Re: Calling a dissector: Type for data parameter

[João Valverde via Wireshark-dev <wireshark-dev () wireshark org>]

It would be nice to fix this in a way that could be used from Lua, to 
make Lua dissectors first-class citizens and allow them to talk to C 
dissectors (and vice-versa).

On 30/05/21 11:36, Graham Bloice wrote:
> When I made that change to MQTT I failed to notice that it called 
> other dissectors with different data pointers, and although 
> specifically modified for SparkplugB, felt that passing the topic as 
> data was sufficiently general to be useful.
>
> As others have noted, I guess the issue here is that to be more 
> specific about when to call a sub-dissector those sub-dissectors need 
> to register with the "parent" dissector using arbitrary fields and 
> arbitrary values to determine when and how the sub-dissector should be 
> called.
>
> A more minimal solution could be for MQTT to declare an enum of "data" 
> pointer types and a special registration routine and sub-dissectors 
> register with that routine passing in one of the enum types and then 
> in MQTT before calling a sub-dissector check that "data pointer type" 
> table.
>
> On Sat, 29 May 2021 at 08:33, Anders Broman <a.broman58 () gmail com 
> <mailto:a.broman58 () gmail com>> wrote:
>
>     Hi,
>     Yes the method is fragile. At the time of development I think it
>     was proposed to pass a struct containing a string and the void
>     pointer where the string could be used as a identifier. But that
>     was voted down.
>     Regards
>     Anders
>
>     Den lör 29 maj 2021 09:26Guy Harris <gharris () sonic net
>     <mailto:gharris () sonic net>> skrev:
>
>         On May 29, 2021, at 12:12 AM, Anders Broman
>         <a.broman58 () gmail com <mailto:a.broman58 () gmail com>> wrote:
>
>         > Shouldn't the caller be calling with the right data type or
>         NULL? So a bug in the MQTT disector?
>
>         How can the MQTT dissector determine what the right data type
>         *is* - especially given that the dissectors aren't wired in,
>         there's a UAT preference that lets the user configure it.
>
>         This is where the current mechanism for passing data between
>         dissectors goes crashing to the ground.
>
>         MQTT passes a topic string, which is just a string, to the
>         dissectors it calls.
>
>         JSON expects to be passed a pointer to an http_message_info_t.
>
>         JSON registers its non-heuristic dissector by name, and allows
>         it to be used with Decode As... for UDP ports.
>
>         It might *look* safe if you check the UDP dissector and the
>         dissectors that use "media_type" and "grpc_message_type", but
>         the "registers its non-heuristic dissector by name" mean there
>         are no guarantees, given that another dissector that passes a
>         pointer to something *other* than an http_message_info_t to
>         dissectors that are specified by name in a UAT.
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>
> --
> Graham Bloice
> Software Developer
> Trihedral UK Limited
>
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