allow_signal: kill the bogus ->mm check, add a note about CLONE_SIGHAND

allow_signal() checks ->mm == NULL.  Not sure why.  Perhaps to make sure
current is the kernel thread.  But this helper must not be used unless we
are the kernel thread, kill this check.

Also, document the fact that the CLONE_SIGHAND kthread must not use
allow_signal(), unless the caller really wants to change the parent's
->sighand->action as well.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -375,9 +375,8 @@ static void set_special_pids(struct pid *pid)
 }
 
 /*
- * Let kernel threads use this to say that they
- * allow a certain signal (since daemonize() will
- * have disabled all of them by default).
+ * Let kernel threads use this to say that they allow a certain signal.
+ * Must not be used if kthread was cloned with CLONE_SIGHAND.
  */
 int allow_signal(int sig)
 {
@@ -385,14 +384,14 @@ int allow_signal(int sig)
 		return -EINVAL;
 
 	spin_lock_irq(&current->sighand->siglock);
+	/* This is only needed for daemonize()'ed kthreads */
 	sigdelset(&current->blocked, sig);
-	if (!current->mm) {
-		/* Kernel threads handle their own signals.
-		   Let the signal code know it'll be handled, so
-		   that they don't get converted to SIGKILL or
-		   just silently dropped */
+	/*
+	 * Kernel threads handle their own signals. Let the signal code
+	 * know it'll be handled, so that they don't get converted to
+	 * SIGKILL or just silently dropped.
+	 */
 	current->sighand->action[(sig)-1].sa.sa_handler = (void __user *)2;
-	}
 	recalc_sigpending();
 	spin_unlock_irq(&current->sighand->siglock);
 	return 0;