Hi Mikael,
On Mon, Nov 18, 2019 at 05:20:54PM +0000, Kanstrup, Mikael wrote:
> Hi,
>
> I'm working on dissecting a proprietary protocol that extends Bluetooth HCI_ACL with a LUA dissector. As there's no heuristics dissector list registered for this particular protocol I thought something similar could be achieved with a chained dissector. I retrieve the original HCI_ACL dissector handle and replace it with my own LUA dissector. In LUA dissector apply some heuristics and if it's not my own protocol then call the original HCI_ACL dissector via the handle.
>
> Code looks like this:
>
> local proto_test = Proto("test", "Use chaining as heuristic dissector")
> local proto_default_acl
>
> function is_test_proto(tvb, pinfo)
> -- Apply heuristics to determine if own protocol
> return false
> end
>
> function proto_test.dissector(tvb, pinfo, tree)
> if not is_test_proto(tvb, pinfo) then
> return proto_default_acl:call(tvb, pinfo, tree)
> end
>
> pinfo.cols.protocol = "test"
> tree = tree:add(proto_test, tvb)
> return tvb:len()
> end
>
> function proto_test.init()
> local hci_type = DissectorTable.get("hci_h4.type")
> local pattern = 0x02 -- ACL
> proto_default_acl = hci_type:get_dissector(pattern)
> hci_type:add(pattern, proto_test)
> end
>
> This unfortunately did not work and I was not able to find out why until I started looking at the HCI_ACL dissector code itself.
>
> static gint
> dissect_bthci_acl(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data)
> {
> <...>
> /* Reject the packet if data is NULL */
> if (data == NULL)
> return 0;
>
> The above NULL check is hit for all calls coming from the LUA dissector. The LUA dissector function prototype does not have the data parameter and it appears it's simply lost when chaining calls through LUA.
Correct, Lua always passes a NULL parameter. Due to type-safety,
Lua dissectors cannot pass arbitrary data.
> Any suggestions on how to approach this? Would it be possible to
> extend the LUA dissector interface with another function prototype
> that supports the data parameter? Just support relaying the parameter
> in chained dissectors, not modifying or doing any fancy stuff with it.
There is unfortunately no runtime registration of types for the 'data'
parameter. If you can somehow keep track that a C dissector was from a
particular dissector table, and then recognize that the Lua dissector
was registered with the same table, then hopefully it should be possible
to propagate the data parameter as special case.
Long-term, we might have to revisit how to pass data between (multiple
layers of) dissectors. Not just Lua, but also C. For Lua, see also:
Bug 15931 - Add Lua support for arbitrary data parameter in dissector calls.
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=15931
(+cc Huang)
Discussion about another mechanism to pass data between dissectors:
https://code.wireshark.org/review/35159
Discussion about another (abandoned) new mechanism to pass data between
dissectors:
https://code.wireshark.org/review/34049
--
Kind regards,
Peter Wu
https://lekensteyn.nl
