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

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Date:	Fri, 6 Dec 2013 14:07:53 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
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:

> [...]
> >> [...]  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?
> 
> As I said, we can use function graph tracer's list as the whitelist,
> since it doesn't include any functions invoked from ftrace's event
> handler. (Note that I don't mention the Systemtap or other user here)
>
> Whitelist is just for keeping the people away from the quantitative
> issue, who just want to trace their subsystems except for ftrace.
> [...]

Would you plan to limit kprobes (or just the perf-probe frontend) to
only function-entries also?  If not, and if intra-function
statement-granularity kprobes remain allowed within a
function-granularity whitelist, then you might still have those
"quantitative" problems.

Even worse, kprobes robustness problems can bite even with a small
whitelist, unless you can test the countless subset selections
cartesian-product the aggrevating factors (like other tracing
facilities being in use at the same time, limited memory, high irq
rates, debugging sessions, architectures, whatever).


> [...]  For the long term solution, I think we can introduce some
> kind of performance gatekeeper as systemtap does. Counting the
> miss-hit rate per second and if it go over a threshold, disable next
> miss-hit (or most miss-hit) probe (as OOM killer does).

That would make sense, but again it would not help deal with kprobes
robustness (in the kernel-crashing rather than kernel-slowdown sense).


- FChE
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