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Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs
- From: "Frank Ch. Eigler" <fche at redhat dot com>
- To: Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>
- Cc: Ingo Molnar <mingo at kernel dot org>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, Sandeepa Prabhu <sandeepa dot prabhu at linaro dot org>, x86 at kernel dot org, lkml <linux-kernel at vger dot kernel dot org>, "Steven Rostedt (Red Hat)" <rostedt at goodmis dot org>, systemtap at sourceware dot org, "David S. Miller" <davem at davemloft dot net>
- Date: Fri, 6 Dec 2013 14:07:53 -0500
- Subject: Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs
- Authentication-results: sourceware.org; auth=none
- References: <20131204012841 dot 22118 dot 82992 dot stgit at kbuild-fedora dot novalocal> <20131204084551 dot GA31772 at gmail dot com> <529FBA71 dot 6070107 at hitachi dot com> <y0mvbz3pd3z dot fsf at fche dot csb> <52A16AD7 dot 6040500 at hitachi dot com>
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