[PATCH] vt: rework the console spawning variables

This is such a rare path it took me a while to figure out how to test
this after soring out the locking.

This patch does several things.
- The variables used are moved into a structure and declared in vt_kern.h
- A spinlock is added so we don't have SMP races updating the values.
- Instead of raw pid_t value a struct_pid is used to guard against
  pid wrap around issues, if the daemon to spawn a new console dies.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/keyboard.c

@@ -108,7 +108,11 @@ const int NR_TYPES = ARRAY_SIZE(max_vals);
 struct kbd_struct kbd_table[MAX_NR_CONSOLES];
 static struct kbd_struct *kbd = kbd_table;
 
-int spawnpid, spawnsig;
+struct vt_spawn_console vt_spawn_con = {
+	.lock = SPIN_LOCK_UNLOCKED,
+	.pid  = NULL,
+	.sig  = 0,
+};
 
 /*
  * Variables exported for vt.c
@@ -578,9 +582,13 @@ static void fn_compose(struct vc_data *vc, struct pt_regs *regs)
 
 static void fn_spawn_con(struct vc_data *vc, struct pt_regs *regs)
 {
-	if (spawnpid)
-		if (kill_proc(spawnpid, spawnsig, 1))
-			spawnpid = 0;
+	spin_lock(&vt_spawn_con.lock);
+	if (vt_spawn_con.pid)
+		if (kill_pid(vt_spawn_con.pid, vt_spawn_con.sig, 1)) {
+			put_pid(vt_spawn_con.pid);
+			vt_spawn_con.pid = NULL;
+		}
+	spin_unlock(&vt_spawn_con.lock);
 }
 
 static void fn_SAK(struct vc_data *vc, struct pt_regs *regs)

drivers/char/vt_ioctl.c

@@ -645,13 +645,16 @@ int vt_ioctl(struct tty_struct *tty, struct file * file,
 	 */
 	case KDSIGACCEPT:
 	{
-		extern int spawnpid, spawnsig;
 		if (!perm || !capable(CAP_KILL))
 		  return -EPERM;
 		if (!valid_signal(arg) || arg < 1 || arg == SIGKILL)
 		  return -EINVAL;
-		spawnpid = current->pid;
-		spawnsig = arg;
+
+		spin_lock_irq(&vt_spawn_con.lock);
+		put_pid(vt_spawn_con.pid);
+		vt_spawn_con.pid = get_pid(task_pid(current));
+		vt_spawn_con.sig = arg;
+		spin_unlock_irq(&vt_spawn_con.lock);
 		return 0;
 	}
 

include/linux/vt_kern.h

@@ -84,4 +84,11 @@ void reset_vc(struct vc_data *vc);
 extern char con_buf[CON_BUF_SIZE];
 extern struct semaphore con_buf_sem;
 
+struct vt_spawn_console {
+	spinlock_t lock;
+	struct pid *pid;
+	int sig;
+};
+extern struct vt_spawn_console vt_spawn_con;
+
 #endif /* _VT_KERN_H */