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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
Subject: Re: Exposing host debug capabilities to userspace
Date: Fri, 21 Nov 2014 11:23:19 +0000
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To: Christoffer Dall <christoffer.dall@linaro.org>
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Christoffer Dall <christoffer.dall@linaro.org> writes:

> On Thu, Nov 20, 2014 at 04:55:14PM +0000, Alex Benn=C3=A9e wrote:
<snip>
>>=20
>> * ptrace(PTRACE_GETREGSET, NT_ARM_HW_WATCH)
>>=20
>> 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 befor=
e
>> 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 approa=
ch
> so we should really investigate why this doesn't work and explore way=
s
> to get it working.

=46rom ptrace_attach:

	retval =3D -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 undesirabl=
e
I'm not sure if it has to be verbotten...

--=20
Alex Benn=C3=A9e