kernel-hardening - Re: [RFC][PATCH] nfsd: add +1 to reference counting scheme for struct nfsd4

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Message-ID: <CAEXv5_ifq6gO1eBueYGCL21QBAsCQNZROxGCuEYWSNmXz7OTeQ@mail.gmail.com>
Date: Tue, 14 Feb 2017 08:48:26 -0500
From: David Windsor <dwindsor@...il.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: Jeff Layton <jlayton@...chiereds.net>, linux-nfs@...r.kernel.org, 
	netdev@...r.kernel.org, kernel-hardening@...ts.openwall.com, 
	Bruce Fields <bfields@...ldses.org>, Kees Cook <keescook@...omium.org>, 
	"Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [RFC][PATCH] nfsd: add +1 to reference counting scheme for struct nfsd4_session

On Mon, Feb 13, 2017 at 7:12 AM, Christoph Hellwig <hch@...radead.org> wrote:
> On Mon, Feb 13, 2017 at 06:42:56AM -0500, David Windsor wrote:
>> I'm not sure this is a sound argument for not converting to
>> refcount_t.
>
> It's an argument again the way how this patch was sent.  Please take care
> of all the trivial conversions first, and then do anything non-trivial
> on a case by case basis.

This is actually what we're currently doing.

The vast majority of atomic_t to refcount_t conversions are trivial,
as they most always follow the pattern:

1.  Initialize the atomic_t refcounter to 1.
2.  Perform shared object get/put operations.
3.  Free the shared object when its refcount becomes 0.

We've identified a handful of instances in which steps 1 and 3 are
different from above (i.e. initializing refcounts to 0, freeing
objects when refcount is non-zero, etc.).  The above patch addresses
one of these instances in NFS.

Thanks,
David

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