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

[Stefan Liebler <stli@linux.ibm.com>]

On 9/2/19 9:37 PM, Adhemerval Zanella wrote:
> 
> 
> 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...
> 
As there are systems where ptrace_scope is configured to 1 by default, 
we should adjust the testcase as the FAIL is misleading.
(I've just recognized that Steve Ellcey had also seen this FAIL: 
https://www.sourceware.org/ml/libc-alpha/2019-07/msg00618.html)

The minimum change should be to detect ptrace_scope == 1 and mark the 
test as UNSUPPORTED. Or we change a bit more and let the test also run 
in this scenario. (Either by support_ptrace_process_set_ptracer_any or 
adjusting the subprocess-tree)

Bye
Stefan