[ptrace] please review follow fork/exec changes

[Konstantin Belousov]

On Wed, Feb 15, 2012 at 09:22:10AM -0800, Dmitry Mikulin wrote:
> 
> 
> 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.

Either the debugger loop which calls wait4/waitpid, or the kern_wait loop
resulting from the debugger calling wait*.

Could you, please, describe, how the patched kernel moves the wait'ed
zombie to the orphan list of the debugger ?
For me, it seems that there is another bug, the child appears both on
the childdren list, and on the orphan list of the real parent.
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