[PATCH] First cut at setsid/tty locking

Use the existing "tty_sem" to protect against the process tty changes
too.

drivers/char/tty_io.c

@@ -918,9 +918,11 @@ void disassociate_ctty(int on_exit)
 
 	lock_kernel();
 
+	down(&tty_sem);
 	tty = current->signal->tty;
 	if (tty) {
 		tty_pgrp = tty->pgrp;
+		up(&tty_sem);
 		if (on_exit && tty->driver->type != TTY_DRIVER_TYPE_PTY)
 			tty_vhangup(tty);
 	} else {
@@ -928,6 +930,7 @@ void disassociate_ctty(int on_exit)
 			kill_pg(current->signal->tty_old_pgrp, SIGHUP, on_exit);
 			kill_pg(current->signal->tty_old_pgrp, SIGCONT, on_exit);
 		}
+		up(&tty_sem);
 		unlock_kernel();	
 		return;
 	}
@@ -937,15 +940,19 @@ void disassociate_ctty(int on_exit)
 			kill_pg(tty_pgrp, SIGCONT, on_exit);
 	}
 
+	/* Must lock changes to tty_old_pgrp */
+	down(&tty_sem);
 	current->signal->tty_old_pgrp = 0;
 	tty->session = 0;
 	tty->pgrp = -1;
 
+	/* Now clear signal->tty under the lock */
 	read_lock(&tasklist_lock);
 	do_each_task_pid(current->signal->session, PIDTYPE_SID, p) {
 		p->signal->tty = NULL;
 	} while_each_task_pid(current->signal->session, PIDTYPE_SID, p);
 	read_unlock(&tasklist_lock);
+	up(&tty_sem);
 	unlock_kernel();
 }
 
@@ -1172,12 +1179,6 @@ static int init_dev(struct tty_driver *driver, int idx,
 	struct termios *ltp, **ltp_loc, *o_ltp, **o_ltp_loc;
 	int retval=0;
 
-	/* 
-	 * Check whether we need to acquire the tty semaphore to avoid
-	 * race conditions.  For now, play it safe.
-	 */
-	down(&tty_sem);
-
 	/* check whether we're reopening an existing tty */
 	if (driver->flags & TTY_DRIVER_DEVPTS_MEM) {
 		tty = devpts_get_tty(idx);
@@ -1366,7 +1367,6 @@ static int init_dev(struct tty_driver *driver, int idx,
 	
 	/* All paths come through here to release the semaphore */
 end_init:
-	up(&tty_sem);
 	return retval;
 
 	/* Release locally allocated memory ... nothing placed in slots */
@@ -1562,9 +1562,14 @@ static void release_dev(struct file * filp)
 	 * each iteration we avoid any problems.
 	 */
 	while (1) {
+		/* Guard against races with tty->count changes elsewhere and
+		   opens on /dev/tty */
+		   
+		down(&tty_sem);
 		tty_closing = tty->count <= 1;
 		o_tty_closing = o_tty &&
 			(o_tty->count <= (pty_master ? 1 : 0));
+		up(&tty_sem);
 		do_sleep = 0;
 
 		if (tty_closing) {
@@ -1600,6 +1605,8 @@ static void release_dev(struct file * filp)
 	 * both sides, and we've completed the last operation that could 
 	 * block, so it's safe to proceed with closing.
 	 */
+	 
+	down(&tty_sem);
 	if (pty_master) {
 		if (--o_tty->count < 0) {
 			printk(KERN_WARNING "release_dev: bad pty slave count "
@@ -1613,6 +1620,7 @@ static void release_dev(struct file * filp)
 		       tty->count, tty_name(tty, buf));
 		tty->count = 0;
 	}
+	up(&tty_sem);
 	
 	/*
 	 * We've decremented tty->count, so we need to remove this file
@@ -1761,9 +1769,13 @@ static int tty_open(struct inode * inode, struct file * filp)
 	index  = -1;
 	retval = 0;
 	
+	down(&tty_sem);
+
 	if (device == MKDEV(TTYAUX_MAJOR,0)) {
-		if (!current->signal->tty)
+		if (!current->signal->tty) {
+			up(&tty_sem);
 			return -ENXIO;
+		}
 		driver = current->signal->tty->driver;
 		index = current->signal->tty->index;
 		filp->f_flags |= O_NONBLOCK; /* Don't let /dev/tty block */
@@ -1788,14 +1800,18 @@ static int tty_open(struct inode * inode, struct file * filp)
 			noctty = 1;
 			goto got_driver;
 		}
+		up(&tty_sem);
 		return -ENODEV;
 	}
 
 	driver = get_tty_driver(device, &index);
-	if (!driver)
+	if (!driver) {
+		up(&tty_sem);
 		return -ENODEV;
+	}
 got_driver:
 	retval = init_dev(driver, index, &tty);
+	up(&tty_sem);
 	if (retval)
 		return retval;
 
@@ -1881,7 +1897,10 @@ static int ptmx_open(struct inode * inode, struct file * filp)
 	}
 	up(&allocated_ptys_lock);
 
+	down(&tty_sem);
 	retval = init_dev(ptm_driver, index, &tty);
+	up(&tty_sem);
+	
 	if (retval)
 		goto out;
 

kernel/exit.c

@@ -332,7 +332,9 @@ void daemonize(const char *name, ...)
 	exit_mm(current);
 
 	set_special_pids(1, 1);
+	down(&tty_sem);
 	current->signal->tty = NULL;
+	up(&tty_sem);
 
 	/* Block and flush all signals */
 	sigfillset(&blocked);

kernel/sys.c

@@ -1075,6 +1075,7 @@ asmlinkage long sys_setsid(void)
 	if (!thread_group_leader(current))
 		return -EINVAL;
 
+	down(&tty_sem);
 	write_lock_irq(&tasklist_lock);
 
 	pid = find_pid(PIDTYPE_PGID, current->pid);
@@ -1088,6 +1089,7 @@ asmlinkage long sys_setsid(void)
 	err = process_group(current);
 out:
 	write_unlock_irq(&tasklist_lock);
+	up(&tty_sem);
 	return err;
 }