Messages in this thread | | | Subject | Re: [PATCH -tip v4 0/6] kprobes: introduce NOKPROBE_SYMBOL() and fixes crash bugs | From | (Frank Ch. Eigler) | Date | Thu, 05 Dec 2013 09:49:04 -0500 |
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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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