linux-kernel - Re: [BUG] SLOB's krealloc() seems bust

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Date:	Tue, 07 Oct 2008 12:13:39 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-mm <linux-mm@...ck.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	akpm <akpm@...uxfoundation.org>
Subject: Re: [BUG] SLOB's krealloc() seems bust


On Tue, 2008-10-07 at 19:57 +0300, Pekka Enberg wrote:
> Hi Matt,
> 
> On Tue, Oct 7, 2008 at 7:37 PM, Matt Mackall <mpm@...enic.com> wrote:
> > SLOB: fix bogus ksize calculation
> >
> > SLOB's ksize calculation was braindamaged and generally harmlessly
> > underreported the allocation size. But for very small buffers, it could
> > in fact overreport them, leading code depending on krealloc to overrun
> > the allocation and trample other data.
> >
> > Signed-off-by: Matt Mackall <mpm@...enic.com>
> > Tested-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> >
> > diff -r 5e32b09a1b2b mm/slob.c
> > --- a/mm/slob.c Fri Oct 03 14:04:43 2008 -0500
> > +++ b/mm/slob.c Tue Oct 07 11:27:47 2008 -0500
> > @@ -515,7 +515,7 @@
> >
> >        sp = (struct slob_page *)virt_to_page(block);
> >        if (slob_page(sp))
> > -               return ((slob_t *)block - 1)->units + SLOB_UNIT;
> > +               return (((slob_t *)block - 1)->units - 1) * SLOB_UNIT;
> 
> Hmm. I don't understand why we do the "minus one" thing here. Aren't
> we underestimating the size now?

The first -1 takes us to the object header in front of the object
pointer. The second -1 subtracts out the size of the header.

But it's entirely possible I'm off by one, so I'll double-check. Nick?

-- 
Mathematics is the supreme nostalgia of our time.

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