Re: perf/tracepoint: another fuzzer generated lockup

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2013/11/15 23:28), Frederic Weisbecker wrote:
> On Fri, Nov 15, 2013 at 09:15:21AM -0500, Steven Rostedt wrote:
>> On Fri, 15 Nov 2013 13:28:33 +0100
>> Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>> On Fri, Nov 15, 2013 at 10:16:18AM +0900, Masami Hiramatsu wrote:
>>>> Kprobes itself can detect nested call by using per-cpu current-running
>>>> kprobe pointer. And if it is nested, it just skips calling handlers.
>>>> Anyway, I don't recommend to probe inside the handlers, but yes,
>>>> you can trace perf-handler by ftrace B). I actually traced a kprobe-bug
>>>> by kprobe-tracer last night, that was amazing :)
>>>
>>> Ah, ok, so that would avoid the worst problems. Good. Should we still
>>> mark the entire perf swevent path as __kprobes just to be sure?
>>
>> I wouldn't unless we can prove that it breaks. It's sometimes nice to
>> be able to debug the debugging facilities with the debugging
>> facilities ;-)
> 
> Even with the existing __kprobes annotations, I'm sure we can find many ways to
> break the kernel.
> 
> We can reproduce that bug with irq_work recursion with setting a kprobe in
> the end of the irq_work() arch low level handler for example. Or simply
> somewhere in irq_exit().
>
> I think we could do dangerous things with breakpoints as well. Setting breakpoints
> in do_debug() or stuffs like that.
> 
> So keeping up with __kprobes annotations to every potential dangerous site
> is not viable IMHO. It's important to maintain basic sanity with tagging sites
> that are used by kprobes itself but we can't really prevent from any issue.
> 
> At some point it's up to the user to know what he's doing and avoid recursion.
> As long as kprobes can be set only by root.

Hmm, it would be better to add some documentation how users can avoid
such thing.

> OTOH it would be nice to detect these kind of behaviour (thinking about irq_work for
> example) and warn when something wrong is suspected.

Agreed.
FYI, kprobes has a recursion detection counter and it is reported via
debugfs/tracing/kprobe_profile :)

Thank you,

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com