netdev - Re: [Lguest] [PATCH 4/5] lguest: use KVM hypercalls

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Message-ID: <20090415134610.GA11683@gondor.apana.org.au>
Date:	Wed, 15 Apr 2009 21:46:10 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	Matias Zabaljauregui <zabaljauregui@...il.com>, odie@...aau.dk,
	Rusty Russell <rusty@...tcorp.com.au>, lguest@...abs.org,
	virtualization@...ts.osdl.org,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [Lguest] [PATCH 4/5] lguest: use KVM hypercalls

On Wed, Apr 15, 2009 at 06:35:58AM -0700, Eric W. Biederman wrote:
> 
> Because as far as I can tell we would just leak that refcount.
> 
> The poll code does not appear to call back into any of the file
> methods when it frees itself from the wait queue.

OK my suggestion was stupid.

But I still don't see how this race is possible at all.

So process A has a tun fd open and is spinning in poll(2).  Now
process B comes along and deletes that tun device.  Process A's
fd should have a netdev reference that keeps the device and
associated structures alive.

Oh I see what's going on.  We're automatically detaching the
device in uninit.  This is just wrong.  Just because process B
deleted the netdev, process A should not be involutarily detached.

Does anything actually rely on this behaviour?

If not we should just change it to not do that.

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