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SubjectRe: [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@sgi.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@sgi.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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