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

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Message-ID: <546CAF5A.4060901@linaro.org>
Date:	Wed, 19 Nov 2014 09:55:22 -0500
From:	David Long <dave.long@...aro.org>
To:	Will Deacon <will.deacon@....com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	William Cohen <wcohen@...hat.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/5] arm64: Kprobes with single stepping support

On 11/19/14 06:25, Will Deacon wrote:
> On Wed, Nov 19, 2014 at 11:21:24AM +0000, Sandeepa Prabhu wrote:
>> On 18 November 2014 20:26, Will Deacon <will.deacon@....com> wrote:
>>
>>> One thing I noticed looking through this patch is that we're effectively
>>> reinventing a bunch of the instruction decoding logic that we already have
>>> in the kernel (introduced since Sandeepa last sent his patch).
>>>
>>> Could you take a look at include/asm/insn.h and kernel/insn.c please, and
>>> see if you can at least consolidate some of this? Some of it should be easy
>>> (i.e. reusing masks, using existing #defines to construct BRK encodings),
>>> but I appreciate there may be places where kprobes needs to add extra bits,
>>> in which case I'd really like to keep this all together if at all possible.
>>>
>>> We're currently in a position where the module loader, BPF jit, ftrace and
>>> the proposed alternative patching scheme are all using the same instruction
>>> manipulation functions, so we should try to continue that trend if we can.
>> Will,
>>
>> kernel/insn.c support generating instruction encodings(forming opcodes
>> with given specifications), so for kprobes, only BRK encoding can use
>> this mechanism.
>> For instruction simulation, the instruction behavior should be
>> simulated on saved pt_regs, which is not supported on insn.c routines,
>> so still need probes-simulate-insn.c. Please point me if I am missing
>> something here.
>
> I was thinking of the magic hex numbers in the kprobes decode tables, which
> seem to correspond directly to the instruction classes described in insn.c
>
> Keeping the actual emulation code separate makes sense.
>
> Will

Of course that follows the model of the much more complex arm32 
kprobes/uprobes decoding.  I can have a go at replacing it with insn.c 
calls.

-dl

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