netdev - Re: [RFC PATCH v1] tun: Cleanup error handling in tun_set

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Message-ID: <20090806143959.GA29323@gondor.apana.org.au>
Date:	Fri, 7 Aug 2009 00:39:59 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Paul Moore <paul.moore@...com>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH v1] tun: Cleanup error handling in tun_set_iff()

On Thu, Aug 06, 2009 at 07:27:13AM -0700, Eric W. Biederman wrote:
>
> Summarizing:
> 
> tun = __tun_get(tfile);
> if (!tun) { // No tun we are not attached.
> 	 < -------------------- race opportunity
> 	rtnl_lock();
>         tun_set_iff();
>         rtnl_unlock();
> }
> ...
> 
> We don't test if we are attached under the rtnl
> until we get to tun_attach();
> 
> So two threads can both do:
> 
> tun = __tun_get(tfile);
> if (!tun) {
> 	rtnl_lock();
>         tun_set_iff();
>             dev = __dev_get_by_name(net, "not_an_interface_name");
>             if (!dev) {
>                dev = alloc_netdev(....);
>                ...;
>                register_netdev(dev);
>                ...;
>                err = tun_attach(..);
>             }
> 
> 
> Only one thread is in tun_set_iff() at a time, but the other thread
> could have attached the file to a device before the one in tun_attach().

Right, I see what you mean.  However I don't think this is possible
because the ioctl runs under the big kernel lock.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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