linux-kernel - Re: [PATCH 3/3] Compare namespaces when comparing addresses in auth

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Date:	Mon, 11 Apr 2011 08:29:06 -0500
From:	Rob Landley <rlandley@...allels.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-nfs@...r.kernel.org>,
	<containers@...ts.linux-foundation.org>,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Tim Spriggs <tims@...irise.org>,
	Kir Kolyshkin <kir@...allels.com>,
	Pavel Emelyanov <xemul@...allels.com>
Subject: Re: [PATCH 3/3] Compare namespaces when comparing addresses in auth_unix
 cache.

On 04/08/2011 10:08 AM, Rob Landley wrote:
> On 04/04/2011 10:46 PM, Serge E. Hallyn wrote:
>> Does this need to take a reference?  Or is there no way for an
>> entry to outlive its netns?  It sort of looks like
>> svcauth_unix_info_release will ensure that doesn't happen, but
>> I'm not convinced because other parts of the kernel can get
>> to ip_map_init through the struct cache_detail.
> 
> When I wrote this I thought the transport's get_net() and put_net()
> would pin it, but after re-reading, the sunrpc code is disgustingly
> convoluted enough that I can't easily reconstruct my earlier reasoning.
>  I'll add a get_net() and put_net() just to not have to worry about it.

Ah-ha!

Stanislav Kinsbursky helped me reconstruct some of the reasoning: we
don't need to take a reference because we never actually dereference the
struct net *, all we do is feed them to net_eq() which just compares the
pointers for equality.  (The inline function exists so it can compile to
a constant "return 1" when configured out.)

So if the network context did go away (which still shouldn't happen
between the rpc_xprt and the struct nfs_client having references to it)
we still wouldn't have a use-after-free problem because we're not
looking at the memory, just the pointer.

So I shouldn't need to add get_net() and put_net() to the cache.  Sound
about right?

Rob
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