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

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Date:	Thu, 05 Dec 2013 09:49:04 -0500
From:	fche@...hat.com (Frank Ch. Eigler)
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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

Hi, Masami -

masami.hiramatsu.pt wrote:

> [...]
> 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.

Why do you think so?  We have had problems with single kprobes in the
"wrong" spot.  The main reason I showed spraying them widely is to get
wide coverage with minimal information/effort, not to suggest that the
number of concurrent probes per se is a problem.  (We have had
systemtap scripts probing some areas of the kernel with thousands of
active kprobes, e.g. for statement-by-statement variable-watching
jobs, and these have worked fine.)

> It doesn't crash the kernel but slows down so much, because every
> probes hit many other nested miss-hit probes. 

(kprobes does have code to detect & handle reentrancy.)

> This gives us a big performance impact. [...]

Sure, but I'd expect to see pure slowdowns show their impact with
time-related problems like watchdogs firing or timeouts.

> [...]  Then, I'd like to propose this new whitelist feature in
> kprobe-tracer (not raw kprobe itself). And a sysctl knob for
> disabling the whitelist.  That knob will be
> /proc/sys/debug/kprobe-event-whitelist and disabling it will mark
> kernel tainted so that we can check it from bug reports.

How would one assemble a reliable whitelist, if we haven't fully
characterized the problems that make the blacklist necessary?

- FChE
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