Re: Exposing host debug capabilities to userspace

["Alex Bennée" <alex.bennee@linaro.org>]

Christoffer Dall <christoffer.dall@linaro.org> writes:

> On Thu, Nov 20, 2014 at 04:55:14PM +0000, Alex Bennée wrote:
<snip>
>> 
>> * ptrace(PTRACE_GETREGSET, NT_ARM_HW_WATCH)
>> 
>> This is used by GDB to access the host details in debug-monitors.
>> However the ptrace API really wants you to attach to a process before
>> calling PTRACE_GETREGSET. Currently I've tried attaching to the
>> thread_id of the vCPU but this fails with EPERM, I suspect because
>> attaching to your own threads likely upsets the kernel.
>
> Can you confirm your suspicion?  This seems like a rather good approach
> so we should really investigate why this doesn't work and explore ways
> to get it working.

From ptrace_attach:

	retval = -EPERM;
	if (unlikely(task->flags & PF_KTHREAD))
		goto out;
	if (same_thread_group(task, current))
		goto out;

I think this is what is triggering my EPERM. I'm going to dig into the
history of code around that bit. While I can see it might be undesirable
I'm not sure if it has to be verbotten...

-- 
Alex Bennée