linux-kernel - Re: [PATCH 1/1] x86: Add check for node passed to node_to

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Date:	Fri, 27 Jun 2008 19:24:59 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Mike Travis" <travis@....com>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	mm-commits@...r.kernel.org, "Yinghai Lu" <yhlu.kernel@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] x86: Add check for node passed to node_to_cpumask V3

On Fri, Jun 27, 2008 at 7:10 PM, Mike Travis <travis@....com> wrote:
> Subject: [PATCH 1/1] x86: Add check for node passed to node_to_cpumask V3
>
>  * When CONFIG_DEBUG_PER_CPU_MAPS is set, the node passed to
>    node_to_cpumask and node_to_cpumask_ptr should be validated.
>    If invalid, then a dump_stack is performed and a zero cpumask
>    is returned.
>
> Based on "Fri Jun 27 10:06:06 PDT 2008" tip/master...  ;-)
>
> Signed-off-by: Mike Travis <travis@....com>
> ---
> V2: Slightly different version to remove a compiler warning.
> V3: Redone to reflect moving setup.c -> setup_percpu.c
> ---
>  arch/x86/kernel/setup_percpu.c |   26 +++++++++++++++++++++++---
>  include/asm-x86/topology.h     |    7 ++++++-
>  2 files changed, 29 insertions(+), 4 deletions(-)
>
> --- linux-2.6.tip.orig/arch/x86/kernel/setup_percpu.c
> +++ linux-2.6.tip/arch/x86/kernel/setup_percpu.c
> @@ -346,6 +346,10 @@ int early_cpu_to_node(int cpu)
>        return per_cpu(x86_cpu_to_node_map, cpu);
>  }
>
> +
> +/* empty cpumask */
> +static const cpumask_t cpu_mask_none;
> +
>  /*
>  * Returns a pointer to the bitmask of CPUs on Node 'node'.
>  */
> @@ -358,13 +362,23 @@ cpumask_t *_node_to_cpumask_ptr(int node
>                dump_stack();
>                return &cpu_online_map;
>        }
> -       BUG_ON(node >= nr_node_ids);
> -       return &node_to_cpumask_map[node];
> +       if (node >= nr_node_ids) {
> +               printk(KERN_WARNING
> +                       "_node_to_cpumask_ptr(%d): node > nr_node_ids(%d)\n",
> +                       node, nr_node_ids);
> +               dump_stack();
> +               return (cpumask_t *)&cpu_mask_none;

Hm, I am wondering about this.

There exists an option DEBUG_RODATA ("Write protect kernel read-only
data structures"). I'm guessing this RO protections means that we'll
get page faults (and thus BUG/panic) if this "none" mask is ever
attempted to be changed.

So if CONFIG_DEBUG_RODATA=y, I believe we'll see first this warning,
then a panic (after all). Would it be better to make cpu_mask_none
non-const, in spirit of trying to continue as far as possible? I don't
really know if it matters, though. It seems that fedora kernels at
least ship with a default of DEBUG_RODATA=y.

What you could also do is to make it non-const and then zero it each
time it is requested. Again, this is an error situation in either
case, so maybe it's not worth fussing about.


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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