Date: Fri, 7 Feb 2020 19:00:44 +0000 (UTC) From: Kyle Evans <kevans@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-12@freebsd.org Subject: svn commit: r357660 - in stable: 11/share/man/man4 11/sys/fs/devfs 11/sys/kern 12/share/man/man4 12/sys/fs/devfs 12/sys/kern Message-ID: <202002071900.017J0i7H099809@repo.freebsd.org>
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Author: kevans Date: Fri Feb 7 19:00:43 2020 New Revision: 357660 URL: https://svnweb.freebsd.org/changeset/base/357660 Log: MFC r355248: tty: implement TIOCNOTTY Generally, it's preferred that an application fork/setsid if it doesn't want to keep its controlling TTY, but it could be that a debugger is trying to steal it instead -- so it would hook in, drop the controlling TTY, then do some magic to set things up again. In this case, TIOCNOTTY is quite handy and still respected by at least OpenBSD, NetBSD, and Linux as far as I can tell. I've dropped the note about obsoletion, as I intend to support TIOCNOTTY as long as it doesn't impose a major burden. Modified: stable/12/share/man/man4/tty.4 stable/12/sys/fs/devfs/devfs_vnops.c stable/12/sys/kern/tty.c Directory Properties: stable/12/ (props changed) Changes in other areas also in this revision: Modified: stable/11/share/man/man4/tty.4 stable/11/sys/fs/devfs/devfs_vnops.c stable/11/sys/kern/tty.c Directory Properties: stable/11/ (props changed) Modified: stable/12/share/man/man4/tty.4 ============================================================================== --- stable/12/share/man/man4/tty.4 Fri Feb 7 17:47:08 2020 (r357659) +++ stable/12/share/man/man4/tty.4 Fri Feb 7 19:00:43 2020 (r357660) @@ -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 @@ Simulate typed input. 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 .It Dv TIOCNOTTY Fa void -This call is obsolete but left for compatibility.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 @@ first opened a terminal device, it acquired that termi 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 @@ system call which will place the process into its own 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 Modified: stable/12/sys/fs/devfs/devfs_vnops.c ============================================================================== --- stable/12/sys/fs/devfs/devfs_vnops.c Fri Feb 7 17:47:08 2020 (r357659) +++ stable/12/sys/fs/devfs/devfs_vnops.c Fri Feb 7 19:00:43 2020 (r357660) @@ -802,9 +802,16 @@ out: 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); } Modified: stable/12/sys/kern/tty.c ============================================================================== --- stable/12/sys/kern/tty.c Fri Feb 7 17:47:08 2020 (r357659) +++ stable/12/sys/kern/tty.c Fri Feb 7 19:00:43 2020 (r357660) @@ -1185,6 +1185,71 @@ tty_rel_gone(struct tty *tp) 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 */ @@ -1699,6 +1764,8 @@ tty_generic_ioctl(struct tty *tp, u_long cmd, void *da 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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