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

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Date:	Tue, 7 Oct 2008 19:57:31 +0300
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Matt Mackall" <mpm@...enic.com>
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

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? Side note, why aren't we using
slob_units() here?

>        else
>                return sp->page.private;
>  }
>
>
>
>
>
> --
> Mathematics is the supreme nostalgia of our time.
>
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