MCTP: Add NCSI (and ethernet) over MCTP dissection

This change adds support for trivially-encapsulated MCTP protocols,
starting with NCSI-over-MCTP.

We need to handle this slightly different from the existing MCTP-based
protocols (MCTP control protocol and NVMe-MI), as the inner protocol is
unaware of the type byte and (optional) checksum tailer. So, add a new
dissector table, "mctp.encap-type" for these, meaning we can just hook
into the raw NC-SI dissector.

We also add the type definition for MCTP-over-ethernet, as defined in
the NCSI-over-MCTP specification.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>

epan/dissectors/packet-eth.c

@@ -35,6 +35,7 @@
 #include "packet-vxlan.h"
 #include "packet-nsh.h"
 #include "packet-acdr.h"
+#include "packet-mctp.h"
 #include <epan/crc32-tvb.h>
 #include <wiretap/erf_record.h>
 
@@ -1197,6 +1198,7 @@ proto_reg_handoff_eth(void)
 
   dissector_add_uint("acdr.media_type", ACDR_Control, eth_withoutfcs_handle);
   dissector_add_uint("acdr.media_type", ACDR_DSP_SNIFFER, eth_withoutfcs_handle);
+  dissector_add_uint("mctp.encap-type", MCTP_TYPE_ETHERNET, eth_withoutfcs_handle);
 
   /*
    * This is to handle the output for the Cisco CMTS "cable intercept"

epan/dissectors/packet-mctp.c

@@ -95,6 +95,7 @@ static const value_string flag_vals[] = {
 };
 
 static dissector_table_t mctp_dissector_table;
+static dissector_table_t mctp_encap_dissector_table;
 static reassembly_table mctp_reassembly_table;
 
 static int
@@ -208,9 +209,17 @@ dissect_mctp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree,
     }
 
     if (next_tvb) {
+        int rc;
+
         type = tvb_get_guint8(next_tvb, 0);
-        dissector_try_uint_new(mctp_dissector_table, type & 0x7f, next_tvb,
-                               pinfo, tree, true, NULL);
+        rc = dissector_try_uint_new(mctp_dissector_table, type & 0x7f,
+                                    next_tvb, pinfo, tree, true, NULL);
+
+        if (!rc && !(type & 0x80)) {
+            tvbuff_t *encap_tvb = tvb_new_subset_remaining(next_tvb, 1);
+            dissector_try_uint_new(mctp_encap_dissector_table, type,
+                                   encap_tvb, pinfo, tree, true, NULL);
+        }
     }
 
     pinfo->fragmented = save_fragmented;
@@ -330,9 +339,31 @@ proto_register_mctp(void)
     proto_register_field_array(proto_mctp, hf, array_length(hf));
     proto_register_subtree_array(ett, array_length(ett));
 
+    /* We have two dissector tables here, both keyed off the type byte, but
+     * with different decode semantics:
+     *
+     * mctp.type: for protocols that are "MCTP-aware" - they perform their
+     *    own decoding of the type byte, including the IC bit, and possibly the
+     *    message integrity check (which is type-specific!). For example,
+     *    NVMe-MI, which includes the type byte in packet specifications
+     *
+     * mctp.encap-type: for procotols that are trivially encapsulated in a
+     *    MCTP message, and do not handle the type byte themselves. For
+     *    example, NC-SI over MCTP, which just wraps a NC-SI packet within
+     *    a MCTP message.
+     *
+     * it doesn't make sense to allow encap-type decoders to also have the IC
+     * bit set, as there is no specification for what format the message
+     * integrity check is in. So, we disallow the IC bit in the type field
+     * for those dissectors.
+     */
     mctp_dissector_table = register_dissector_table("mctp.type", "MCTP type",
                                                     proto_mctp, FT_UINT8,
                                                     BASE_HEX);
+    mctp_encap_dissector_table = register_dissector_table("mctp.encap-type",
+                                                          "MCTP encapsulated type",
+                                                          proto_mctp, FT_UINT8,
+                                                          BASE_HEX);
 
     reassembly_table_register(&mctp_reassembly_table,
                               &addresses_reassembly_table_functions);

epan/dissectors/packet-ncsi.c

@@ -26,6 +26,7 @@
 #include <epan/expert.h>
 #include <epan/addr_resolv.h>
 #include <epan/pci-ids.h>
+#include <epan/dissectors/packet-mctp.h>
 
 void proto_reg_handoff_ncsi(void);
 void proto_register_ncsi(void);
@@ -1670,6 +1671,7 @@ void
 proto_reg_handoff_ncsi(void)
 {
     dissector_add_uint("ethertype", ETHERTYPE_NCSI, ncsi_handle);
+    dissector_add_uint("mctp.encap-type", MCTP_TYPE_NCSI, ncsi_handle);
 }
 
 /*