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

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Message-ID: <20121129204531.GK15094@google.com>
Date:	Thu, 29 Nov 2012 12:45:31 -0800
From:	Kent Overstreet <koverstreet@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Benjamin LaHaise <bcrl@...ck.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 12:42:17PM -0800, Andi Kleen wrote:
> Kent Overstreet <koverstreet@...gle.com> writes:
> 
> > 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.
> 
> The regular atomic_t is limited in ways that you are not.
> See my original mail.

I don't follow, can you explain?
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