linux-kernel - Re: [PATCH v2] slab: Add POISON_POINTER_DELTA to ZERO_SIZE

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Date:   Fri, 18 Nov 2016 11:47:09 -0600 (CST)
From:   Christoph Lameter <cl@...ux.com>
To:     Michael Ellerman <mpe@...erman.id.au>
cc:     akpm@...ux-foundation.org, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        keescook@...omium.org
Subject: Re: [PATCH v2] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR

On Thu, 17 Nov 2016, Michael Ellerman wrote:

> Currently ZERO_OR_NULL_PTR() uses a trick of doing a single check that
> x <= ZERO_SIZE_PTR, and ignoring the fact that it also matches 1-15.

Well yes that was done so we do not add too many branches all over the
kernel.....


> That no longer really works once we add the poison delta, so split it
> into two checks. Assign x to a temporary to avoid evaluating it
> twice (suggested by Kees Cook).

And now you are doing just that.