linux-kernel - Re: [RFC PATCH 1/2] Marker probes in futex.c

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Date:	Wed, 16 Apr 2008 11:39:14 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	prasad@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>
Subject: Re: [RFC PATCH 1/2] Marker probes in futex.c

* Arjan van de Ven (arjan@...radead.org) wrote:
> On Wed, 16 Apr 2008 10:48:14 -0400
> Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> 
> > * Arjan van de Ven (arjan@...radead.org) wrote:
> > > On Wed, 16 Apr 2008 10:00:09 -0400
> > > Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > > 
> > > 
> > > > > > > If we want to support NMI context and have the ability to
> > > > > > > instrument preemptable code without too much headache, we
> > > > > > > must insure that every modification will leave the code in a
> > > > > > > "correct" state and that we do not grow the size of any
> > > > > > > reachable instruction.  Also, we must insure gcc did not put
> > > > > > > code between these instructions. Modifying non-relocatable
> > > > > > > instructions would also be a pain, since we would have to
> > > > > > > deal with instruction pointer relocation in the breakpoint
> > > > > > > code when the code modification is being done.
> > > > > 
> > > > > you also need to make sure no cpu is executing that code ever.. 
> > > > > but you already deal with that right?
> > > > > 
> > > > 
> > > > By "insure that every modification will leave the code in a
> > > > "correct" state", I mean that at any given time before, during or
> > > > after the code modification, if an NMI comes on any CPU and try
> > > > to run the modified code, it should have a valid version of the
> > > > code to execute. Does it make more sense ?
> > > 
> > > I understand your words. My concern is that I don't quite
> > > understand how you guarantee that you'll not be executing the code
> > > you're modifying. Just saying "it's consistent before and after"
> > > sounds nice but probably isn't enough to be safe.
> > > 
> > Ah, I see. I insert a breakpoint and execute a bypass rather than the
> > code being modified. I only put back the 1st instruction byte after
> > the rest of the instruction has been modified.
> 
> sorry but I'm not convinced that that is safe without a real exclusion mechanism.

I'll post the patchset soon. Please have a look at the rather lengthy
comment in arch/x86/kernel/immediate.c for details. I'll be glad to
answer any question that remains.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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