Re: [PATCH] Add UNSUPPORTED check in elf/tst-pldd.

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 29/08/2019 05:47, Florian Weimer wrote:
> * Stefan Liebler:
> 
>> On 8/28/19 11:24 AM, Florian Weimer wrote:
>>> * Stefan Liebler:
>>>
>>>>   static void
>>>>   target_process (void *arg)
>>>>   {
>>>> +  if (ptrace_scope == 1)
>>>> +    {
>>>> +      /* YAMA is configured to "restricted ptrace".
>>>> +	 Disable the restriction for this subprocess.  */
>>>> +      support_ptrace_process_set_ptracer_any ();
>>>> +    }
>>>> +
>>>>     pause ();
>>>>   }
>>>
>>> I think this has a race condition if pldd attaches to the process before
>>> the support_ptrace_process_set_ptracer_any call.  I have no idea how
>>> hard it is in practice to hit this race.  It should be possible to use a
>>> process-shared barrier or some other form of synchronization to avoid
>>> this issue.
>>>
>>> Thanks,
>>> Florian
>>>
>>
>> I've added a synchronization with stdatomic.h on a shared memory mapping.
>> I've not used pthread* functions as I don't want to link against
>> libpthread.so. Then further adjustments are needed.
>>
>> Or should I just restrict the test ptrace_scope 0 as Adhemerval has
>> proposed in his post?
> 
> Is it possible to create a process tree like this?
> 
> 
>   parent (performs output checks)
>     subprocess 1 (becomes pldd via execve)
>       subprocess 2
> 
> If you execve pldd from subprocess 1, wouldn't subprocess 2 in its
> ptrace scope for ptrace_scope < 2?

Do we really need that ad-hoc support on tst-pldd to make it support 
ptrace_scope 1?

I don't oppose the support Stefan has added on latest iteration to
make it work, but this is a lot of code to support a very specific
scenario...