Have different MTP2 dissectors for different metadata and CRC.

Have separate MTP2 dissectors for:

	MTP2 frames with no pseudo-header and no CRC;

	MTP2 frames with a pseudo-header and no CRC;

	MTP2 frames with no pseudo-header and a CRC;

and call the appropriate dissector in the appropriate places.

While we're at it, get rid of a global variable - pass the "use extended
sequence numbers" flag down through the dissection code path, rather
than having it as a global.

Change-Id: Id8da1fbe3529e3ffadd5c30646cbc922f506a01f
Reviewed-on: https://code.wireshark.org/review/5679
Reviewed-by: Guy Harris <guy@alum.mit.edu>

2 files changed, 45 insertions, 28 deletions

diff --git a/epan/dissectors/packet-erf.c b/epan/dissectors/packet-erf.c
index 1438d33e36..033e184a61 100644
--- a/epan/dissectors/packet-erf.c
+++ b/epan/dissectors/packet-erf.c
@@ -1949,7 +1949,7 @@ proto_reg_handoff_erf(void)
   chdlc_handle  = find_dissector("chdlc");
   ppp_handle    = find_dissector("ppp_hdlc");
   frelay_handle = find_dissector("fr");
-  mtp2_handle   = find_dissector("mtp2");
+  mtp2_handle   = find_dissector("mtp2_with_crc");
 
   /* Get handle for ATM dissector */
   atm_untruncated_handle = find_dissector("atm_untruncated");
diff --git a/epan/dissectors/packet-mtp2.c b/epan/dissectors/packet-mtp2.c
index c979688507..ffa720de76 100644
--- a/epan/dissectors/packet-mtp2.c
+++ b/epan/dissectors/packet-mtp2.c
@@ -67,7 +67,6 @@ static gint ett_mtp2       = -1;
 
 static dissector_handle_t mtp3_handle;
 static gboolean use_extended_sequence_numbers_default = FALSE;
-static gboolean use_extended_sequence_numbers         = FALSE;
 
 #define BSN_BIB_LENGTH          1
 #define FSN_FIB_LENGTH          1
@@ -105,7 +104,7 @@ static gboolean use_extended_sequence_numbers         = FALSE;
 #define EXTENDED_SPARE_MASK     0xfe00
 
 static void
-dissect_mtp2_header(tvbuff_t *su_tvb, proto_item *mtp2_tree)
+dissect_mtp2_header(tvbuff_t *su_tvb, proto_item *mtp2_tree, gboolean use_extended_sequence_numbers)
 {
   if (mtp2_tree) {
     if (use_extended_sequence_numbers) {
@@ -245,7 +244,8 @@ static const value_string status_field_acro_vals[] = {
 #define SF_EXTRA_LENGTH                 1
 
 static void
-dissect_mtp2_lssu(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_tree)
+dissect_mtp2_lssu(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_tree,
+                  gboolean use_extended_sequence_numbers)
 {
   guint8 sf = 0xFF;
   guint8 sf_offset, sf_extra_offset;
@@ -272,7 +272,8 @@ dissect_mtp2_lssu(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_tree)
 }
 
 static void
-dissect_mtp2_msu(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_item, proto_item *tree)
+dissect_mtp2_msu(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_item,
+                 proto_item *tree, gboolean use_extended_sequence_numbers)
 {
   gint sif_sio_length;
   tvbuff_t *sif_sio_tvb;
@@ -297,12 +298,14 @@ dissect_mtp2_msu(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_item, pr
 }
 
 static void
-dissect_mtp2_su(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_item, proto_item *mtp2_tree, proto_tree *tree,gboolean validate_crc)
+dissect_mtp2_su(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_item,
+                proto_item *mtp2_tree, proto_tree *tree, gboolean validate_crc,
+                gboolean use_extended_sequence_numbers)
 {
   guint16 li;
   tvbuff_t  *next_tvb = NULL;
 
-  dissect_mtp2_header(su_tvb, mtp2_tree);
+  dissect_mtp2_header(su_tvb, mtp2_tree, use_extended_sequence_numbers);
   if (validate_crc)
     next_tvb = mtp2_decode_crc16(su_tvb, mtp2_tree, pinfo);
 
@@ -317,52 +320,60 @@ dissect_mtp2_su(tvbuff_t *su_tvb, packet_info *pinfo, proto_item *mtp2_item, pro
   case 1:
   case 2:
     if (validate_crc)
-      dissect_mtp2_lssu(next_tvb, pinfo, mtp2_tree);
+      dissect_mtp2_lssu(next_tvb, pinfo, mtp2_tree, use_extended_sequence_numbers);
     else
-      dissect_mtp2_lssu(su_tvb, pinfo, mtp2_tree);
+      dissect_mtp2_lssu(su_tvb, pinfo, mtp2_tree, use_extended_sequence_numbers);
     break;
   default:
     /* In some capture files (like .rf5), CRC are not present */
     /* So, to avoid trouble, give the complete buffer if CRC validation is disabled */
     if (validate_crc)
-      dissect_mtp2_msu(next_tvb, pinfo, mtp2_item, tree);
+      dissect_mtp2_msu(next_tvb, pinfo, mtp2_item, tree, use_extended_sequence_numbers);
     else
-      dissect_mtp2_msu(su_tvb, pinfo, mtp2_item, tree);
+      dissect_mtp2_msu(su_tvb, pinfo, mtp2_item, tree, use_extended_sequence_numbers);
     break;
   }
 }
 
 static void
-dissect_mtp2_common(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, gboolean validate_crc)
+dissect_mtp2_common(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree,
+                    gboolean validate_crc, gboolean use_extended_sequence_numbers)
 {
   proto_item *mtp2_item;
   proto_tree *mtp2_tree;
 
-  if ((pinfo->phdr->pkt_encap != WTAP_ENCAP_MTP2_WITH_PHDR) ||
-      (pinfo->pseudo_header->mtp2.annex_a_used == MTP2_ANNEX_A_USED_UNKNOWN))
-    use_extended_sequence_numbers = use_extended_sequence_numbers_default;
-  else
-    use_extended_sequence_numbers = (pinfo->pseudo_header->mtp2.annex_a_used == MTP2_ANNEX_A_USED);
-
   col_set_str(pinfo->cinfo, COL_PROTOCOL, "MTP2");
 
   mtp2_item = proto_tree_add_item(tree, proto_mtp2, tvb, 0, -1, ENC_NA);
   mtp2_tree = proto_item_add_subtree(mtp2_item, ett_mtp2);
 
-  dissect_mtp2_su(tvb, pinfo, mtp2_item, mtp2_tree, tree, validate_crc);
+  dissect_mtp2_su(tvb, pinfo, mtp2_item, mtp2_tree, tree, validate_crc,
+                  use_extended_sequence_numbers);
+}
+
+/* Dissect MTP2 frame without CRC16 and with a pseudo-header */
+static void
+dissect_mtp2_with_phdr(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
+{
+  if (pinfo->pseudo_header->mtp2.annex_a_used == MTP2_ANNEX_A_USED_UNKNOWN)
+    dissect_mtp2_common(tvb, pinfo, tree, FALSE, use_extended_sequence_numbers_default);
+  else
+    dissect_mtp2_common(tvb, pinfo, tree, FALSE,
+                        (pinfo->pseudo_header->mtp2.annex_a_used == MTP2_ANNEX_A_USED));
+}
+
+/* Dissect MTP2 frame with CRC16 included at end of payload */
+static void
+dissect_mtp2_with_crc(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
+{
+  dissect_mtp2_common(tvb, pinfo, tree, TRUE, use_extended_sequence_numbers_default);
 }
 
-/* Dissect MTP2 frame with/without CRC16 included at end of payload */
+/* Dissect MTP2 frame without CRC16 included at end of payload */
 static void
 dissect_mtp2(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree)
 {
-  /* If the link extension indicates the FCS presence, then the Checkbits
-   * have to be proceeded in the MTP2 dissector */
-  if ( pinfo->fd->lnk_t == WTAP_ENCAP_ERF ) {
-    dissect_mtp2_common(tvb, pinfo, tree, TRUE);
-  } else {
-    dissect_mtp2_common(tvb, pinfo, tree, FALSE);
-  }
+  dissect_mtp2_common(tvb, pinfo, tree, FALSE, use_extended_sequence_numbers_default);
 }
 
 void
@@ -401,6 +412,7 @@ proto_register_mtp2(void)
 
   proto_mtp2 = proto_register_protocol("Message Transfer Part Level 2", "MTP2", "mtp2");
   mtp2_handle = register_dissector("mtp2", dissect_mtp2, proto_mtp2);
+  register_dissector("mtp2_with_crc", dissect_mtp2_with_crc, proto_mtp2);
 
   proto_register_field_array(proto_mtp2, hf, array_length(hf));
   proto_register_subtree_array(ett, array_length(ett));
@@ -420,8 +432,13 @@ proto_register_mtp2(void)
 void
 proto_reg_handoff_mtp2(void)
 {
+  dissector_handle_t mtp2_with_phdr_handle;
+
   dissector_add_uint("wtap_encap", WTAP_ENCAP_MTP2, mtp2_handle);
-  dissector_add_uint("wtap_encap", WTAP_ENCAP_MTP2_WITH_PHDR, mtp2_handle);
+  mtp2_with_phdr_handle = create_dissector_handle(dissect_mtp2_with_phdr,
+                                                  proto_mtp2);
+  dissector_add_uint("wtap_encap", WTAP_ENCAP_MTP2_WITH_PHDR,
+                                   mtp2_with_phdr_handle);
 
   mtp3_handle   = find_dissector("mtp3");
 }