Re: [PATCH v5 3/3] kprobes: arm: enable OPTPROBES for ARM 32

From: Will Deacon
Date: Wed Sep 03 2014 - 06:32:18 EST

On Wed, Sep 03, 2014 at 11:18:04AM +0100, Masami Hiramatsu wrote:
> (2014/09/02 22:49), Jon Medhurst (Tixy) wrote:
> > 1. On SMP systems it's very slow because of kprobe's use of stop_machine
> > for applying and removing probes, this forces the system to idle and
> > wait for the next scheduler tick for each probe change.
> Hmm, agreed. It seems that arm32 limitation of self-modifying code on SMP.
> I'm not sure how we can handle it, but I guess;
> - for some processors which have better coherent cache for SMP, we can
> atomically replace the breakpoint code with original code.

Except that it's not an architected breakpoint instruction, as I mentioned
before. It's also not really a property of the cache.

> - Even if we get an "undefined instruction" exception, its handler can
> ask kprobes if the address is under modifying or not. And if it is,
> we can just return from the exception to retry the execution.

It's not as simple as that -- you could potentially see an interleaving of
the two instructions. The architecture is even broader than that:

Concurrent modification and execution of instructions can lead to the
resulting instruction performing any behavior that can be achieved by
executing any sequence of instructions that can be executed from the
same Exception level,

There are additional guarantees for some instructions (like the architected
BKPT instruction).

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