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Index: head/share/man/man4/tty.4
===================================================================
--- head/share/man/man4/tty.4
+++ head/share/man/man4/tty.4
@@ -28,7 +28,7 @@
.\" @(#)tty.4 8.3 (Berkeley) 4/19/94
.\" $FreeBSD$
.\"
-.Dd January 11, 2017
+.Dd November 27, 2019
.Dt TTY 4
.Os
.Sh NAME
@@ -194,7 +194,6 @@
Pretend as if the terminal received the character pointed to by
.Fa cp .
.It Dv TIOCNOTTY Fa void
-This call is obsolete but left for compatibility.
In the past, when a process that did not have a controlling terminal (see
.Em The Controlling Terminal
in
@@ -203,7 +202,7 @@
controlling terminal.
For some programs this was a hazard as they
did not want a controlling terminal in the first place, and this
-provided a mechanism to disassociate the controlling terminal from
+provides a mechanism to disassociate the controlling terminal from
the calling process.
It
.Em must
@@ -228,6 +227,14 @@
has the effect of disassociating it from the controlling terminal.
This is the new and preferred method for programs to lose their controlling
terminal.
+.Pp
+However, environmental restrictions may prohibit the process from being able to
+.Fn fork
+and call the
+.Fn setsid
+system call to disassociate it from the controlling terminal.
+In this case, it must use
+.Dv TIOCNOTTY .
.It Dv TIOCSTOP Fa void
Stop output on the terminal (like typing ^S at the keyboard).
.It Dv TIOCSTART Fa void
Index: head/sys/fs/devfs/devfs_vnops.c
===================================================================
--- head/sys/fs/devfs/devfs_vnops.c
+++ head/sys/fs/devfs/devfs_vnops.c
@@ -829,9 +829,16 @@
error = ENOTTY;
if (error == 0 && com == TIOCSCTTY) {
- /* Do nothing if reassigning same control tty */
+ /*
+ * Do nothing if reassigning same control tty, or if the
+ * control tty has already disappeared. If it disappeared,
+ * it's because we were racing with TIOCNOTTY. TIOCNOTTY
+ * already took care of releasing the old vnode and we have
+ * nothing left to do.
+ */
sx_slock(&proctree_lock);
- if (td->td_proc->p_session->s_ttyvp == vp) {
+ if (td->td_proc->p_session->s_ttyvp == vp ||
+ td->td_proc->p_session->s_ttyp == NULL) {
sx_sunlock(&proctree_lock);
return (0);
}
Index: head/sys/kern/tty.c
===================================================================
--- head/sys/kern/tty.c
+++ head/sys/kern/tty.c
@@ -1195,6 +1195,71 @@
tty_rel_free(tp);
}
+static int
+tty_drop_ctty(struct tty *tp, struct proc *p)
+{
+ struct session *session;
+ struct vnode *vp;
+
+ /*
+ * This looks terrible, but it's generally safe as long as the tty
+ * hasn't gone away while we had the lock dropped. All of our sanity
+ * checking that this operation is OK happens after we've picked it back
+ * up, so other state changes are generally not fatal and the potential
+ * for this particular operation to happen out-of-order in a
+ * multithreaded scenario is likely a non-issue.
+ */
+ tty_unlock(tp);
+ sx_xlock(&proctree_lock);
+ tty_lock(tp);
+ if (tty_gone(tp)) {
+ sx_xunlock(&proctree_lock);
+ return (ENODEV);
+ }
+
+ /*
+ * If the session doesn't have a controlling TTY, or if we weren't
+ * invoked on the controlling TTY, we'll return ENOIOCTL as we've
+ * historically done.
+ */
+ session = p->p_session;
+ if (session->s_ttyp == NULL || session->s_ttyp != tp) {
+ sx_xunlock(&proctree_lock);
+ return (ENOTTY);
+ }
+
+ if (!SESS_LEADER(p)) {
+ sx_xunlock(&proctree_lock);
+ return (EPERM);
+ }
+
+ PROC_LOCK(p);
+ SESS_LOCK(session);
+ vp = session->s_ttyvp;
+ session->s_ttyp = NULL;
+ session->s_ttyvp = NULL;
+ session->s_ttydp = NULL;
+ SESS_UNLOCK(session);
+
+ tp->t_sessioncnt--;
+ p->p_flag &= ~P_CONTROLT;
+ PROC_UNLOCK(p);
+ sx_xunlock(&proctree_lock);
+
+ /*
+ * If we did have a vnode, release our reference. Ordinarily we manage
+ * these at the devfs layer, but we can't necessarily know that we were
+ * invoked on the vnode referenced in the session (i.e. the vnode we
+ * hold a reference to). We explicitly don't check VBAD/VI_DOOMED here
+ * to avoid a vnode leak -- in circumstances elsewhere where we'd hit a
+ * VI_DOOMED vnode, release has been deferred until the controlling TTY
+ * is either changed or released.
+ */
+ if (vp != NULL)
+ vrele(vp);
+ return (0);
+}
+
/*
* Exposing information about current TTY's through sysctl
*/
@@ -1709,6 +1774,8 @@
MPASS(tp->t_session);
*(int *)data = tp->t_session->s_sid;
return (0);
+ case TIOCNOTTY:
+ return (tty_drop_ctty(tp, td->td_proc));
case TIOCSCTTY: {
struct proc *p = td->td_proc;

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