Re: ptrace.2: BUGS (missing WIFEXITED notification)

[Denys Vlasenko <dvlasenk-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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".

>> PTRACE_EVENT_EXIT stop will happen for it (if requested), but the
>> subsequent WIFEXITED notification will not be delivered until all other
>> threads exit. As explained above, if one of other threads calls
>> execve(2), the death of the thread group leader will never be reported.
>> If the execed thread is not traced by this tracer, the tracer will never
>> know that execve(2) happened. One possible workaround is to
>> PTRACE_DETACH the thread group leader instead of restarting it in this
>> case. Last confirmed on 2.6.38.6."
>>
>> I wanted to write that we've also noticed the same thing not only for
>> _exit() but also for terminating signals, however we also came across
>> this bit in the manual source:
>>
>> .\" Note from Denys Vlasenko:
>> .\" Here "exits" means any kind of death - _exit, exit_group,
>> .\" signal death. Signal death and exit_group cases are trivial,
>> .\" though: since signal death and exit_group kill all other threads
>> .\" too, "until all other threads exit" thing happens rather soon
>> .\" in these cases. Therefore, only _exit presents observably
>> .\" puzzling behavior to ptrace users: thread leader _exit's,
>> .\" but WIFEXITED isn't reported! We are trying to explain here
>> .\" why it is so.
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