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

[Sandeepa Prabhu]

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.
>
>>> 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

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