linux-kernel - Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference

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Message-Id: <200806241136.52430.rusty@rustcorp.com.au>
Date:	Tue, 24 Jun 2008 11:36:51 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mike Travis <travis@....com>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Adrian Bunk <bunk@...nel.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org, Gautham R Shenoy <ego@...ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference

On Tuesday 24 June 2008 02:58:44 Mike Travis wrote:
> Rusty Russell wrote:
> > On Monday 23 June 2008 02:29:07 Vegard Nossum wrote:
> >> And the (cpu < nr_cpu_ids) fails because the CPU has just been
> >> offlined (or failed to initialize, but it's the same thing), while
> >> NR_CPUS is the value that was compiled in as CONFIG_NR_CPUS (so the
> >> former check will always be true).
> >>
> >> I don't think it is valid to ask for a per_cpu() variable on a CPU
> >> which does not exist, though
> >
> > Yes it is.  As long as cpu_possible(cpu), per_cpu(cpu) is valid.
> >
> > The number check should be removed: checking cpu_possible() is
> > sufficient.
> >
> > Hope that helps,
> > Rusty.
>
> I don't see a check for index being out of range in cpu_possible().

You're right.  It assumes cpu is < NR_CPUS.  Hmm, I have no idea what's going 
on.  nr_cpu_ids (ignore that it's a horrible name for a bad idea) should be 
fine to test against.

Vegard's analysis is flawed: just because cpu is offline, it still must be < 
nr_cpu_ids, which is based on possible cpus.  Unless something crazy is 
happening, but a quick grep doesn't reveal anyone manipulating nr_cpu_ids.

If changing this fixes the bug, something else is badly wrong...
Rusty.

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