Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts

Peter Zijlstra
2009-05-22T09:02:14Z

On Fri, 2009-05-22 at 10:57 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
> > On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:
> >
> > > The equivalence of contexts is detected by keeping a pointer in
> > > each cloned context pointing to the context it was cloned from.
> > > To cope with the situation where a context is changed by adding
> > > or removing counters after it has been cloned, we also keep a
> > > generation number on each context which is incremented every time
> > > a context is changed. When a context is cloned we take a copy
> > > of the parent's generation number, and two cloned contexts are
> > > equivalent only if they have the same parent and the same
> > > generation number. In order that the parent context pointer
> > > remains valid (and is not reused), we increment the parent
> > > context's reference count for each context cloned from it.
> >
> > > + u32 generation;
> >
> > Suppose someone writes a malicious proglet that inherits the
> > counters, puts the child to sleep, does 2^32 mods on the counter
> > set, and then wakes up the child.
> >
> > Would that merely corrupt the results, or make the kernel explode?
>
> i think it should be an u64.

So at to make the attack infeasible by death of the attacker and burnout
of the solar-system? :-)