linux-kernel - Re: 2.6.28-rc6-git1 -- BUG: unable to handle kernel paging request at ffff8800be8b0019

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Message-ID: <84144f020811270613t3f0258ddxac52abb9a447bf40@mail.gmail.com>
Date:	Thu, 27 Nov 2008 16:13:11 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Hugh Dickins" <hugh@...itas.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Miles Lane" <miles.lane@...il.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Christoph Lameter" <cl@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>, "Tejun Heo" <htejun@...il.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Vegard Nossum" <vegard.nossum@...il.com>
Subject: Re: 2.6.28-rc6-git1 -- BUG: unable to handle kernel paging request at ffff8800be8b0019

Hi Hugh,

(I'm adding Vegard to the cc as he pointed out the same thing.)

On Thu, Nov 27, 2008 at 3:54 PM, Hugh Dickins <hugh@...itas.com> wrote:
> I think you're looking at a 2.6.28-rc5 sprint_symbol() there:
> the world has moved on since those days.  I changed it to use the
> supplied "buffer" instead of local "namebuf" in 2.6.28-rc6, so we
> have to wonder if my patch is to blame - though I don't see it.
>
> Sorry, I'm eating lunch then about to go out for a couple of hours:
> can't look into it now, but maybe this info will help you to make
> better sense of what's going on.

Oh, right. I think I see where this is going. The buffer is coming
from sysfs and is PAGE_SIZE long. In SLUB, we do check for overflows
but list_locations() allows us to get as close as 100 bytes bytes from
the edge of the page. Unfortunately, kallsyms_lookup() does:

  const char *kallsyms_lookup(unsigned long addr,
                              unsigned long *symbolsize,
                              unsigned long *offset,
                              char **modname, char *namebuf)
  {
          namebuf[KSYM_NAME_LEN - 1] = 0;
          namebuf[0] = 0;

where KSYM_NAME_LEN is 128 which will tip us over the page edge
potentially triggering an oops. The proper fix is probably to pass a
length of namebuf to kallsyms_lookup() but it's probably best to
revert the patch this late in the release cycle?

                        Pekka
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