Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support

[David Long <dave.long@linaro.org>]

On 07/21/2016 01:23 PM, Marc Zyngier wrote:
> On 21/07/16 17:33, David Long wrote:
>> On 07/20/2016 12:09 PM, Marc Zyngier wrote:
>>> On 08/07/16 17:35, David Long wrote:
>>>> From: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>
>>>>
>>>> Add support for basic kernel probes(kprobes) and jump probes
>>>> (jprobes) for ARM64.
>>>>
>>>> Kprobes utilizes software breakpoint and single step debug
>>>> exceptions supported on ARM v8.
>>>>
>>>> A software breakpoint is placed at the probe address to trap the
>>>> kernel execution into the kprobe handler.
>>>>
>>>> ARM v8 supports enabling single stepping before the break exception
>>>> return (ERET), with next PC in exception return address (ELR_EL1). The
>>>> kprobe handler prepares an executable memory slot for out-of-line
>>>> execution with a copy of the original instruction being probed, and
>>>> enables single stepping. The PC is set to the out-of-line slot address
>>>> before the ERET. With this scheme, the instruction is executed with the
>>>> exact same register context except for the PC (and DAIF) registers.
>>>>
>>>> Debug mask (PSTATE.D) is enabled only when single stepping a recursive
>>>> kprobe, e.g.: during kprobes reenter so that probed instruction can be
>>>> single stepped within the kprobe handler -exception- context.
>>>> The recursion depth of kprobe is always 2, i.e. upon probe re-entry,
>>>> any further re-entry is prevented by not calling handlers and the case
>>>> counted as a missed kprobe).
>>>>
>>>> Single stepping from the x-o-l slot has a drawback for PC-relative accesses
>>>> like branching and symbolic literals access as the offset from the new PC
>>>> (slot address) may not be ensured to fit in the immediate value of
>>>> the opcode. Such instructions need simulation, so reject
>>>> probing them.
>>>>
>>>> Instructions generating exceptions or cpu mode change are rejected
>>>> for probing.
>>>>
>>>> Exclusive load/store instructions are rejected too.  Additionally, the
>>>> code is checked to see if it is inside an exclusive load/store sequence
>>>> (code from Pratyush).
>>>>
>>>> System instructions are mostly enabled for stepping, except MSR/MRS
>>>> accesses to "DAIF" flags in PSTATE, which are not safe for
>>>> probing.
>>>>
>>>> This also changes arch/arm64/include/asm/ptrace.h to use
>>>> include/asm-generic/ptrace.h.
>>>>
>>>> Thanks to Steve Capper and Pratyush Anand for several suggested
>>>> Changes.
>>>>
>>>> Signed-off-by: Sandeepa Prabhu <sandeepa.s.prabhu@gmail.com>
>>>> Signed-off-by: David A. Long <dave.long@linaro.org>
>>>> Signed-off-by: Pratyush Anand <panand@redhat.com>
>>>> Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
>>>> ---
>>>>    arch/arm64/Kconfig                      |   1 +
>>>>    arch/arm64/include/asm/debug-monitors.h |   5 +
>>>>    arch/arm64/include/asm/insn.h           |   2 +
>>>>    arch/arm64/include/asm/kprobes.h        |  60 ++++
>>>>    arch/arm64/include/asm/probes.h         |  34 +++
>>>>    arch/arm64/include/asm/ptrace.h         |  14 +-
>>>>    arch/arm64/kernel/Makefile              |   2 +-
>>>>    arch/arm64/kernel/debug-monitors.c      |  16 +-
>>>>    arch/arm64/kernel/probes/Makefile       |   1 +
>>>>    arch/arm64/kernel/probes/decode-insn.c  | 143 +++++++++
>>>>    arch/arm64/kernel/probes/decode-insn.h  |  34 +++
>>>>    arch/arm64/kernel/probes/kprobes.c      | 525 ++++++++++++++++++++++++++++++++
>>>>    arch/arm64/kernel/vmlinux.lds.S         |   1 +
>>>>    arch/arm64/mm/fault.c                   |  26 ++
>>>>    14 files changed, 859 insertions(+), 5 deletions(-)
>>>>    create mode 100644 arch/arm64/include/asm/kprobes.h
>>>>    create mode 100644 arch/arm64/include/asm/probes.h
>>>>    create mode 100644 arch/arm64/kernel/probes/Makefile
>>>>    create mode 100644 arch/arm64/kernel/probes/decode-insn.c
>>>>    create mode 100644 arch/arm64/kernel/probes/decode-insn.h
>>>>    create mode 100644 arch/arm64/kernel/probes/kprobes.c
>>>>
>>>
>>> [...]
>>>
>>>> diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
>>>> new file mode 100644
>>>> index 0000000..79c9511
>>>> --- /dev/null
>>>> +++ b/arch/arm64/include/asm/kprobes.h
>>>> @@ -0,0 +1,60 @@
>>>> +/*
>>>> + * arch/arm64/include/asm/kprobes.h
>>>> + *
>>>> + * Copyright (C) 2013 Linaro Limited
>>>> + *
>>>> + * This program is free software; you can redistribute it and/or modify
>>>> + * it under the terms of the GNU General Public License version 2 as
>>>> + * published by the Free Software Foundation.
>>>> + *
>>>> + * This program is distributed in the hope that it will be useful,
>>>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> + * General Public License for more details.
>>>> + */
>>>> +
>>>> +#ifndef _ARM_KPROBES_H
>>>> +#define _ARM_KPROBES_H
>>>> +
>>>> +#include <linux/types.h>
>>>> +#include <linux/ptrace.h>
>>>> +#include <linux/percpu.h>
>>>> +
>>>> +#define __ARCH_WANT_KPROBES_INSN_SLOT
>>>> +#define MAX_INSN_SIZE			1
>>>> +#define MAX_STACK_SIZE			128
>>>
>>> Where is that value coming from? Because even on my 6502, I have a 256
>>> byte stack.
>>>
>>
>> Although I don't claim to know the original author's thoughts I would
>> guess it is based on the seven other existing implementations for
>> kprobes on various architectures, all of which appear to use either 64
>> or 128 for MAX_STACK_SIZE.  The code is not trying to duplicate the
>> whole stack.
>
> I get that (this was supposed to be a humorous comment, but I guess
> after spending too much time tracking this thing, my own sense of humour
> was becoming limited).
>

It was only meant to be factual.

> My main worry is that whatever value you pick, it is always going to be
> wrong. This is used to preserve arguments that are passed on the stack,
> as opposed to passed by registers). We have no idea of what is getting
> passed there so saving nothing, 128 bytes or 2kB is about the same. It
> is always wrong.
 >
> A much better solution would be to check the frame pointer, and copy the
> delta between FP and SP, assuming it fits inside the allocated buffer.
> If it doesn't, or if FP is invalid, we just skip the hook, because we
> can't reliably execute it.

Well, this is the way it works literally everywhere else. It is a 
documented limitation (Documentation/kprobes.txt). Said documentation 
may need to be changed along with the suggested fix.

While it might be nice if there were less of a limitation it doesn't 
feel wise to me to be making this change at this time. It feels like an 
enhancement to consider amongst future improvements for all architectures.

>
> Thanks,
>
> 	M.
>

Thanks,
-dl