Date: Fri, 18 Nov 2016 11:47:09 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
To: Michael Ellerman <mpe@ellerman.id.au>
cc: akpm@linux-foundation.org, penberg@kernel.org, rientjes@google.com,
        iamjoonsoo.kim@lge.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        keescook@chromium.org
Subject: Re: [PATCH v2] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR
In-Reply-To: <1479376267-18486-1-git-send-email-mpe@ellerman.id.au>
Message-ID: <alpine.DEB.2.20.1611181146330.26818@east.gentwo.org>
References: <1479376267-18486-1-git-send-email-mpe@ellerman.id.au>
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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.