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

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Message-Id: <1214441979.25608.86.camel@ymzhang>
Date:	Thu, 26 Jun 2008 08:59:39 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Travis <travis@....com>,
	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>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer
	dereference


On Tue, 2008-06-24 at 16:06 +0800, Zhang, Yanmin wrote:
> On Tue, 2008-06-24 at 11:36 +1000, Rusty Russell wrote:
> > 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.
> 
> In function _cpu_up, the panic happens when calling __raw_notifier_call_chain
> at the second time. Kernel doesn't panic when calling it at the first time. If
> just say because of nr_cpu_ids, that's not right.
> 
> By checking source codes, I find function do_boot_cpu is the culprit.
> Consider below call chain:
>  _cpu_up=>__cpu_up=>smp_ops.cpu_up=>native_cpu_up=>do_boot_cpu.
> 
> So do_boot_cpu is called in the end. In do_boot_cpu, if boot_error==true,
> cpu_clear(cpu, cpu_possible_map) is executed. So later on, when _cpu_up
> calls __raw_notifier_call_chain at the second time to report CPU_UP_CANCELED,
> because this cpu is already cleared from cpu_possible_map, get_cpu_sysdev returns
> NULL.
> 
> Many resources are related to cpu_possible_map, so it's better not to change it.
> 
> Below patch against 2.6.26-rc7 fixes it by removing the bit clearing in cpu_possible_map.
> 
> Vegard, would you like to help test it?
> 
> Signed-off-by: Zhang Yanmin <yanmin_zhang@...ux.intel.com>
> 
> ---
> 
> diff -Nraup linux-2.6.26-rc7/arch/x86/kernel/smpboot.c linux-2.6.26-rc7_cpuhotplug/arch/x86/kernel/smpboot.c
> --- linux-2.6.26-rc7/arch/x86/kernel/smpboot.c	2008-06-24 09:03:54.000000000 +0800
> +++ linux-2.6.26-rc7_cpuhotplug/arch/x86/kernel/smpboot.c	2008-06-24 09:04:45.000000000 +0800
> @@ -996,7 +996,6 @@ do_rest:
>  #endif
>  		cpu_clear(cpu, cpu_callout_map); /* was set by do_boot_cpu() */
>  		cpu_clear(cpu, cpu_initialized); /* was set by cpu_init() */
> -		cpu_clear(cpu, cpu_possible_map);
>  		cpu_clear(cpu, cpu_present_map);
>  		per_cpu(x86_cpu_to_apicid, cpu) = BAD_APICID;
>  	}
> 
Andrew,

Would you like to pick up this patch? Rusty Russell <rusty@...tcorp.com.au> acked it.

Thanks,
yanmin


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