Messages in this thread | | | Date | Wed, 16 Apr 2008 11:39:14 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 1/2] Marker probes in futex.c |
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* Arjan van de Ven (arjan@infradead.org) wrote: > On Wed, 16 Apr 2008 10:48:14 -0400 > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote: > > > * Arjan van de Ven (arjan@infradead.org) wrote: > > > On Wed, 16 Apr 2008 10:00:09 -0400 > > > Mathieu Desnoyers <mathieu.desnoyers@polymtl.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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