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

[Stefan Liebler]

On 8/29/19 10:47 AM, 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?
Yes, this is possible.
I've rearranged the subprocesses. See attached patch.
Now we have a new function pldd_process which forks target_process,
stores the pid of target_prcess to a shared memory mapping as do_test 
needs to know this pid.

Afterwards it execve to pldd which successfully ptrace target_process in 
case of "restricted ptrace".

Please review the usage of support-subprocess-functions.

Bye,
Stefan
> 
> Thanks,
> Florian
> 

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