linux-kernel - Re: [PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text (but move it)

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Date:	Wed, 02 May 2012 19:40:32 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 6/9][RFC] kprobes: Allow probe on ftrace reserved text
 (but move it)

On Wed, 2012-05-02 at 16:40 -0400, Frank Ch. Eigler wrote:
> rostedt wrote:
> 
> > [...]  Added KPROBE_FLAG_MOVED (as suggested by Masami) that is set
> > when the address is moved to get around an ftrace nop. [...]
> 
> Steve, perhaps my earlier comments on this got lost during the mailing
> list outage.

I saw it, but it didn't really specify what you wanted. Here's your
comment:


> I suspect Masami intended that this flag is later used during int3
> processing to subtract MCOUNT_INSN_SIZE back out from the pt_regs->ip
> during kprobe_handler() if this flag was set.

This is what I thought too, but to me it sounded like Masami could do
the work. I was just setting up a flag to make it possible.

> 
> The gist is that a KPROBE_FLAG_MOVED being set this way accomplishes
> very little since nothing is looking for that flag.  Instead, you
> should patch {arch/*}/kernel/kprobe.c kprobe_handler() to subtract
> MCOUNT_INSN_SIZE back out from pt_regs->ip if KPROBE_FLAG_MOVED was
> set.  That way, kprobes clients need do not perceive the int3 movement.

I basically thought that Masami wanted me to add the flag, and then
others could look for this and do the adjustment. I'm not the kprobes
author. I was just adding a flag that Masami and others could use to do
such updates.

I'm not sure if the adjustment is fine with everyone, as it may cause
repercussions that I don't know about. 

Perhaps that could be another patch (want to write it?)

-- Steve


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