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

Rusty Russell
2008-06-24T01:42:27Z

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.