linux-kernel - Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support

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Date:	Mon, 11 Nov 2013 11:09:42 +0530
From:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Will Deacon <will.deacon@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"steve.capper@...aro.org" <steve.capper@...aro.org>,
	"nico@...aro.org" <nico@...aro.org>,
	"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"Vijaya.Kumar@...iumnetworks.com" <Vijaya.Kumar@...iumnetworks.com>,
	Jiang Liu <liuj97@...il.com>
Subject: Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support

On 9 November 2013 14:40, Masami Hiramatsu
<masami.hiramatsu.pt@...achi.com> wrote:
> (2013/11/09 1:56), Will Deacon wrote:
>> Hi Sandeepa,
>>
>> On Thu, Oct 17, 2013 at 12:17:47PM +0100, Sandeepa Prabhu wrote:
>>> Add support for basic kernel probes(kprobes), jump probes (jprobes)
>>> for ARM64.
>>
>> I think this series will conflict quite heavily with the jump_label series,
>> since they both introduce some common instruction manipulation code. On the
>> debug side, there will also be conflicts with the kgdb series, so it might
>> make sense for us to merge those two first, then you can rebase on a stable
>> branch from us.
>
> [...]
>
>> In fact, how do you avoid a race with hardware breakpoints? E.g., somebody
>> places a hardware breakpoint on an instruction in the kernel for which
>> kprobes has patched in a brk. We take the hardware breakpoint, disable the
>> breakpoint and set up a single step before returning to the brk. The brk
>> then traps, but we must take care not to disable single-step and/or unmask
>> debug exceptions, because that will cause the hardware breakpoint code to
>> re-arm its breakpoint before we've stepped off the brk instruction.
>
> Hmm, frankly to say, this kind of race issue is not seriously discussed
> on x86 too, since kgdb is still a special tool (not used on the production
> system).
> I think under such situation kgdb operator must have full control of the
> system, and he can (and has to) avoid such kind of race.
Masami,

Hmm I think in same lines, but not sure if we expect kprobes to be
able to work fool-proof along with kgdb or hw breakpoints ?

Thanks,
Sandeepa
>
> Thank you,
>
> --
> Masami HIRAMATSU
> IT Management Research Dept. Linux Technology Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt@...achi.com
>
>
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