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List:       linux-man
Subject:    Re: ptrace.2: BUGS (missing WIFEXITED notification)
From:       "Michael Kerrisk (man-pages)" <mtk.manpages () gmail ! com>
Date:       2015-06-18 6:49:59
Message-ID: 55826A17.8000804 () gmail ! com
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Vegard (and Quentin): Ping!

On 05/15/2015 02:05 PM, Michael Kerrisk (man-pages) wrote:
> Hi Vegard,
> 
> On 15 May 2015 at 12:12, Vegard Nossum <vegard.nossum@oracle.com> wrote:
>> On 05/14/2015 06:39 PM, Denys Vlasenko wrote:
>>>
>>> On 05/14/2015 06:28 PM, Quentin Casasnovas wrote:
>>>>
>>>> On Thu, May 14, 2015 at 03:52:36PM +0200, Denys Vlasenko wrote:
>>>>>
>>>>> On 05/14/2015 03:44 PM, Michael Kerrisk (man-pages) wrote:
>>>>>>
>>>>>> Hi Denys,
>>>>>>
>>>>>> Do you have any thoughts on the below?
>>>>>
>>>>>
>>>>> Yes, the poster is right: this part needs fixing, the behavior is
>>>>> the same on any kind of process termination.
>>>>>
>>>>>
>>>>>> On 05/12/2015 04:31 PM, Vegard Nossum wrote:
>>>>>>>
>>>>>>> We hit another edge case in the ptrace() interface and after several
>>>>>>> hours of chasing it down, we found that it was already described in
>>>>>>> the
>>>>>>> "BUGS" section:
>>>>>>>
>>>>>>> "If a thread group leader is traced and exits by calling _exit(2), a
>>>>>
>>>>>
>>>>> I think a possible fix is just to replace "exits by calling _exit(2)"
>>>>> part
>>>>> of the above text with "terminates".
>>>>>
>>>>
>>>> Should we also add a little paragraph detailing that waitpid() would hang
>>>> indefinitely if one thread terminates while the others are in
>>>> ptrace-stop?
>>>
>>>
>>> It implies this by saying "but the subsequent WIFEXITED notification
>>> will not be delivered until all other threads exit".
>>>
>>> If another thread is in ptrace-stop, it did not exit yet. Therefore,
>>> WIFEXITED notification to the thread group leader will not be delivered.
>>> Therefore, waitpid() on it would hang.
>>
>>
>> While I agree that the information in the current man page is strictly
>> speaking sufficient, I personally still think it would be an improvement
>> to mention it explicitly (i.e. my proposed change #2 in the original
>> e-mail). Just because I think it's a sort of non-obvious pitfall; out of
>> hand, you don't expect a call to waitpid() on a process that has exited
>> to hang. That's just my opinion, though.
> 
> That sounds okay to me. Would you and/or Quentin be willing to put
> together a patch to the man page?
> 
> Thanks,
> 
> Michael
> 
> 
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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