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

[Sandeepa Prabhu]

On 13 November 2013 12:25, Sandeepa Prabhu <sandeepa.prabhu at linaro.org> wrote:
>>>> 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.
>>
>>>> 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)
Hi Will, Masami,

I am not sure of 'perf' right now (my minimal rootfs doesn't have) but
I tried to test hardware breakpoints using sample modules
"samples/hw_breakpoint/" on arm64 upstream branch. This should use
same kernel api as perf I believe.

1.  Placing watchpoint ( attr.bp_type = HW_BREAKPOINT_W |
HW_BREAKPOINT_R) upon vfs_symlink symbol, but seems watch-point is not
triggering at all.
2.  Placing text breakpoint (modified sample module with attr.bp_type
= HW_BREAKPOINT_X) upon vfs_symlink, and run "ln -s /dev/null
/tmp/foo".  This time, breakpoint hit but exception is re-cursing
infinitely!

I have attached the kernel logs for reference. So wanted to check if
hw breakpoint/watch-points are working on the upstream branch? Has it
been tested recently with sample modules  or perf/ptrace?

Thanks,
Sandeepa
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