linux-kernel - Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per

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Date:	Wed, 28 Nov 2007 00:01:57 +0200
From:	pageexec@...email.hu
To:	Andrew Morton <akpm@...ux-foundation.org>, Andi Kleen <ak@...e.de>,
	travis@....com
CC:	Christoph Lameter <clameter@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm: Prevent dereferencing non-allocated per_cpu variables

On 27 Nov 2007 at 13:50, travis@....com wrote:

> Change loops controlled by 'for (i = 0; i < NR_CPUS; i++)' to use
> 'for_each_possible_cpu(i)' when there's a _remote possibility_ of
> dereferencing a non-allocated per_cpu variable involved.

actually, it's not that remote, it happens every time
NR_CPUS > num_possible_cpus(). i ran into this myself
on a dual core box with NR_CPUS=4. due to my rewrite
of the i386 per-cpu segment handling, i actually got
a NULL deref where the vanilla kernel would be accessing
the area of [__per_cpu_start, __per_cpu_end] for each
non-possible CPU (which doesn't crash per se but is
still not correct somehow i think).

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