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Re: [ptrace] please review follow fork/exec changes Dmitry Mikulin Wed Feb 15 10:01:25 2012

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
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
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.

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