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

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Date:	Fri, 15 Nov 2013 16:39:36 +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 03:55:42PM +0000, Sandeepa Prabhu wrote:
> On 13 November 2013 20:01, Will Deacon <will.deacon@....com> wrote:
> > On Wed, Nov 13, 2013 at 06:55:33AM +0000, Sandeepa Prabhu wrote:
> >> 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?
> This is generic test module (samples/hw_breakpoint/data_breakpoint.ko)
> which places watchpoint for bothe read/write.
> Atleast watchpt should have triggered for Read right? I also tried
> with othe functions like do_fork, vfs_read etc but no hit.

You'd need to place something for exec if you want to see anything on the
instruction side. A read by the instruction fetcher does not trigger a read
watchpoint on ARM.

> >> 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).
> [This test was done on upstream branch, without kprobes patches.]
> Hmm, then this is expected with test breakpoint right? is this
> handling to be done by perf and ptrace?

perf stat doesn't register an overflow handler, so the hw_breakpoint
backend will handle the step. ptrace registers a handler which sends a
SIGTRAP to the debugger (e.g. gdb), which handles the step manually
(probably using a PTRACE_SINGLESTEP request).

> I did not see arm64 support in linux/tools/perf/,  there are multiple
> patches in mailing list though. Are you aware of any version of perf
> that work with arm64?

The perf tool should work fine on arm64 using mainline. Are you seeing
problems?

Will
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