Make tcsetsid(3) work on revoked TTYs.

Right now the only way to make tcsetsid(3)/TIOCSCTTY work, is by
ensuring the session leader is dead. This means that an application that
catches SIGHUPs and performs a sleep prevents us from assigning a new
session leader.

Change the code to make it work on revoked TTYs as well. This allows us
to change init(8) to make the shutdown script run in a more clean
environment.

1 files changed, 6 insertions, 3 deletions

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index 162ae37..bf800be 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -1487,15 +1487,18 @@ tty_generic_ioctl(struct tty *tp, u_long cmd, void *data, struct thread *td)
 		}
 
 		if (p->p_session->s_ttyvp != NULL ||
-		    (tp->t_session != NULL && tp->t_session->s_ttyvp != NULL)) {
+		    (tp->t_session != NULL && tp->t_session->s_ttyvp != NULL &&
+		    tp->t_session->s_ttyvp->v_type != VBAD)) {
 			/*
 			 * There is already a relation between a TTY and
 			 * a session, or the caller is not the session
 			 * leader.
 			 *
 			 * Allow the TTY to be stolen when the vnode is
-			 * NULL, but the reference to the TTY is still
-			 * active.
+			 * invalid, but the reference to the TTY is
+			 * still active.  This allows immediate reuse of
+			 * TTYs of which the session leader has been
+			 * killed or the TTY revoked.
 			 */
 			sx_xunlock(&proctree_lock);
 			return (EPERM);