[PATCH RFC 2/6] arm64: Kprobes with single stepping support

[Sandeepa Prabhu]

On 12 November 2013 15:47, Masami Hiramatsu
<masami.hiramatsu.pt at hitachi.com> wrote:
> (2013/11/12 17:44), Sandeepa Prabhu wrote:
>> On 12 November 2013 12:57, Masami Hiramatsu
>> <masami.hiramatsu.pt at hitachi.com> wrote:
>>> (2013/11/12 15:23), Sandeepa Prabhu wrote:
>>>>>>> OK, I've ensured that the hw_breakpoint (from perf) can work
>>>>>>> with kprobes (from ftrace) at the same address on x86.
>>>>>>> So if arm64 already support hw_breakpoint on perf, kprobes should
>>>>>>> work with it.
>>>>>>
>>>>>> Single-stepping on x86 is different to the step behaviour on arm64 afaik. On
>>>>>> ARM, we have to manually remove the breakpoint, perform a single-step, then
>>>>>> add the breakpoint again. If we re-enable debug exceptions in the kprobe
>>>>>> handler, the step will complete early and we'll never step off the
>>>>>> breakpoint.
>>>>>
>>>>> I'm unsure about arm64's debug feature behavior, what does happen when
>>>>> it performs a single-step on sw-breakpoint?
>>>>>
>>>>>> Sandeepa: I think you need to retry Masami's test on the arm64 model, since
>>>>>> I'm fairly sure it won't work as expected without some additional code.
>>>>>
>>>>> OK, anyway, for testing same one, we need to port ftrace first. So the next
>>>
>>> Sorry for confusion, s/next/fallback is what I meant. Making a kprobe module
>>> can be done without ftrace port.
>> Yes, got it, all my verification until now are done using sample
>> modules only,  looking out for perf (or some other mechanism: ptrace?)
>> that uses v8 hw breakpoint.
>
> Yes, kprobe vs. perf and uprobe vs. ptrace :)
>
>
>>>>> plan is to make a kprobe module to put a probe (which just printk something)
>>>>> on a specific function (e.g. vfs_symlink), and run perf record with
>>>>> hw-breakpoint as below
>>>>>
>>>>> $ perf record -e "mem:0xXXXXXX:k" ln -s /dev/null /tmp/foo
>>>>>
>>>>> Note that 0xXXXXXX is the address of vfs_symlink.
>>>>>
>>>>> After that, you can see the message in dmesg and also check the perf result
>>>>> with "sudo perf script --dump" (you can find a PERF_RECORD_SAMPLE entry if
>>>>> it works)
>>>> Thanks for steps, ARM64 ftrace patches are under review on arm mailing
>>>> list, I can contact the (linaro) developer implementing ftrace on
>>>> what's supported and then figure-out a way to test this concurrency of
>>>> kprobes breakpoint and hardware breakpoint.
>>>
>>> Would you mean this? :)
>>> http://www.spinics.net/lists/arm-kernel/msg278477.html
>>>
>>> Wow, it seems that this also has some works around instruction
>>> manipulation (and confusable filenames...)
>> I referred to: http://lwn.net/Articles/572323/  which is another
>> implementation and on LAKML
>
> OK, I'll check that (and looks good at a glance).
> By the way, I concern about Linaro guys who looks working a bit far
> from the LKML and original feature maintainers. Please contact them,
> I'm sure they don't bite your hand :)
Hmm sure, will convey to our developers/leads :-)

>
> BTW, I'm currently trying a general housecleaning of __kprobes
> annotations. It may also have impact on your patch.
> https://lkml.org/lkml/2013/11/8/187
Hmm, we can help testing your patchset on arm64 platforms. Also have
many doubts on the changes you are working [blacklisting probes etc]

Basically I had tried placing kprobe on memcpy() and the model hung
(insmod never returned back!). Fast-model I have does not have option
of any debug so no clue what happened!.
memcpy() is low-level call being used internally within kprobes, so
probably we cannot handle probe on that routine, but then how to make
sure all such API are rejected by kprobe sub-system ?

~Sandeepa
>
> Thank you,
>
> --
> Masami HIRAMATSU
> IT Management Research Dept. Linux Technology Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt at hitachi.com
>
>