WIP: gm: Decode GSM GM MS RA Capabilities IE with RLCMAC CSN1 decoder

Tested with a sample pcap file containing an GSM GM packet (Attach
Request)  with an MS Radio Access Capabilities IE containing several entries.

TODO:
* Check if by dropping the ad-hoc decoder we lose support/features.
* Drop de_gmm_ms_radio_acc_cap and replace it with
  de_gmm_ms_radio_acc_cap_rlcmac(), since there are other users of that
  function in other protocol dissectors. Then all benefit from the
  change. More than 1k lines of code can be dropped.
* Some general clean up required

Change-Id: I096eafcb5ca31d0ad1fa63561f43853ee4e7a40f

epan/dissectors/packet-csn1.c

@@ -531,25 +531,26 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
         csnStream_t arT = *ar;
         proto_item   *ti;
         proto_tree   *test_tree;
-
-        test_tree = proto_tree_add_subtree_format(tree, tvb, bit_offset>>3, 1, ett_csn1, &ti, "%s", pDescr->sz);
-
-        csnStreamInit(&arT, bit_offset, remaining_bits_len, ar->pinfo);
-        Status = csnStreamDissector(test_tree, &arT, (const CSN_DESCR*)pDescr->descr.ptr, tvb, pvDATA(data, pDescr->offset), ett_csn1);
-
-        if (Status >= 0)
+        if (pDescr->may_be_null && remaining_bits_len == 0)
         {
-          proto_item_set_len(ti,((arT.bit_offset-1)>>3) - (bit_offset>>3)+1);
-          remaining_bits_len  = arT.remaining_bits_len;
-          bit_offset          = arT.bit_offset;
-          pDescr++;
+          proto_tree_add_none_format(tree, hf_null_data, tvb, 0, 0, "[NULL data]: %s Not Present", pDescr->sz);
+        } else {
+          test_tree = proto_tree_add_subtree_format(tree, tvb, bit_offset>>3, 1, ett_csn1, &ti, "%s", pDescr->sz);
+          csnStreamInit(&arT, bit_offset, remaining_bits_len, ar->pinfo);
+          Status = csnStreamDissector(test_tree, &arT, (const CSN_DESCR*)pDescr->descr.ptr, tvb, pvDATA(data, pDescr->offset), ett_csn1);
+          if (Status >= 0)
+          {
+            proto_item_set_len(ti,((arT.bit_offset-1)>>3) - (bit_offset>>3)+1);
+            remaining_bits_len  = arT.remaining_bits_len;
+            bit_offset          = arT.bit_offset;
+          }
+          else
+          {
+            /* Has already been processed: ProcessError("csnStreamDissector", Status, pDescr);  */
+            return Status;
+          }
         }
-        else
-        {
-          /* Has already been processed: ProcessError("csnStreamDissector", Status, pDescr);  */
-          return Status;
-        }
-
+        pDescr++;
         break;
       }
 
@@ -1002,22 +1003,25 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
             proto_item   *ti;
             proto_tree   *test_tree;
 
-            test_tree = proto_tree_add_subtree(tree, tvb, bit_offset>>3, 1, ett_csn1, &ti, pDescr->sz);
-
-            csnStreamInit(&arT, bit_offset, remaining_bits_len, ar->pinfo);
-            Status = csnStreamDissector(test_tree, &arT, (const CSN_DESCR *)pDescr->descr.ptr, tvb, pvDATA(data, pDescr->offset), ett_csn1);
-            if (Status >= 0)
+            if (pDescr->may_be_null && remaining_bits_len == 0)
             {
-              proto_item_set_len(ti,((arT.bit_offset-1)>>3) - (bit_offset>>3)+1);
-              remaining_bits_len = arT.remaining_bits_len;
-              bit_offset         = arT.bit_offset;
-              pDescr++;
+              proto_tree_add_none_format(tree, hf_null_data, tvb, 0, 0, "[NULL data]: %s Not Present", pDescr->sz);
+            } else {
+              test_tree = proto_tree_add_subtree(tree, tvb, bit_offset>>3, 1, ett_csn1, &ti, pDescr->sz);
+              csnStreamInit(&arT, bit_offset, remaining_bits_len, ar->pinfo);
+              Status = csnStreamDissector(test_tree, &arT, (const CSN_DESCR *)pDescr->descr.ptr, tvb, pvDATA(data, pDescr->offset), ett_csn1);
+              if (Status >= 0)
+              {
+                proto_item_set_len(ti,((arT.bit_offset-1)>>3) - (bit_offset>>3)+1);
+                remaining_bits_len = arT.remaining_bits_len;
+                bit_offset         = arT.bit_offset;
+              }
+              else
+              { /* return error code Has already been processed:  */
+                return Status;
+              }
             }
-            else
-            { /* return error code Has already been processed:  */
-              return Status;
-            }
-
+            pDescr++;
             break;
           }
 
@@ -1066,7 +1070,7 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
 
       case CSN_NEXT_EXIST:
       {
-        guint8 fExist;
+        guint8 isnull;
 
         pui8  = pui8DATA(data, pDescr->offset);
 
@@ -1085,18 +1089,30 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
         /* the "regular" M_NEXT_EXIST description element */
         proto_tree_add_bits_item(tree, *(pDescr->hf_ptr), tvb, bit_offset, 1, ENC_BIG_ENDIAN);
 
-        fExist = 0x00;
+        isnull = 1;
         if (tvb_get_bits8(tvb, bit_offset, 1))
         {
-          fExist = 0x01;
+          if (remaining_bits_len == 1)
+          {
+             /* If { 1 < end > } and all next items may be null, store it as { 0 } */
+            const CSN_DESCR* pDescrNext = pDescr + 1;
+            guint8 i;
+            for (i = 0; i < pDescr->i; i++, pDescrNext++)
+            {
+              if (!pDescrNext->may_be_null)
+                isnull = 0;
+            }
+          } else {
+            isnull = 0;
+          }
         }
 
-        *pui8     = fExist;
+        *pui8     = !isnull;
 
         remaining_bits_len --;
         bit_offset++;
 
-        if (fExist == 0)
+        if (isnull)
         { /* Skip 'i' entries */
           pDescr += pDescr->i;
         }
@@ -1107,7 +1123,7 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
 
       case CSN_NEXT_EXIST_LH:
       {
-        guint8 fExist;
+        guint8 isnull;
         pui8  = pui8DATA(data, pDescr->offset);
 
         /* this if-statement represents the M_NEXT_EXIST_OR_NULL_LH description element */
@@ -1125,14 +1141,29 @@ csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pDescr, t
         /* the "regular" M_NEXT_EXIST_LH description element */
         proto_tree_add_bits_item(tree, *(pDescr->hf_ptr), tvb, bit_offset, 1, ENC_BIG_ENDIAN);
 
-        fExist = tvb_get_masked_bits8(tvb, bit_offset, 1);
+        isnull = 1;
+        if (tvb_get_masked_bits8(tvb, bit_offset, 1))
+        {
+          if (remaining_bits_len == 1) {
+             /* If { 1 < end > } and all next items may be null, store it as { 0 } */
+            const CSN_DESCR* pDescrNext = pDescr + 1;
+            guint8 i;
+            for (i = 0; i < pDescr->i; i++, pDescrNext++)
+            {
+              if (!pDescrNext->may_be_null)
+                isnull = 0;
+            }
+          } else {
+            isnull = 0;
+          }
+        }
 
-        *pui8++   = fExist;
+        *pui8++   = !isnull;
 
         remaining_bits_len --;
         bit_offset++;
 
-        if (fExist == 0)
+        if (isnull)
         { /* Skip 'i' entries */
           pDescr += pDescr->i;
         }

epan/dissectors/packet-csn1.h

@@ -477,6 +477,17 @@ gint16 csnStreamDissector(proto_tree *tree, csnStream_t* ar, const CSN_DESCR* pD
 #define M_TYPE(_STRUCT, _MEMBER, _MEMBER_TYPE)\
         {CSN_TYPE, 0, {(const void*)CSNDESCR_##_MEMBER_TYPE}, offsetof(_STRUCT, _MEMBER), FALSE, #_MEMBER, NULL, 0, NULL, NULL, NULL}
 
+/******************************************************************************
+ * M_TYPE_OR_NULL(Par1, Par2, Par3)
+ * Similar to the M_TYPE except that not only the request set of bits but also the
+ * end of the message may be encountered when looking for the next element in
+ * the message.
+ * Covers the case {null | 0 | 1 < IE >}
+ *****************************************************************************/
+#define M_TYPE_OR_NULL(_STRUCT, _MEMBER, _MEMBER_TYPE)\
+        {CSN_TYPE, 0, {(const void*)CSNDESCR_##_MEMBER_TYPE}, offsetof(_STRUCT, _MEMBER), TRUE, #_MEMBER, NULL, 0, NULL, NULL, NULL}
+
+
 /******************************************************************************
  * M_TYPE_LABEL(Par1, Par2, Par3, Par4)
  * Same as M_TYPE but allows to define a custom string for the subtree

epan/dissectors/packet-gsm_a_gm.c

@@ -91,6 +91,7 @@
 #include <epan/expert.h>
 #include <epan/ipproto.h>
 #include <epan/etypes.h>
+#include "packet-gsm_rlcmac.h"
 #include "packet-ber.h"
 #include "packet-gsm_a_common.h"
 #include "packet-e212.h"
@@ -609,6 +610,7 @@ static expert_field ei_gsm_a_gm_missing_mandatory_element = EI_INIT;
 static dissector_handle_t rrc_irat_ho_info_handle;
 static dissector_handle_t lte_rrc_ue_eutra_cap_handle;
 static dissector_handle_t nbifom_handle;
+static dissector_handle_t rlcmac_racap_handle;
 
 static dissector_table_t gprs_sm_pco_subdissector_table; /* GPRS SM PCO PPP Protocols */
 
@@ -1663,6 +1665,16 @@ static const value_string gsm_a_gm_ec_pch_mon_support_vals[] = {
 	{0, NULL}
 };
 
+guint16
+de_gmm_ms_radio_acc_cap_rlcmac(tvbuff_t *tvb, proto_tree *tree, packet_info *pinfo, guint32 offset, guint len, gchar *add_string _U_, int string_len _U_)
+{
+	tvbuff_t *payload_tvb;
+	payload_tvb = tvb_new_subset_length(tvb, offset, len);
+
+	call_dissector(rlcmac_racap_handle, payload_tvb, pinfo, tree);
+	return len;
+}
+
 guint16
 de_gmm_ms_radio_acc_cap(tvbuff_t *tvb, proto_tree *tree, packet_info *pinfo, guint32 offset, guint len, gchar *add_string _U_, int string_len _U_)
 {
@@ -6372,7 +6384,7 @@ guint16 (*gm_elem_fcn[])(tvbuff_t *tvb, proto_tree *tree, packet_info *pinfo _U_
 	de_gmm_imeisv_req,                 /* IMEISV Request */
 	de_gmm_rec_npdu_lst,               /* Receive N-PDU Numbers List */
 	de_gmm_ms_net_cap,                 /* MS Network Capability */
-	de_gmm_ms_radio_acc_cap,           /* MS Radio Access Capability */
+	de_gmm_ms_radio_acc_cap_rlcmac,           /* MS Radio Access Capability */
 	de_gmm_cause,                      /* GMM Cause */
 	de_gmm_rai,                        /* Routing Area Identification */
 	de_gmm_rai2,                       /* Routing Area Identification 2 */
@@ -9681,6 +9693,7 @@ proto_reg_handoff_gsm_a_gm(void)
 	rrc_irat_ho_info_handle = find_dissector_add_dependency("rrc.irat.irat_ho_info", proto_a_gm);
 	lte_rrc_ue_eutra_cap_handle = find_dissector_add_dependency("lte-rrc.ue_eutra_cap", proto_a_gm);
 	nbifom_handle = find_dissector_add_dependency("nbifom", proto_a_gm);
+	rlcmac_racap_handle = find_dissector_add_dependency("gsm_rlcmac_racap", proto_a_gm);
 }
 
 /*

epan/dissectors/packet-gsm_rlcmac.c

@@ -2750,7 +2750,7 @@ CSN_DESCR_BEGIN       (Content_t)
   /* additions in release 7 */
   M_UINT_OR_NULL      (Content_t,  DTM_Handover_Capability,  1, &hf_content_dtm_handover_capability),
   M_NEXT_EXIST_OR_NULL(Content_t, Exist_DownlinkDualCarrierCapability_r7, 1, &hf_content_multislot_capability_reduction_for_dl_dual_carrier_exist),
-  M_TYPE              (Content_t, DownlinkDualCarrierCapability_r7, DownlinkDualCarrierCapability_r7_t),
+  M_TYPE_OR_NULL      (Content_t, DownlinkDualCarrierCapability_r7, DownlinkDualCarrierCapability_r7_t),
 
   M_UINT_OR_NULL      (Content_t,  FlexibleTimeslotAssignment,  1, &hf_content_flexible_timeslot_assignment),
   M_UINT_OR_NULL      (Content_t,  GAN_PS_HandoverCapability,  1, &hf_content_gan_ps_handover_capability),
@@ -2819,13 +2819,11 @@ CSN_DESCR_END  (MS_RA_capability_value_t)
  * This one would be used to decode for instance MS RA Capabilities IE SGSN->MS on the PCU.
  * However, an ad-hoc decoder is used in this scenario in wireshark: See packet-gsm_a_gm.c de_gmm_ms_radio_acc_cap().
  */
-#if 0
 static const
 CSN_DESCR_BEGIN (MS_Radio_Access_capability_t)
   M_REC_TARRAY_1(MS_Radio_Access_capability_t, MS_RA_capability_value, MS_RA_capability_value_t, Count_MS_RA_capability_value, &hf_ms_ra_capability_value),
   M_PADDING_BITS(MS_Radio_Access_capability_t, &hf_padding),
 CSN_DESCR_END   (MS_Radio_Access_capability_t)
-#endif
 
 /* TS44.060 section 12.30  "MS Radio Access Capability 2". Same as above but without spare bits */
 static const
@@ -9903,6 +9901,20 @@ dissect_gsm_ec_rlcmac_uplink(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree
   return dissect_gsm_rlcmac_uplink(tvb, pinfo, tree, &rlc_mac);
 }
 
+static int
+dissect_gsm_rlcmac_racap(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data _U_)
+{
+	csnStream_t  ar;
+	MS_Radio_Access_capability_t racap = {0};
+        int len = tvb_reported_length(tvb);
+        guint16      bit_length = len * 8;
+
+	csnStreamInit(&ar, 0, bit_length, pinfo);
+
+	csnStreamDissector(tree, &ar, CSNDESCR(MS_Radio_Access_capability_t), tvb, &racap, ett_gsm_rlcmac);
+	return len;
+}
+
 void
 proto_register_gsm_rlcmac(void)
 {
@@ -18620,6 +18632,7 @@ proto_register_gsm_rlcmac(void)
   proto_register_subtree_array(ett, array_length(ett));
   expert_gsm_rlcmac = expert_register_protocol(proto_gsm_rlcmac);
   expert_register_field_array(expert_gsm_rlcmac, ei, array_length(ei));
+  register_dissector("gsm_rlcmac_racap", dissect_gsm_rlcmac_racap, proto_gsm_rlcmac);
   register_dissector("gsm_rlcmac_ul", dissect_gsm_rlcmac_uplink, proto_gsm_rlcmac);
   register_dissector("gsm_rlcmac_dl", dissect_gsm_rlcmac_downlink, proto_gsm_rlcmac);
   register_dissector("gsm_ec_rlcmac_ul", dissect_gsm_ec_rlcmac_uplink, proto_gsm_rlcmac);