linux-kernel - Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE

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Message-ID: <20131210152811.GA1195@gmail.com>
Date:	Tue, 10 Dec 2013 16:28:11 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Sandeepa Prabhu <sandeepa.prabhu@...aro.org>, x86@...nel.org,
	lkml <linux-kernel@...r.kernel.org>,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	systemtap@...rceware.org, "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and
 fixes crash bugs


* Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:

> (2013/12/05 19:21), Ingo Molnar wrote:
> > 
> > * Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:
> > 
> >>> So we need both a maintainable and a sane/safe solution, and I'd 
> >>> like to apply the whole thing at once and be at ease that the 
> >>> solution is round. We should have done this years ago.
> >>
> >> For the safeness of kprobes, I have an idea; introduce a whitelist 
> >> for dynamic events. AFAICS, the biggest unstable issue of kprobes 
> >> comes from putting *many* probes on the functions called from 
> >> tracers.
> > 
> > If the number of 'noprobe' annotations is expected to explode then 
> > maybe another approach should be considered.
> 
> No, since this is a "quantitative" issue, the annotation helps us.
> 
> > For example in perf we detect recursion. Could kprobes do that and 
> > detect hitting a probe while running kprobes code, and ignore it [do 
> > an early return]?
> 
> Yes, the kprobe itself already has recursion detector and it rejects
> calling handler.

So why are annotations needed at all? What can happen if an annotation 
is missing and a piece of code is probed which is also used by the 
kprobes code internally - do we crash, lock up, misbehave or handle it 
safely?

Thanks,

	Ingo
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