Re: [PATCH v3 1/5] arm64: Kprobes with single stepping support

[David Long]

On 11/18/14 08:28, Jon Medhurst (Tixy) wrote:
> On Tue, 2014-11-18 at 01:32 -0500, David Long wrote:
> > From: Sandeepa Prabhu <sandeepa.prabhu@xxxxxxxxxx>
> >
> > Add support for basic kernel probes(kprobes) and jump probes
> > (jprobes) for ARM64.
> >
> > Kprobes will utilize software breakpoint and single step debug
> > exceptions supported on ARM v8.
> >
> > software breakpoint is placed at the probe address to trap the
> > kernel execution into kprobe handler.
> >
> > ARM v8 support single stepping to be enabled while exception return
> > (ERET) with next PC in exception return address (ELR_EL1).
> > kprobe handler prepares a executable memory slot for out-of-line
> > execution with the copy of the original instruction under probe, and
> > enable single stepping from the instruction slot. With this scheme,
> > the instruction is executed with the exact same register context
> > 'except PC' that points to instruction slot.
> >
> > Debug mask(PSTATE.D) is enabled only when single stepping a recursive
> > kprobes i.e. during kprobes reenter so that probes instruction can be
> > single stepped within the kprobe handler -exception- context.
>
> Does this mean that at the point the probed instruction is
> single-stepped there is nothing extra that has been pushed on on the
> kernel stack by any kprobes handling code? I just want to check that you
> aren't going to hit the problems that the 32-bit kprobes code is
> currently being fixed for [1].
>
> The simulated instructions in patch 2 don't access the stack, so they
> are safe from the problem.
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/303525.html

Took me a while to read through the background on this.  I believe it is 
true nothing extra is put on the stack in the single-step case.

-dl

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