Re: [ptrace] please review follow fork/exec changes

On 02/15/2012 08:32 AM, Konstantin Belousov wrote:
> On Mon, Feb 13, 2012 at 02:50:45PM -0800, Dmitry Mikulin wrote:
>>>>> It seems that now wait4(2) can be called from the real (non-debugger)
>>>>> parent first and result in the call to proc_reap(), isn't it ? We would
>>>>> then just reparent the child back to the caller, still leaving the
>>>>> zombie and confusing debugger.
>>>> When either gdb or the real parent gets to proc_reap() the process
>>>> wouldn't
>>>> get destroyed, it'll get caught by the following clause:
>>>>      if (p->p_oppid&&   (t = pfind(p->p_oppid)) != NULL) {
>>>>
>>>> and the real parent with get the child back into the children's list while
>>>> gdb will get it into the orphan list. The second time around when
>>>> proc_reap() is entered, p->p_oppid will be 0 and the process will get
>>>> really reaped. Does it make sense? And proc_reparent() attempts to keep
>>>> the
>>>> orphan list clean and not have the same entries and the list of siblings.
>>> Right, this is what I figured. But I asked about some further implication
>>> of this change:
>>>
>>> if real parent spuriosly calls wait4(2) on the child pid after the child
>>> exited, but before the debugger called the wait4(), then exactly the
>>> code you noted above will be run. This results in the child being fully
>>> returned to the original parent.
>>>
>>> Next, the wait4() call from debugger gets an error, and zombie will be
>>> kept around until parent calls wait4() for this pid once more.
>>>
>>> Am I missed something ?
>> In this case the process will move from gdb's child list to gdb's orphan
>> list when the real parent does a wait4(). Next time around the wait loop in
>> gdb it'll be caught by the orphan's proc_reap().
> I do not see how the next debugger loop could find this process at all,
> since the first wait4() call reparented it to the original parent.

Not the debugger loop, the kern_wait() loop. The child get re-parented to the original parent but moves to the orphan list of the debugger process.