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

From: David Long
Date: Thu Nov 20 2014 - 23:28:40 EST

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.


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.


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