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

[Sandeepa Prabhu]

>>> 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
> 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.

Thanks,
Sandeepa
>
> Thank you,
>
> --
> Masami HIRAMATSU
> IT Management Research Dept. Linux Technology Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
>
>
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