Messages in this thread | | | Date | Wed, 16 Apr 2008 08:32:19 -0700 | From | Arjan van de Ven <> | Subject | Re: [RFC PATCH 1/2] Marker probes in futex.c |
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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.
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