linux-kernel - Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777 warning

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Date:	Fri, 1 Jun 2007 15:33:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <clameter@....com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Srinivasa Ds <srinivasa@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Dinakar Guniguntala <dino@...ibm.com>, pj@....com,
	simon.derr@...l.net, clameter@...ulhu.engr.sgi.com,
	rientjes@...gle.com
Subject: Re: [RFC] [PATCH] cpuset operations causes Badness at mm/slab.c:777
 warning

On Fri, 1 Jun 2007 15:20:05 -0700 (PDT)
Christoph Lameter <clameter@....com> wrote:

> On Fri, 1 Jun 2007, Andrew Morton wrote:
> 
> > If slab was smart enough, it would have poisoned those 8 bytes to some
> > known pattern, and then checked that they still had that pattern when the
> > memory got freed again.
> 
> So this is new feature request?
> 
> > But it isn't smart enough, so the bug went undetected.
> 
> I should make SLUB put poisoning values in unused areas of a kmalloced 
> object?

hm, I hadn't thought of it that way actually.  I was thinking it was
specific to kmalloc(0) but as you point out, the situation is
generalisable.

Yes, if someone does kmalloc(42) and we satisfy the allocation from the
size-64 slab, we should poison and then check the allegedly-unused 22
bytes.

Please ;)

(vaguely stunned that we didn't think of doing this years ago).

It'll be a large patch, I expect?

Actually, I have this vague memory that slab would take that kmalloc(42)
and would then return kmalloc(64)+22, so the returned memory is
"right-aligned".  This way the existing overrun-detection is applicable to
all kmallocs.  Maybe I dreamed it.

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