fork: Move and describe why the code examines PIDNS_ADDING

Normally this would be something that would be handled by handling
signals that are sent to a group of processes but in this case the
forking process is not a member of the group being signaled.  Thus
special code is needed to prevent a race with pid namespaces exiting,
and fork adding new processes within them.

Move this test up before the signal restart just in case signals are
also pending.  Fatal conditions should take presedence over restarts.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>

kernel/fork.c

@@ -1922,6 +1922,12 @@ static __latent_entropy struct task_struct *copy_process(
 
 	rseq_fork(p, clone_flags);
 
+	/* Don't start children in a dying pid namespace */
+	if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
+		retval = -ENOMEM;
+		goto bad_fork_cancel_cgroup;
+	}
+
 	/*
 	 * Process group and session signals need to be delivered to just the
 	 * parent before the fork or both the parent and the child after the
@@ -1935,10 +1941,7 @@ static __latent_entropy struct task_struct *copy_process(
 		retval = -ERESTARTNOINTR;
 		goto bad_fork_cancel_cgroup;
 	}
-	if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
-		retval = -ENOMEM;
-		goto bad_fork_cancel_cgroup;
-	}
+
 
 	init_task_pid_links(p);
 	if (likely(p->pid)) {