linux-kernel - Re: [PATCH 22/25] Generic dynamic per cpu refcounting

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Message-ID: <20121129202231.GJ15094@google.com>
Date:	Thu, 29 Nov 2012 12:22:31 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, zab@...hat.com,
	jmoyer@...hat.com, axboe@...nel.dk, viro@...iv.linux.org.uk
Subject: Re: [PATCH 22/25] Generic dynamic per cpu refcounting

On Thu, Nov 29, 2012 at 02:34:52PM -0500, Benjamin LaHaise wrote:
> On Thu, Nov 29, 2012 at 11:29:25AM -0800, Kent Overstreet wrote:
> > There's some kind of symmetry going on here, and if I'd been awake more
> > in college I could probably say exactly why it works, but it does.
> 
> I think the catch is that using only a 32 bit counter is something the 
> user could arbitrarily control the sum of all parts.  I think a 64 bit 
> counter may be required to ensure no overflow occurs.  Otherwise, an 
> overflow could result in a premature free when there are still 2^32 
> objects active thanks to a malicious user (possible on systems with lots 
> of memory these days -- remote, but possible).

That's no different from regular atomic_t - but you're right, we
should be using size_t for anything userspace can manipulate.

Not gonna worry about it in this patch though because the refcount was
an atomic_t before and userspace can only do one get per thread.
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