csn1: Avoid storing existence bit as true if content was actually NULL

If we decode Exist bit as "1" but we are at the end of the message, and
all the Next items we'd read are expected to be possibly NULL, then swap
the Exist bit in the decoded structure as "0" in order to tell the
decoder user that the related information structure is actually unset,
as if "0" was received.

This patch is a port from patch fixing same issue in the osmo-pcu.git copy of
csn1 decoder:
https://git.osmocom.org/osmo-pcu/commit/?id=1859ec38cc4f4e3788e495a100fdec3787d25020
And fixup patch for that one:
https://git.osmocom.org/osmo-pcu/commit/?id=9ecdc11eb6b983748ae2fd6a1d07849c8106826f

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_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),