packet-vnc.c: Improve re-assembly.

Essentially:

  When more data is needed to continue dissecting a PDU, use
  DESEGMENT_ONE_MORE_SEGMENT instead of repeatedly requesting
  additional bytes (for one or a few more fields).

  - Improves the efficiency of the dissection;

  - Prevents 'one-pass' tshark dissection from redissecting
    the PDU repeatedly many, many times with each time dissecting
    the PDU with one or a few more additional fields.
    This generated *lots* of (repeated) output since a reassembled
    VNC PDU can contain many fields (each of short length).

  - (A comment in packet-tcp.c states, in effect, that repeatedly
     requesting a specific amount of more bytes to dissect a PDU
     will "break reassembly" although I note that the reassembly did
     seem to work (in-efficiently)).

Note: Although this patch improves the handling of reassembly, the
      dissector has significant issues. For example. see Bug #5366.

I expect this fixes the Bug #10134 issue: "Cannot allocate memory";

Before the fix, 'tshark -nVxr' for the input file generated trees with
multiple hundreds of thousands of entries and generated reassembled
PDUs consisting of many, many small fragments.

Change-Id: I970037c346fbaa4bffa5726fd5bee5f69396eabf
Reviewed-on: https://code.wireshark.org/review/2471
Reviewed-by: Bill Meier <wmeier@newsguy.com>

1 files changed, 16 insertions, 7 deletions

diff --git a/epan/dissectors/packet-vnc.c b/epan/dissectors/packet-vnc.c
index c3b5f3f42d..04449309fb 100644
--- a/epan/dissectors/packet-vnc.c
+++ b/epan/dissectors/packet-vnc.c
@@ -1590,7 +1590,7 @@ vnc_server_to_client(tvbuff_t *tvb, packet_info *pinfo, gint *offset,
 {
 	gint start_offset;
 	guint8 message_type;
-	gint bytes_needed = 0, length_remaining;
+	gint bytes_needed = 0;
 
 	proto_item *ti = NULL;
 	proto_tree *vnc_server_message_type_tree;
@@ -1639,13 +1639,9 @@ vnc_server_to_client(tvbuff_t *tvb, packet_info *pinfo, gint *offset,
 		break;
 	}
 
-	if(bytes_needed > 0 && vnc_preference_desegment &&
-	   pinfo->can_desegment) {
-		length_remaining = tvb_length_remaining(tvb, *offset);
-
+	if(bytes_needed > 0 && vnc_preference_desegment && pinfo->can_desegment) {
 		pinfo->desegment_offset = start_offset;
-		pinfo->desegment_len = bytes_needed - length_remaining;
-		return;
+		pinfo->desegment_len = DESEGMENT_ONE_MORE_SEGMENT;
 	}
 }
 
@@ -4625,3 +4621,16 @@ proto_reg_handoff_vnc(void)
 		}
 	}
 }
+
+/*
+ * Editor modelines  -  http://www.wireshark.org/tools/modelines.html
+ *
+ * Local variables:
+ * c-basic-offset: 8
+ * tab-width: 8
+ * indent-tabs-mode: t
+ * End:
+ *
+ * vi: set shiftwidth=8 tabstop=8 noexpandtab:
+ * :indentSize=8:tabSize=8:noTabs=false:
+ */