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

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Message-Id: <1242982920.26820.622.camel@twins>
Date:	Fri, 22 May 2009 11:02:00 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] perf_counter: optimize context switch between
 identical inherited contexts

On Fri, 2009-05-22 at 10:57 +0200, Ingo Molnar wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.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? :-)

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