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

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Date:	Mon, 11 Nov 2013 19:51:52 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	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>,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support

(2013/11/11 16:54), Masami Hiramatsu wrote:
>>>> 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 ?
> 
> For hw breakpoint, yes, we finally get check each other to safely
> use it even if one rejects the other one at some points(address).
> Since the hw breakpoint is already open for normal user via perf,
> we should do it. But the policy still needs to be discussed.

OK, I've ensured that the hw_breakpoint (from perf) can work
with kprobes (from ftrace) at the same address on x86.
So if arm64 already support hw_breakpoint on perf, kprobes should
work with it.

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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