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

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Message-ID: <527DFC1C.1020107@hitachi.com>
Date:	Sat, 09 Nov 2013 18:10:52 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Will Deacon <will.deacon@....com>
Cc:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>,
	"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>,
	"jiang.liu@...wei.com" <jiang.liu@...wei.com>,
	"Vijaya.Kumar@...iumnetworks.com" <Vijaya.Kumar@...iumnetworks.com>
Subject: Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping support

(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.

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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