pid: remove the child_reaper special case in init/main.c

This patchset is a cleanup and a preparation to unshare the pid namespace.
These prerequisites prepare for Eric's patchset to give a file descriptor
to a namespace and join an existing namespace.

This patch:

It turns out that the existing assignment in copy_process of the
child_reaper can handle the initial assignment of child_reaper we just
need to generalize the test in kernel/fork.c

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Acked-by: Serge E. Hallyn <serge@hallyn.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pid.h

@@ -140,6 +140,17 @@ static inline struct pid_namespace *ns_of_pid(struct pid *pid)
 	return ns;
 }
 
+/*
+ * is_child_reaper returns true if the pid is the init process
+ * of the current namespace. As this one could be checked before
+ * pid_ns->child_reaper is assigned in copy_process, we check
+ * with the pid number.
+ */
+static inline bool is_child_reaper(struct pid *pid)
+{
+	return pid->numbers[pid->level].nr == 1;
+}
+
 /*
  * the helpers to get the pid's id seen from different namespaces
  *

init/main.c

@@ -787,15 +787,6 @@ static int __init kernel_init(void * unused)
 	 * init can run on any cpu.
 	 */
 	set_cpus_allowed_ptr(current, cpu_all_mask);
-	/*
-	 * Tell the world that we're going to be the grim
-	 * reaper of innocent orphaned children.
-	 *
-	 * We don't want people to have to make incorrect
-	 * assumptions about where in the task array this
-	 * can be found.
-	 */
-	init_pid_ns.child_reaper = current;
 
 	cad_pid = task_pid(current);
 

kernel/fork.c

@@ -1296,7 +1296,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 		tracehook_finish_clone(p, clone_flags, trace);
 
 		if (thread_group_leader(p)) {
-			if (clone_flags & CLONE_NEWPID)
+			if (is_child_reaper(pid))
 				p->nsproxy->pid_ns->child_reaper = p;
 
 			p->signal->leader_pid = pid;