LAPDm: Support the LAPDm B4 Frame Format

The B4 Frame Format is used on the downlink SACCH and has no length
field.

While the comment on top of packet-lapdm.c claimed ever since its
introduction in 2009 that B4 was a supported format, in fact it was not
supported yet.  This patch makes handling the length field conditional
to a frame format that has a length field, and introduces lapdm_data_t
that can be passed using call_dissector_with_data().

The GSMTAP dissector is updated to use this mechanism to specify the
frame format based on the channel type.

Change-Id: I52cb1cedbc8c7baf65e70d3e050e8932573647aa
Reviewed-on: https://code.wireshark.org/review/21767
Reviewed-by: Jaap Keuter <jaap.keuter@xs4all.nl>
Petri-Dish: Jaap Keuter <jaap.keuter@xs4all.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>

4 files changed, 102 insertions, 12 deletions

diff --git a/epan/dissectors/Makefile.am b/epan/dissectors/Makefile.am
index b5df4cdda3..5f49065059 100644
--- a/epan/dissectors/Makefile.am
+++ b/epan/dissectors/Makefile.am
@@ -1621,6 +1621,7 @@ DISSECTOR_INCLUDES = \
 	packet-kerberos.h	\
 	packet-klm.h		\
 	packet-l2tp.h		\
+	packet-lapdm.h		\
 	packet-lbm.h		\
 	packet-lbtrm.h		\
 	packet-lbtru.h		\
diff --git a/epan/dissectors/packet-gsmtap.c b/epan/dissectors/packet-gsmtap.c
index c5ddd0539a..6494caf78e 100644
--- a/epan/dissectors/packet-gsmtap.c
+++ b/epan/dissectors/packet-gsmtap.c
@@ -43,6 +43,7 @@
 #include <epan/packet.h>
 
 #include "packet-gsmtap.h"
+#include "packet-lapdm.h"
 #include "packet-tetra.h"
 
 void proto_register_gsmtap(void);
@@ -318,6 +319,17 @@ dissect_sacch_l1h(tvbuff_t *tvb, proto_tree *tree)
 	proto_tree_add_item(l1h_tree, hf_sacch_l1h_ta, tvb, 1, 1, ENC_BIG_ENDIAN);
 }
 
+static void
+handle_lapdm(guint8 sub_type, tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
+{
+	lapdm_data_t ld;
+
+	ld.hdr_type = LAPDM_HDR_FMT_B;
+	/* only downlink SACCH frames use B4 header format */
+	if (sub_type & GSMTAP_CHANNEL_ACCH && pinfo->p2p_dir == P2P_DIR_RECV)
+		ld.hdr_type = LAPDM_HDR_FMT_B4;
+	call_dissector_with_data(sub_handles[GSMTAP_SUB_UM_LAPDM], tvb, pinfo, tree, &ld);
+}
 
 static void
 handle_tetra(int channel _U_, tvbuff_t *payload_tvb _U_, packet_info *pinfo _U_, proto_tree *tree _U_)
@@ -489,8 +501,8 @@ dissect_gsmtap(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _
 		case GSMTAP_CHANNEL_SDCCH8:
 		case GSMTAP_CHANNEL_TCH_F:
 		case GSMTAP_CHANNEL_TCH_H:
-			sub_handle = GSMTAP_SUB_UM_LAPDM;
-			break;
+			handle_lapdm(sub_type, payload_tvb, pinfo, tree);
+			return tvb_captured_length(tvb);
 		case GSMTAP_CHANNEL_PACCH:
 			if (pinfo->p2p_dir == P2P_DIR_SENT) {
 				sub_handle = GSMTAP_SUB_UM_RLC_MAC_UL;
diff --git a/epan/dissectors/packet-lapdm.c b/epan/dissectors/packet-lapdm.c
index 3135a1166f..d351edd76e 100644
--- a/epan/dissectors/packet-lapdm.c
+++ b/epan/dissectors/packet-lapdm.c
@@ -50,6 +50,7 @@
  */
 
 #include "config.h"
+#include "packet-lapdm.h"
 
 #include <epan/packet.h>
 #include <epan/prefs.h>
@@ -130,6 +131,7 @@ static gboolean reassemble_lapdm = TRUE;
 #define LAPDM_LEN_SHIFT         2
 
 #define LAPDM_HEADER_LEN 3
+#define LAPDM_HEADER_LEN_B4 2
 
 #define LAPDM_SAPI_RR_CC_MM     0
 #define LAPDM_SAPI_SMS          3
@@ -201,27 +203,57 @@ static const fragment_items lapdm_frag_items = {
     "fragments"
 };
 
+static gboolean hdr_has_length(enum lapdm_hdr_type hdr_type)
+{
+    switch (hdr_type) {
+    case LAPDM_HDR_FMT_A:
+    case LAPDM_HDR_FMT_B:
+        return TRUE;
+    default:
+        return FALSE;
+    }
+}
+
 
 static int
-dissect_lapdm(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
+dissect_lapdm(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data)
 {
     proto_tree *lapdm_tree, *addr_tree, *length_tree;
     proto_item *lapdm_ti, *addr_ti, *length_ti;
-    guint8 addr, length, cr, sapi, len, n_s;
+    guint8 addr, length, header_len, cr, sapi, len, n_s;
     int control;
     gboolean m;
     tvbuff_t *payload;
     int available_length;
     gboolean is_response = FALSE;
+    enum lapdm_hdr_type hdr_type = LAPDM_HDR_FMT_B;
+
+    if (data) {
+        lapdm_data_t *ld = (lapdm_data_t *) data;
+        hdr_type = ld->hdr_type;
+    }
+
+    switch (hdr_type) {
+    case LAPDM_HDR_FMT_A:
+    case LAPDM_HDR_FMT_B:
+        length = tvb_get_guint8(tvb, 2);
+        header_len = LAPDM_HEADER_LEN;
+        break;
+    case LAPDM_HDR_FMT_B4:
+        length = 0;
+        header_len = LAPDM_HEADER_LEN_B4;
+        break;
+    default:
+        return 0;
+    }
 
     /* Check that there's enough data */
-    if (tvb_captured_length(tvb) < LAPDM_HEADER_LEN)
+    if (tvb_captured_length(tvb) < header_len)
         return 0;
 
     col_set_str(pinfo->cinfo, COL_PROTOCOL, "LAPDm");
 
     addr = tvb_get_guint8(tvb, 0);
-    length = tvb_get_guint8(tvb, 2);
 
     cr = addr & LAPDM_CR;
     if (pinfo->p2p_dir == P2P_DIR_RECV) {
@@ -232,7 +264,7 @@ dissect_lapdm(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U
     }
 
     if (tree) {
-        lapdm_ti = proto_tree_add_item(tree, proto_lapdm, tvb, 0, LAPDM_HEADER_LEN, ENC_NA);
+        lapdm_ti = proto_tree_add_item(tree, proto_lapdm, tvb, 0, header_len, ENC_NA);
         lapdm_tree = proto_item_add_subtree(lapdm_ti, ett_lapdm);
 
         addr_ti = proto_tree_add_uint(lapdm_tree, hf_lapdm_address, tvb, 0, 1, addr);
@@ -252,7 +284,8 @@ dissect_lapdm(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U
                                    ett_lapdm_control, &lapdm_cf_items, NULL /* LAPDm doesn't support extended */, NULL, NULL,
                                    is_response, FALSE, FALSE);
 
-    if (tree) {
+    /* dissect length field (if present) */
+    if (tree && hdr_has_length(hdr_type)) {
         length_ti = proto_tree_add_uint(lapdm_tree, hf_lapdm_length, tvb,
                                         2, 1, length);
         length_tree = proto_item_add_subtree(length_ti, ett_lapdm_length);
@@ -262,18 +295,24 @@ dissect_lapdm(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U
         proto_tree_add_uint(length_tree, hf_lapdm_el, tvb, 2, 1, length);
     }
 
+    if (hdr_has_length(hdr_type)) {
+        len = (length & LAPDM_LEN) >> LAPDM_LEN_SHIFT;
+        m = (length & LAPDM_M) >> LAPDM_M_SHIFT;
+    } else {
+        len = tvb_captured_length(tvb) - header_len;
+        m = 0;
+    }
+
     sapi = (addr & LAPDM_SAPI) >> LAPDM_SAPI_SHIFT;
-    len = (length & LAPDM_LEN) >> LAPDM_LEN_SHIFT;
     n_s = (control & XDLC_N_S_MASK) >> XDLC_N_S_SHIFT;
-    m = (length & LAPDM_M) >> LAPDM_M_SHIFT;
-    available_length = tvb_captured_length(tvb) - LAPDM_HEADER_LEN;
+    available_length = tvb_captured_length(tvb) - header_len;
 
     /* No point in doing anything if no payload
      */
     if( !MIN(len, available_length) )
         return 2;
 
-    payload = tvb_new_subset_length_caplen(tvb, LAPDM_HEADER_LEN, MIN(len,available_length), len);
+    payload = tvb_new_subset_length_caplen(tvb, header_len, MIN(len,available_length), len);
 
     /* Potentially segmented I frame
      */
diff --git a/epan/dissectors/packet-lapdm.h b/epan/dissectors/packet-lapdm.h
new file mode 100644
index 0000000000..7b399fd334
--- /dev/null
+++ b/epan/dissectors/packet-lapdm.h
@@ -0,0 +1,38 @@
+/* packet-lapdm.h
+ *
+ * Wireshark - Network traffic analyzer
+ * By Gerald Combs <gerald@wireshark.org>
+ * Copyright 1998 Gerald Combs
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#ifndef __PACKET_BTL2CAP_H__
+#define __PACKET_BTL2CAP_H__
+
+/* See GSM TS 04.06 */
+enum lapdm_hdr_type {
+    LAPDM_HDR_FMT_A,
+    LAPDM_HDR_FMT_B,
+    LAPDM_HDR_FMT_Bter,
+    LAPDM_HDR_FMT_B4,
+    LAPDM_HDR_FMT_C,
+};
+
+typedef struct _lapdm_data_t {
+    enum lapdm_hdr_type hdr_type;
+} lapdm_data_t;
+
+#endif