Add a VT_LOCKSWITCH ioctl that disallows vty switching. Something

like this can be emulated by VT_SETMODEing to VT_PROCESS and never
releasing the vty, but this has a number of problems, most notably
that a process must stay resident for the lock to be in effect.

Reviewed by:	roam, sheldonh

sys/dev/syscons/syscons.c

@@ -981,6 +981,13 @@ scioctl(dev_t dev, u_long cmd, caddr_t data, int flag, struct thread *td)
 	*(int *)data = scp->index + 1;
 	return 0;
 
+    case VT_LOCKSWITCH:		/* prevent vty switching */
+	if ((*(int *)data) & 0x01)
+	    sc->flags |= SC_SCRN_VTYLOCK;
+	else
+	    sc->flags &= ~SC_SCRN_VTYLOCK;
+	return 0;
+
     case KDENABIO:      	/* allow io operations */
 	error = suser(td);
 	if (error != 0)
@@ -2075,6 +2082,13 @@ sc_switch_scr(sc_softc_t *sc, u_int next_scr)
 
     DPRINTF(5, ("sc0: sc_switch_scr() %d ", next_scr + 1));
 
+    /* prevent switch if previously requested */
+    if (sc->flags & SC_SCRN_VTYLOCK) {
+	    sc_bell(sc->cur_scp, sc->cur_scp->bell_pitch,
+		sc->cur_scp->bell_duration);
+	    return EPERM;
+    }
+
     /* delay switch if the screen is blanked or being updated */
     if ((sc->flags & SC_SCRN_BLANKED) || sc->write_in_progress
 	|| sc->blink_in_progress || sc->videoio_in_progress) {

sys/dev/syscons/syscons.h

@@ -194,6 +194,7 @@ typedef struct sc_softc {
 #define	SC_SCRN_IDLE	(1 << 5)
 #define	SC_SCRN_BLANKED	(1 << 6)
 #define	SC_SAVER_FAILED	(1 << 7)
+#define	SC_SCRN_VTYLOCK	(1 << 8)
 
 #define	SC_INIT_DONE	(1 << 16)
 #define	SC_SPLASH_SCRN	(1 << 17)

sys/sys/consio.h

@@ -335,6 +335,9 @@ typedef struct vt_mode vtmode_t;
 /* get the index of the vty */
 #define VT_GETINDEX	_IOR('v', 8, int)
 
+/* prevent switching vtys */
+#define VT_LOCKSWITCH	_IOW('v', 9, int)
+
 /*
  * Video mode switching ioctl.  See sys/fbio.h for mode numbers.
  */