linux-kernel - Re: [PATCH 0/3][RFC] tracing/kprobes: prevent jprobes from crashing function graph tracer

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Date:	Sun, 01 Nov 2009 09:48:45 -0500
From:	Masami Hiramatsu <mhiramat@...hat.com>
To:	"Frank Ch. Eigler" <fche@...hat.com>
CC:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"David S. Miller" <davem@...emloft.net>,
	Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: Re: [PATCH 0/3][RFC] tracing/kprobes: prevent jprobes from crashing
 function graph tracer

Frank Ch. Eigler wrote:
> Steven Rostedt<rostedt@...dmis.org>  writes:
>
>> [...]  Jprobes and the function graph tracer use the same mechanism
>> to trace the exit of a function. Unfortunately, only one can be done
>> at a time.  The function graph tracer replaces the return address
>> with its own handler, but so does jprobes. The two are not
>> compatible. [...]
>
> What about kretprobes?  It too uses the same mechanism.

Exactly, kretprobe uses similar mechanism with func-graph tracer.

Fortunately, it doesn't cause any problem, because kretprobe doesn't
skip any function-return, and func-graph tracer(mcount) always intrudes
a function after kretprobe.

---
call func
	<-- kretprobe prehandler changes return address
	func()
	<-- func-graph prehandler changes return address
	...
	return
	--> func-graph trampoline sets ip to "kretprobe trampoline"
	--> kretprobe trampoline sets ip to "real return address"
(return address)
---

So that kretprobe handler doesn't conflict with function graph tracer.

Thank you,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@...hat.com

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