Re: Chained CAN dissecector: Can not get reference for CAN dissector

[Michael Mann <mmann78 () netscape net>]

The CAN ID is passed as the "data" parameter to the subdissector from the CAN dissector.  The C code of a subdissector 
to handle it would be as follows:
 
struct can_identifier
{
    guint32 id;
};
 
static int subdissector_function(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data)
{
    struct can_identifier can_id;
    can_id = *((struct can_identifier*)data);
    if (can_id.id != 42)
    {
        return 0;
    }
    ...
}

I'm not as familiar with Lua to know how to setup the equivalent, but you shouldn't need to "override" the CAN 
dissector to get the CAN ID.
 
 
-----Original Message-----
From: Sebastian Schildt <sebastian () frozenlight de>
To: wireshark-dev <wireshark-dev () wireshark org>
Sent: Tue, Dec 15, 2015 5:37 am
Subject: [Wireshark-dev] Chained CAN dissecector: Can not get reference for CAN dissector


Hello Wiresharkers, 

I have a problem (obviously :) ) . I want to create a CAN dissector (in Lua). What I already achieved is creating a Can 
subdissector: So my dissector gets called  for CAN payload. However, I need access to the CAN identfier, so I figured 
my dissector needs to be on the same "level" as the CAN dissector (right?)

Therefore I tried to use the "chained dissector" pattern, where my dissector would call the CAN dissector and then does 
some further processing. For this my dissector must call the CAN dissector, but I do not know how to get a reference to 
it.

I tried 

original_can_dissector = Dissector.get("CAN")
or
original_can_dissector = Dissector.get("can")

that did not work, then I used lua to print

for key,value in pairs(Dissector.list()) do print(key,value) end

but I did not find any reference to CAN. Is the CAN dissector hidden in some DissectorTable? But then which one? I 
couldn't find it.

The relevant lines in the C code seem to be
proto_can = proto_register_protocol(
"Controller Area Network",      /* name       */
"CAN",                          /* short name */
"can"           /* abbrev     */
);
And
dissector_handle_t can_handle;

can_handle = create_dissector_handle(dissect_socketcan, proto_can);
dissector_add_uint("wtap_encap", WTAP_ENCAP_SOCKETCAN, can_handle);
dissector_add_uint("sll.ltype", LINUX_SLL_P_CAN, can_handle);

but I am not very familiar with Wireshark source, so I am guestimating here.
Also, after solving the problem of getting a reference to the original CAN dissector I guess I need to replicate the 
add_uint calls in Lua to register my own dissector. If anyone has the correct syntax up his sleeve I'd also appreciate 
it :)

Thanks

Sebastian


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