Wireshark-dev: [Wireshark-dev] Tshark: proto_tree not created on first pass with tap defined
From: Paul Offord <[email protected]>
Date: Fri, 10 Feb 2017 08:15:45 +0000



I’ve been investigating a problem with transum, a post-dissector.  If you run tshark with transum it throws Access Violations.  I’m starting tshark with these arguments:


-2 -q -ta -o transum.tsumenabled:TRUE -T fields -E separator=, -E quote=d -E header=y -e frame.number -e _ws.col.Time -e ip.src -e ip.dst -e tcp.srcport -e tcp.dstport -e _ws.col.Info -r "C:\traces\Contoso_01\web01\web01_00001_20161012151754.pcap"


NB: The -2 flag indicates that tshark should make two passes.


The reason transum throws Access Violations is because the dissect_transum dissector is called on the first pass with a NULL proto_tree pointer.  I’ll add defensive code to transum to avoid the Access Violation but there is an underlying problem.


It's normal for a dissector to be called with a NULL proto_tree pointer on the first pass *unless* a tap has been registered.  Transum registers a tap, and so when using Wireshark, each first-pass call to dissect_transum includes a pointer to a proto_tree.  With tshark, even though the tap is registered the proto_tree pointer is still NULL.


When running Wirehsark, the decision to create a proto_tree is made in cf_read of file.c with this code:


  /* Get the union of the flags for all tap listeners. */

  tap_flags = union_of_tap_listener_flags();

  create_proto_tree =

    (dfcode != NULL || have_filtering_tap_listeners() || (tap_flags & TL_REQUIRES_PROTO_TREE));


  [lines removed from listing]


  epan_dissect_init(&edt, cf->epan, create_proto_tree, FALSE);



When running tshark with the parameters above the decision to create a proto_tree is made in load_cap_file(…) function of tshark.c with this code:


  /* Do we have any tap listeners with filters? */

  filtering_tap_listeners = have_filtering_tap_listeners();


  /* Get the union of the flags for all tap listeners. */

  tap_flags = union_of_tap_listener_flags();


  if (perform_two_pass_analysis) {

    frame_data *fdata;


   [lines removed from listing]


    if (do_dissection) {

       gboolean create_proto_tree = FALSE;


      /* If we're going to be applying a filter, we'll need to

         create a protocol tree against which to apply the filter. */

      if (cf->rfcode || cf->dfcode)

        create_proto_tree = TRUE;


      tshark_debug("tshark: create_proto_tree = %s", create_proto_tree ? "TRUE" : "FALSE");


      /* We're not going to display the protocol tree on this pass,

         so it's not going to be "visible". */

      edt = epan_dissect_new(cf->epan, create_proto_tree, FALSE);




Neither, cf->rfcode or cf->dfcode are true and so the tree isn’t created.  I think the code should be:


      if (cf->rfcode || cf->dfcode || filtering_tap_listeners)

        create_proto_tree = TRUE;



Am I right?  Have I misunderstood something about tshark?


Thanks and regards…Paul


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