[WIP] packet: handle return value of new-style dissectorsreassembly-fixes

Before this patch, any non-zero return value from a new-style dissector
would be treated as success. The actual meaning of the return value
("need more bytes" or "accepted X bytes for PDU") was ignored.

After this patch, non-zero values will set desegment_offset and
desegment_len (if these were not set before and if reassembly is
allowed by the parent protocol).

Regression alert: many dissectors return bogus values. These fall in
the categories:

 - Calculate offset + value, return this. Bug: the data may actually
   be less (or more).
 - Return less than tvb_captured_length(tvb). Bug: if padding/junk
   follows a PDU, then it needs to be dropped. When a smaller value is
   returned, the new code assumes that it is part of the next segment.

Warning: currently, dissectors need to handle cases where multiple PDUs
are present in a single segment (otherwise the remainder gets ignored).
The HTTP dissector handles this with an internal loop for example.

To be done: fix dissectors to return a negative value rather than
setting desegment_len.

DO NOT MERGE: two getenv() for debugging.

Change-Id: Id6d5ea195d620ab2bfb18c39f86b598b7b2b4797

1 files changed, 31 insertions, 2 deletions

diff --git a/epan/packet.c b/epan/packet.c
index e938884126..7fba6fb9d7 100644
--- a/epan/packet.c
+++ b/epan/packet.c
@@ -597,8 +597,11 @@ struct dissector_handle {
  * old style dissector :
  *   length of the payload or 1 of the payload is empty
  * new dissector :
- *   >0  this protocol was successfully dissected and this was this protocol.
+ *   >0  the protocol accepted the data, this is the number of bytes that were
+ *       processed. If not equal to tvb_captured_length(), then the remaining
+ *       data will be used for the next PDU (if reassembly is enabled).
  *   0   this packet did not match this protocol.
+ *   <0  this protocol accepted the data, but needs more for a PDU.
  *
  * The only time this function will return 0 is if it is a new style dissector
  * and if the dissector rejected the packet.
@@ -621,11 +624,37 @@ call_dissector_through_handle(dissector_handle_t handle, tvbuff_t *tvb,
 		EP_CHECK_CANARY(("before calling handle->dissector.new_d for %s",handle->name));
 		ret = (*handle->dissector.new_d)(tvb, pinfo, tree, data);
 		EP_CHECK_CANARY(("after calling handle->dissector.new_d for %s",handle->name));
+		/* If the parent protocol can handle desegmentation (such as
+		 * TCP), then set params according to the dissector return
+		 * value, but *only* if desegment_len was not already set */
+		if (!getenv("WS_DISABLE_EXPERIMENT"))
+		if (pinfo->can_desegment && pinfo->desegment_len == 0) {
+			if (ret < 0) {
+				/* need more bytes, so request another segment.
+				 * Note that desegment_offset may be set if the
+				 * dissector could already extract some PDUs */
+				pinfo->desegment_len = DESEGMENT_ONE_MORE_SEGMENT;
+			} else if (ret > 0) {
+				/* succesfully dissected, check for more
+				 * segments or verify boundary */
+				guint caplen = tvb_captured_length(tvb);
+
+				if (getenv("WS_BULLSHIT") && (guint) ret < caplen) {
+					/* not all data was consumed, assume
+					 * that more data is necessary */
+					pinfo->desegment_offset = ret;
+					pinfo->desegment_len = DESEGMENT_ONE_MORE_SEGMENT;
+				} else {
+					/* ret must not be larger than available */
+					DISSECTOR_ASSERT((guint) ret <= caplen);
+				}
+			}
+		}
 	} else {
 		EP_CHECK_CANARY(("before calling handle->dissector.old for %s",handle->name));
 		(*handle->dissector.old)(tvb, pinfo, tree);
 		EP_CHECK_CANARY(("after calling handle->dissector.old for %s",handle->name));
-		ret = tvb_length(tvb);
+		ret = tvb_captured_length(tvb);
 		if (ret == 0) {
 			/*
 			 * XXX - a tvbuff can have 0 bytes of data in