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

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Date:	Thu, 06 Aug 2009 08:02:22 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Herbert Xu <herbert@...dor.apana.org.au>
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()

Herbert Xu <herbert@...dor.apana.org.au> writes:

> 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.

Why not?  We can sleep on that code path.
Although now that you mention it we should use unlocked_ioctl unless
we actually need the BKL.

Eric
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