text: // SPDX-License-Identifier: GPL-2.0
  /*
   *  Copyright (C) 1991, 1992  Linus Torvalds
   */
  
  #include <linux/types.h>
  #include <linux/errno.h>
  #include <linux/signal.h>
  #include <linux/sched/signal.h>
  #include <linux/sched/task.h>
  #include <linux/tty.h>
  #include <linux/fcntl.h>
  #include <linux/uaccess.h>
  #include "tty.h"
  
  static int is_ignored(int sig)
  {
  	return (sigismember(&current->blocked, sig) ||
  		current->sighand->action[sig-1].sa.sa_handler == SIG_IGN);
  }
  
  /**
   *	tty_check_change	-	check for POSIX terminal changes
   *	@tty: tty to check
   *	@sig: signal to send
   *
   *	If we try to write to, or set the state of, a terminal and we're
   *	not in the foreground, send a SIGTTOU.  If the signal is blocked or
   *	ignored, go ahead and perform the operation.  (POSIX 7.2)
   *
   *	Locking: ctrl_lock
   */
  int __tty_check_change(struct tty_struct *tty, int sig)
  {
  	unsigned long flags;
  	struct pid *pgrp, *tty_pgrp;
  	int ret = 0;
  
  	if (current->signal->tty != tty)
  		return 0;
  
  	rcu_read_lock();
  	pgrp = task_pgrp(current);
  
  	spin_lock_irqsave(&tty->ctrl_lock, flags);
  	tty_pgrp = tty->pgrp;
  	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
  
  	if (tty_pgrp && pgrp != tty_pgrp) {
  		if (is_ignored(sig)) {
  			if (sig == SIGTTIN)
  				ret = -EIO;
  		} else if (is_current_pgrp_orphaned())
  			ret = -EIO;
  		else {
  			kill_pgrp(pgrp, sig, 1);
  			set_thread_flag(TIF_SIGPENDING);
  			ret = -ERESTARTSYS;
  		}
  	}
  	rcu_read_unlock();
  
  	if (!tty_pgrp)
  		tty_warn(tty, "sig=%d, tty->pgrp == NULL!\n", sig);
  
  	return ret;
  }
  
  int tty_check_change(struct tty_struct *tty)
  {
  	return __tty_check_change(tty, SIGTTOU);
  }
  EXPORT_SYMBOL(tty_check_change);
  
  void proc_clear_tty(struct task_struct *p)
  {
  	unsigned long flags;
  	struct tty_struct *tty;
  
  	spin_lock_irqsave(&p->sighand->siglock, flags);
  	tty = p->signal->tty;
  	p->signal->tty = NULL;
  	spin_unlock_irqrestore(&p->sighand->siglock, flags);
  	tty_kref_put(tty);
  }
  
  /**
   * proc_set_tty -  set the controlling terminal
   *	@tty: tty structure
   *
   * Only callable by the session leader and only if it does not already have
   * a controlling terminal.
   *
   * Caller must hold:  tty_lock()
   *		      a readlock on tasklist_lock
   *		      sighand lock
   */
  static void __proc_set_tty(struct tty_struct *tty)
  {
  	unsigned long flags;
  
  	spin_lock_irqsave(&tty->ctrl_lock, flags);
  	/*
  	 * The session and fg pgrp references will be non-NULL if
  	 * tiocsctty() is stealing the controlling tty
  	 */
  	put_pid(tty->session);
  	put_pid(tty->pgrp);
  	tty->pgrp = get_pid(task_pgrp(current));
  	tty->session = get_pid(task_session(current));
  	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
  	if (current->signal->tty) {
  		tty_debug(tty, "current tty %s not NULL!!\n",
  			  current->signal->tty->name);
  		tty_kref_put(current->signal->tty);
  	}
  	put_pid(current->signal->tty_old_pgrp);
  	current->signal->tty = tty_kref_get(tty);
  	current->signal->tty_old_pgrp = NULL;
  }
  
  static void proc_set_tty(struct tty_struct *tty)
  {
  	spin_lock_irq(&current->sighand->siglock);
  	__proc_set_tty(tty);
  	spin_unlock_irq(&current->sighand->siglock);
  }
  
  /*
   * Called by tty_open() to set the controlling tty if applicable.
   */
  void tty_open_proc_set_tty(struct file *filp, struct tty_struct *tty)
  {
  	read_lock(&tasklist_lock);
  	spin_lock_irq(&current->sighand->siglock);
  	if (current->signal->leader &&
  	    !current->signal->tty &&
  	    tty->session == NULL) {
  		/*
  		 * Don't let a process that only has write access to the tty
  		 * obtain the privileges associated with having a tty as
  		 * controlling terminal (being able to reopen it with full
  		 * access through /dev/tty, being able to perform pushback).
  		 * Many distributions set the group of all ttys to "tty" and
  		 * grant write-only access to all terminals for setgid tty
  		 * binaries, which should not imply full privileges on all ttys.
  		 *
  		 * This could theoretically break old code that performs open()
  		 * on a write-only file descriptor. In that case, it might be
  		 * necessary to also permit this if
  		 * inode_permission(inode, MAY_READ) == 0.
  		 */
  		if (filp->f_mode & FMODE_READ)
  			__proc_set_tty(tty);
  	}
  	spin_unlock_irq(&current->sighand->siglock);
  	read_unlock(&tasklist_lock);
  }
  
  struct tty_struct *get_current_tty(void)
  {
  	struct tty_struct *tty;
  	unsigned long flags;
  
  	spin_lock_irqsave(&current->sighand->siglock, flags);
  	tty = tty_kref_get(current->signal->tty);
  	spin_unlock_irqrestore(&current->sighand->siglock, flags);
  	return tty;
  }
  EXPORT_SYMBOL_GPL(get_current_tty);
  
  /*
   * Called from tty_release().
   */
  void session_clear_tty(struct pid *session)
  {
  	struct task_struct *p;
  
  	do_each_pid_task(session, PIDTYPE_SID, p) {
  		proc_clear_tty(p);
  	} while_each_pid_task(session, PIDTYPE_SID, p);
  }
  
  /**
   *	tty_signal_session_leader	- sends SIGHUP to session leader
   *	@tty: controlling tty
   *	@exit_session: if non-zero, signal all foreground group processes
   *
   *	Send SIGHUP and SIGCONT to the session leader and its process group.
   *	Optionally, signal all processes in the foreground process group.
   *
   *	Returns the number of processes in the session with this tty
   *	as their controlling terminal. This value is used to drop
   *	tty references for those processes.
   */
  int tty_signal_session_leader(struct tty_struct *tty, int exit_session)
  {
  	struct task_struct *p;
  	int refs = 0;
  	struct pid *tty_pgrp = NULL;
  
  	read_lock(&tasklist_lock);
  	if (tty->session) {
  		do_each_pid_task(tty->session, PIDTYPE_SID, p) {
  			spin_lock_irq(&p->sighand->siglock);
  			if (p->signal->tty == tty) {
  				p->signal->tty = NULL;
  				/*
  				 * We defer the dereferences outside of
  				 * the tasklist lock.
  				 */
  				refs++;
  			}
  			if (!p->signal->leader) {
  				spin_unlock_irq(&p->sighand->siglock);
  				continue;
  			}
  			__group_send_sig_info(SIGHUP, SEND_SIG_PRIV, p);
  			__group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
  			put_pid(p->signal->tty_old_pgrp);  /* A noop */
  			spin_lock(&tty->ctrl_lock);
  			tty_pgrp = get_pid(tty->pgrp);
  			if (tty->pgrp)
  				p->signal->tty_old_pgrp = get_pid(tty->pgrp);
  			spin_unlock(&tty->ctrl_lock);
  			spin_unlock_irq(&p->sighand->siglock);
  		} while_each_pid_task(tty->session, PIDTYPE_SID, p);
  	}
  	read_unlock(&tasklist_lock);
  
  	if (tty_pgrp) {
  		if (exit_session)
  			kill_pgrp(tty_pgrp, SIGHUP, exit_session);
  		put_pid(tty_pgrp);
  	}
  
  	return refs;
  }
  
  /**
   *	disassociate_ctty	-	disconnect controlling tty
   *	@on_exit: true if exiting so need to "hang up" the session
   *
   *	This function is typically called only by the session leader, when
   *	it wants to disassociate itself from its controlling tty.
   *
   *	It performs the following functions:
   *	(1)  Sends a SIGHUP and SIGCONT to the foreground process group
   *	(2)  Clears the tty from being controlling the session
   *	(3)  Clears the controlling tty for all processes in the
   *		session group.
   *
   *	The argument on_exit is set to 1 if called when a process is
   *	exiting; it is 0 if called by the ioctl TIOCNOTTY.
   *
   *	Locking:
   *		BTM is taken for hysterical raisons, and held when
   *		  called from no_tty().
   *		  tty_mutex is taken to protect tty
   *		  ->siglock is taken to protect ->signal/->sighand
   *		  tasklist_lock is taken to walk process list for sessions
   *		    ->siglock is taken to protect ->signal/->sighand
   */
  void disassociate_ctty(int on_exit)
  {
  	struct tty_struct *tty;
  
  	if (!current->signal->leader)
  		return;
  
  	tty = get_current_tty();
  	if (tty) {
  		if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY) {
  			tty_vhangup_session(tty);
  		} else {
  			struct pid *tty_pgrp = tty_get_pgrp(tty);
  
  			if (tty_pgrp) {
  				kill_pgrp(tty_pgrp, SIGHUP, on_exit);
  				if (!on_exit)
  					kill_pgrp(tty_pgrp, SIGCONT, on_exit);
  				put_pid(tty_pgrp);
  			}
  		}
  		tty_kref_put(tty);
  
  	} else if (on_exit) {
  		struct pid *old_pgrp;
  
  		spin_lock_irq(&current->sighand->siglock);
  		old_pgrp = current->signal->tty_old_pgrp;
  		current->signal->tty_old_pgrp = NULL;
  		spin_unlock_irq(&current->sighand->siglock);
  		if (old_pgrp) {
  			kill_pgrp(old_pgrp, SIGHUP, on_exit);
  			kill_pgrp(old_pgrp, SIGCONT, on_exit);
  			put_pid(old_pgrp);
  		}
  		return;
  	}
  
  	spin_lock_irq(&current->sighand->siglock);
  	put_pid(current->signal->tty_old_pgrp);
  	current->signal->tty_old_pgrp = NULL;
  	tty = tty_kref_get(current->signal->tty);
  	spin_unlock_irq(&current->sighand->siglock);
  
  	if (tty) {
  		unsigned long flags;
  
  		tty_lock(tty);
  		spin_lock_irqsave(&tty->ctrl_lock, flags);
  		put_pid(tty->session);
  		put_pid(tty->pgrp);
  		tty->session = NULL;
  		tty->pgrp = NULL;
  		spin_unlock_irqrestore(&tty->ctrl_lock, flags);
  		tty_unlock(tty);
  		tty_kref_put(tty);
  	}
  
  	/* Now clear signal->tty under the lock */
  	read_lock(&tasklist_lock);
  	session_clear_tty(task_session(current));
  	read_unlock(&tasklist_lock);
  }
  
  /*
   *
   *	no_tty	- Ensure the current process does not have a controlling tty
   */
  void no_tty(void)
  {
  	/*
  	 * FIXME: Review locking here. The tty_lock never covered any race
  	 * between a new association and proc_clear_tty but possibly we need
  	 * to protect against this anyway.
  	 */
  	struct task_struct *tsk = current;
  
  	disassociate_ctty(0);
  	proc_clear_tty(tsk);
  }
  
  /**
   *	tiocsctty	-	set controlling tty
   *	@tty: tty structure
   *	@file: file structure used to check permissions
   *	@arg: user argument
   *
   *	This ioctl is used to manage job control. It permits a session
   *	leader to set this tty as the controlling tty for the session.
   *
   *	Locking:
   *		Takes tty_lock() to serialize proc_set_tty() for this tty
   *		Takes tasklist_lock internally to walk sessions
   *		Takes ->siglock() when updating signal->tty
   */
  static int tiocsctty(struct tty_struct *tty, struct file *file, int arg)
  {
  	int ret = 0;
  
  	tty_lock(tty);
  	read_lock(&tasklist_lock);
  
  	if (current->signal->leader && (task_session(current) == tty->session))
  		goto unlock;
  
  	/*
  	 * The process must be a session leader and
  	 * not have a controlling tty already.
  	 */
  	if (!current->signal->leader || current->signal->tty) {
  		ret = -EPERM;
  		goto unlock;
  	}
  
  	if (tty->session) {
  		/*
  		 * This tty is already the controlling
  		 * tty for another session group!
  		 */
  		if (arg == 1 && capable(CAP_SYS_ADMIN)) {
  			/*
  			 * Steal it away
  			 */
  			session_clear_tty(tty->session);
  		} else {
  			ret = -EPERM;
  			goto unlock;
  		}
  	}
  
  	/* See the comment in tty_open_proc_set_tty(). */
  	if ((file->f_mode & FMODE_READ) == 0 && !capable(CAP_SYS_ADMIN)) {
  		ret = -EPERM;
  		goto unlock;
  	}
  
  	proc_set_tty(tty);
  unlock:
  	read_unlock(&tasklist_lock);
  	tty_unlock(tty);
  	return ret;
  }
  
  /**
   *	tty_get_pgrp	-	return a ref counted pgrp pid
   *	@tty: tty to read
   *
   *	Returns a refcounted instance of the pid struct for the process
   *	group controlling the tty.
   */
  struct pid *tty_get_pgrp(struct tty_struct *tty)
  {
  	unsigned long flags;
  	struct pid *pgrp;
  
  	spin_lock_irqsave(&tty->ctrl_lock, flags);
  	pgrp = get_pid(tty->pgrp);
  	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
  
  	return pgrp;
  }
  EXPORT_SYMBOL_GPL(tty_get_pgrp);
  
  /*
   * This checks not only the pgrp, but falls back on the pid if no
   * satisfactory pgrp is found. I dunno - gdb doesn't work correctly
   * without this...
   *
   * The caller must hold rcu lock or the tasklist lock.
   */
  static struct pid *session_of_pgrp(struct pid *pgrp)
  {
  	struct task_struct *p;
  	struct pid *sid = NULL;
  
  	p = pid_task(pgrp, PIDTYPE_PGID);
  	if (p == NULL)
  		p = pid_task(pgrp, PIDTYPE_PID);
  	if (p != NULL)
  		sid = task_session(p);
  
  	return sid;
  }
  
  /**
   *	tiocgpgrp		-	get process group
   *	@tty: tty passed by user
   *	@real_tty: tty side of the tty passed by the user if a pty else the tty
   *	@p: returned pid
   *
   *	Obtain the process group of the tty. If there is no process group
   *	return an error.
   *
   *	Locking: none. Reference to current->signal->tty is safe.
   */
  static int tiocgpgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
  {
  	struct pid *pid;
  	int ret;
  	/*
  	 * (tty == real_tty) is a cheap way of
  	 * testing if the tty is NOT a master pty.
  	 */
  	if (tty == real_tty && current->signal->tty != real_tty)
  		return -ENOTTY;
  	pid = tty_get_pgrp(real_tty);
  	ret =  put_user(pid_vnr(pid), p);
  	put_pid(pid);
  	return ret;
  }
  
  /**
   *	tiocspgrp		-	attempt to set process group
   *	@tty: tty passed by user
   *	@real_tty: tty side device matching tty passed by user
   *	@p: pid pointer
   *
   *	Set the process group of the tty to the session passed. Only
   *	permitted where the tty session is our session.
   *
   *	Locking: RCU, ctrl lock
   */
  static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
  {
  	struct pid *pgrp;
  	pid_t pgrp_nr;
  	int retval = tty_check_change(real_tty);
  
  	if (retval == -EIO)
  		return -ENOTTY;
  	if (retval)
  		return retval;
  
  	if (get_user(pgrp_nr, p))
  		return -EFAULT;
  	if (pgrp_nr < 0)
  		return -EINVAL;
  
  	spin_lock_irq(&real_tty->ctrl_lock);
  	if (!current->signal->tty ||
  	    (current->signal->tty != real_tty) ||
  	    (real_tty->session != task_session(current))) {
  		retval = -ENOTTY;
  		goto out_unlock_ctrl;
  	}
  	rcu_read_lock();
  	pgrp = find_vpid(pgrp_nr);
  	retval = -ESRCH;
  	if (!pgrp)
  		goto out_unlock;
  	retval = -EPERM;
  	if (session_of_pgrp(pgrp) != task_session(current))
  		goto out_unlock;
  	retval = 0;
  	put_pid(real_tty->pgrp);
  	real_tty->pgrp = get_pid(pgrp);
  out_unlock:
  	rcu_read_unlock();
  out_unlock_ctrl:
  	spin_unlock_irq(&real_tty->ctrl_lock);
  	return retval;
  }
  
  /**
   *	tiocgsid		-	get session id
   *	@tty: tty passed by user
   *	@real_tty: tty side of the tty passed by the user if a pty else the tty
   *	@p: pointer to returned session id
   *
   *	Obtain the session id of the tty. If there is no session
   *	return an error.
   */
  static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
  {
  	unsigned long flags;
  	pid_t sid;
  
  	/*
  	 * (tty == real_tty) is a cheap way of
  	 * testing if the tty is NOT a master pty.
  	 */
  	if (tty == real_tty && current->signal->tty != real_tty)
  		return -ENOTTY;
  
  	spin_lock_irqsave(&real_tty->ctrl_lock, flags);
  	if (!real_tty->session)
  		goto err;
  	sid = pid_vnr(real_tty->session);
  	spin_unlock_irqrestore(&real_tty->ctrl_lock, flags);
  
  	return put_user(sid, p);
  
  err:
  	spin_unlock_irqrestore(&real_tty->ctrl_lock, flags);
  	return -ENOTTY;
  }
  
  /*
   * Called from tty_ioctl(). If tty is a pty then real_tty is the slave side,
   * if not then tty == real_tty.
   */
  long tty_jobctrl_ioctl(struct tty_struct *tty, struct tty_struct *real_tty,
  		       struct file *file, unsigned int cmd, unsigned long arg)
  {
  	void __user *p = (void __user *)arg;
  
  	switch (cmd) {
  	case TIOCNOTTY:
  		if (current->signal->tty != tty)
  			return -ENOTTY;
  		no_tty();
  		return 0;
  	case TIOCSCTTY:
  		return tiocsctty(real_tty, file, arg);
  	case TIOCGPGRP:
  		return tiocgpgrp(tty, real_tty, p);
  	case TIOCSPGRP:
  		return tiocspgrp(tty, real_tty, p);
  	case TIOCGSID:
  		return tiocgsid(tty, real_tty, p);
  	}
  	return -ENOIOCTLCMD;
  }
  