proc: add proc_mark_important server code

This is based on a fragment of Guillem Jovers patch presented here:

http://lists.gnu.org/archive/html/bug-hurd/2006-02/msg00081.html

It has been refreshed, updated and the copyright year is adjusted
properly. It has been complemented with the necessary features to
address the issues the original patch set out to address, namely
that killall5 freezes the proc translator before it tries to walk
over /proc/*/stat to decide which process to kill. Prior to this
patch (and the one marking the procfs server as important
process), killall5 would deadlock trying to walk over the proc
file system.

Ironically it would not have killed any process later on even if
it had the chance, since two values obtained from /proc/*/stat
are currently hardcoded to zero in our procfs. Patches addressing
the problem as a whole are prepared and will be sent as a follow
up.

* proc/proc.h (struct proc): Add p_important field.
* proc/pgrp.c (S_proc_getpgrppids): Exclude important system processes.
(S_proc_mark_important): New function.
* proc/mgt.c (create_startup_proc): Mark init as important.

1 files changed, 35 insertions, 0 deletions

diff --git a/proc/mgt.c b/proc/mgt.c
index 1a881d0c..6a10e915 100644
--- a/proc/mgt.c
+++ b/proc/mgt.c
@@ -593,6 +593,8 @@ create_startup_proc (void)
 
   p->p_deadmsg = 1;		/* Force initial "re-"fetch of msgport.  */
 
+  p->p_important = 1;
+
   p->p_noowner = 0;
   p->p_id = make_ids (&zero, 1);
   assert (p->p_id);
@@ -868,3 +870,36 @@ genpid ()
 
   return nextpid++;
 }
+
+/* Implement proc_mark_important as described in <hurd/process.defs>. */
+kern_return_t
+S_proc_mark_important (struct proc *p)
+{
+  if (!p)
+    return EOPNOTSUPP;
+
+  /* Only root may use this interface.  Any children of startup_proc
+     exempt from this restriction, as startup_proc calls this on their
+     behalf.  The kernel process is a notable example of an process
+     that needs this exemption.  That is not an problem however, since
+     all children of /hurd/init are important and we mark them as such
+     anyway.  */
+  if (! check_uid (p, 0) && ! check_owner (startup_proc, p))
+    return EPERM;
+
+  p->p_important = 1;
+  return 0;
+}
+
+/* Implement proc_is_important as described in <hurd/process.defs>.  */
+error_t
+S_proc_is_important (struct proc *callerp,
+		     boolean_t *essential)
+{
+  if (!callerp)
+    return EOPNOTSUPP;
+
+  *essential = callerp->p_important;
+
+  return 0;
+}