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

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Message-ID: <CA+b37P1R8g9V61j4==tfZxQUm1Zym6ssJUAKKHLKXOgTsMa3wQ@mail.gmail.com>
Date:	Wed, 13 Nov 2013 12:38:13 +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>,
	"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 13 November 2013 12:25, Sandeepa Prabhu <sandeepa.prabhu@...aro.org> 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.
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!

I have attached the kernel logs for reference. So wanted to check if
hw breakpoint/watch-points are working on the upstream branch? Has it
been tested recently with sample modules  or perf/ptrace?

Thanks,
Sandeepa

View attachment "kernel_aarch64_20131113-120837.log" of type "text/x-log" (31635 bytes)