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From: Don Lewis <truckman@FreeBSD.org>
Subject: Re: svn commit: r269656 - in head: bin/ps sys/kern sys/sys
To: kib@FreeBSD.org
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On  7 Aug, Konstantin Belousov wrote:
> Author: kib
> Date: Thu Aug  7 05:47:53 2014
> New Revision: 269656
> URL: http://svnweb.freebsd.org/changeset/base/269656
> 
> Log:
>   Correct the problems with the ptrace(2) making the debuggee an orphan.
>   One problem is inferior(9) looping due to the process tree becoming a
>   graph instead of tree if the parent is traced by child. Another issue
>   is due to the use of p_oppid to restore the original parent/child
>   relationship, because real parent could already exited and its pid
>   reused (noted by mjg).
>   
>   Add the function proc_realparent(9), which calculates the parent for
>   given process. It uses the flag P_TREE_FIRST_ORPHAN to detect the head
>   element of the p_orphan list and than stepping back to its container
>   to find the parent process. If the parent has already exited, the
>   init(8) is returned.
>   
>   Move the P_ORPHAN and the new helper flag from the p_flag* to new
>   p_treeflag field of struct proc, which is protected by proctree lock
>   instead of proc lock, since the orphans relationship is managed under
>   the proctree_lock already.
>   
>   The remaining uses of p_oppid in ptrace(PT_DETACH) and process
>   reapping are replaced by proc_realparent(9).

Changing the parent process has always seemed like a hack to me.  It
seems like the debugger should register itself as an additional parent.