[lac] Do not use the reserved LAC 0x0 for two different things

We are using LAC=0 for remembering that a GSM subscriber is
detached. I recently added code to gsm_bts_by_lac that will
return every BTS in case the lac is 0. Harald highlightes
that we would now search for detached subscribers at every
BTS of our network which is clearly not what we want.

Introduce two defines for the two reserved LAC, add a
pointer to the specification, check that our config files
do not contain these reserved values, use the define
and change gsm_bts_by_lac to use the other define.

openbsc/include/openbsc/gsm_04_08.h

@@ -660,6 +660,10 @@ enum chreq_type {
 #define SBIT(a) (1 << a)
 #define ALL_STATES 0xffffffff
 
+/* Table 10.5.3/3GPP TS 04.08: Location Area Identification information element */
+#define GSM_LAC_RESERVED_DETACHED       0x0
+#define GSM_LAC_RESERVED_ALL_BTS        0xfffe
+
 /* GSM 04.08 Bearer Capability: Information Transfer Capability */
 enum gsm48_bcap_itcap {
 	GSM48_BCAP_ITCAP_SPEECH		= 0,

openbsc/src/gsm_data.c

@@ -274,7 +274,7 @@ struct gsm_bts *gsm_bts_by_lac(struct gsm_network *net, unsigned int lac,
 			continue;
 		}
 
-		if (lac == 0 || bts->location_area_code == lac)
+		if (lac == GSM_LAC_RESERVED_ALL_BTS || bts->location_area_code == lac)
 			return bts;
 	}
 	return NULL;

openbsc/src/gsm_subscriber.c

@@ -105,7 +105,7 @@ int subscr_update(struct gsm_subscriber *s, struct gsm_bts *bts, int reason)
 	case GSM_SUBSCRIBER_UPDATE_DETACHED:
 		/* Only detach if we are currently in this area */
 		if (bts->location_area_code == s->lac)
-			s->lac = 0;
+			s->lac = GSM_LAC_RESERVED_DETACHED;
 		dispatch_signal(SS_SUBSCR, S_SUBSCR_DETACHED, s);
 		break;
 	default:

openbsc/src/vty_interface.c

@@ -872,6 +872,13 @@ DEFUN(cfg_bts_lac,
 			lac, VTY_NEWLINE);
 		return CMD_WARNING;
 	}
+
+	if (lac == GSM_LAC_RESERVED_DETACHED || lac == GSM_LAC_RESERVED_ALL_BTS) {
+		vty_out(vty, "%% LAC %d is reserved by GSM 04.08%s",
+			lac, VTY_NEWLINE);
+		return CMD_WARNING;
+	}
+
 	bts->location_area_code = lac;
 
 	return CMD_SUCCESS;