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

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Date:	Wed, 13 Nov 2013 14:31:10 +0000
From:	Will Deacon <will.deacon@....com>
To:	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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: Re: Re: [PATCH RFC 2/6] arm64: Kprobes with single stepping
 support

On Wed, Nov 13, 2013 at 06:55:33AM +0000, Sandeepa Prabhu wrote:
> >>> I'm unsure about arm64's debug feature behavior, what does happen when
> >>> it performs a single-step on sw-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.
> >>>
> >>> OK, anyway, for testing same one, we need to port ftrace first. So the next
> >
> > Sorry for confusion, s/next/fallback is what I meant. Making a kprobe module
> > can be done without ftrace port.
> >
> >>> plan is to make a kprobe module to put a probe (which just printk something)
> >>> on a specific function (e.g. vfs_symlink), and run perf record with
> >>> hw-breakpoint as below
> >>>
> >>> $ perf record -e "mem:0xXXXXXX:k" ln -s /dev/null /tmp/foo
> >>>
> >>> Note that 0xXXXXXX is the address of vfs_symlink.
> >>>
> >>> After that, you can see the message in dmesg and also check the perf result
> >>> with "sudo perf script --dump" (you can find a PERF_RECORD_SAMPLE entry if
> >>> it works)
> Hi Will, Masami,
> 
> I am not sure of 'perf' right now (my minimal rootfs doesn't have) but
> I tried to test hardware breakpoints using sample modules
> "samples/hw_breakpoint/" on arm64 upstream branch. This should use
> same kernel api as perf I believe.
> 
> 1.  Placing watchpoint ( attr.bp_type = HW_BREAKPOINT_W |
> HW_BREAKPOINT_R) upon vfs_symlink symbol, but seems watch-point is not
> triggering at all.

vfs_symlink is a function. Why would you expect to write it?

> 2.  Placing text breakpoint (modified sample module with attr.bp_type
> = HW_BREAKPOINT_X) upon vfs_symlink, and run "ln -s /dev/null
> /tmp/foo".  This time, breakpoint hit but exception is re-cursing
> infinitely!

The problem here is that we expect the overflow handler to deal with the
stepping (like GDB does via ptrace). If you don't register a handler, the
kernel will do the step (like you would get if you used perf stat -e
mem:0xNNNN:x).

Will
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