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

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Date:	Mon, 11 Nov 2013 10:58:12 +0000
From:	Will Deacon <will.deacon@....com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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>,
	"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

On Mon, Nov 11, 2013 at 10:51:52AM +0000, Masami Hiramatsu wrote:
> (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.

Single-stepping on x86 is different to the step behaviour on arm64 afaik. On
ARM, we have to manually remove the breakpoint, perform a single-step, then
add the breakpoint again. If we re-enable debug exceptions in the kprobe
handler, the step will complete early and we'll never step off the
breakpoint.

Sandeepa: I think you need to retry Masami's test on the arm64 model, since
I'm fairly sure it won't work as expected without some additional code.

Will
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