Re: arm64 uprobes support showing signs of life

[William Cohen <wcohen at redhat dot com>]

On 01/07/2015 07:07 AM, Pratyush Anand wrote:
> Hi Will,
> 
> 
> On Monday 05 January 2015 09:36 PM, William Cohen wrote:
>> Hi All,
>>
>> Pratyush has been implementing uprobes support for aarch64 and has
>> posted a set of patches
>> (http://comments.gmane.org/gmane.linux.ports.arm.kernel/382237) for
>> review.  The really new Linux kernels requires a patch to the
>> systemtap runtime because the f_dentry macro has been removed.  With
>> the patched systemtap I was able to run the systemtap testsuite and
>> get some test results exercising the uprobes support:
>>
>> https://web.elastic.org/~dejazilla/viewsummary.php?summary=%3D%27%3C54A9E21C.2080504%40redhat.com%3E%27
>>
>> There looked to be a localized fixes for plt support to eliminate the
>> unsupported systemtap.base/list.exp plt-* tests.
>>
>> A number of the tests appear to fail because of userspace arguments
>> cannot be found for sdt probes.
>>
>> The uprobe patches still need some refinement.  On a number of
>> systemtap "make installcheck" runs the kernel would get stuck spewing
>> out:
>>
>>    Unexpected kernel single-step exception at EL1
> 
> There are at least two functions of arm64/uprobe implementation uprobe_breakpoint_handler and uprobe_single_step_handler, where a kprobe insertion might be causing above issue. Currently I have qualified these functions with __kprobe, so that one can not insert a kprobe there. With this you should not be able to see the above message.
> 
> However, I am still investigating if a kprobe insertion be allowed to these functions or any other function which is called directly from debug exception handler (do_debug_exception)
> 

Hi Pratyush,

Are there any other places that need to be protected from kprobe probing in the arm64 code?

> Update code is here:
> 
> https://github.com/pratyushanand/linux.git : ml_arm64_uprobe_devel_v2
> 
> ~Pratyush

I will give the new kernel a try.  

-Will

> 
>>
>> However, things are looking better for user-space probing on arm64.
>>
>> -Will
>>