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

[Kent Overstreet <koverstreet@google.com>]

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.