trying to understand what prevents one task from calling
net_generic() to pick up the tun_net pointer at the same time some other
task calls tun_net_exit().

 						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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