Re: Lua dissector: How to set sub-field bit widths using preferences?

[Jeff Morriss <jeff.morriss.ws () gmail com>]

On Mon, Sep 3, 2018 at 11:32 AM David Aldrich <david.aldrich.ntml () gmail com>
wrote:

> Our protocol includes a 16-bit field which is sub-divided into 4
> sub-fields.  The width of those sub-fields is variable so I want to specify
> the widths using Wireshark preferences.  I understand how to create and
> read  Wireshark preferences but I am unsure of how to apply them in this
> circumstance.
>
> My code structure looks like this:
>
> my_protocol = Proto("...", "...")
>
> -- Create a preference
> my_protocol.prefs.ru_port_id_width = Pref.uint( "RU_Port_ID width ", 4 )
>
> -- Read the preference
> ru_port_id_width = my_protocol.prefs.ru_port_id_width
>
> -- Specify a field using the preference
> RuPortId_F = ProtoField.uint16("...", "...", base.HEX, NULL, ((2^
> ru_port_id_width)-1))
>
> -- Create a fields table and add the field to it
> xran_protocol.fields = {RuPortId_F}
>
> -- Then specify the dissector function
> function my_protocol.dissector(buffer, pinfo,tree)
>
> -- Show the field in a subtree
> local RtcidPcid_range = buffer(4,2) -- the 16-bit field
> local ecpriRtcidPcid = ecpriRtcidPcid_range:uint()
> subtree:add(RuPortId_F, RtcidPcid_range, RtcidPcid) -- the variable width
> sub-field
> The trouble with this is that the preference is only read at startup. A
> new value can't be specified by the user without restarting Wireshark to
> apply it.

That sounds like a bug if it's true.

But what happens if you read the preference value inside your dissector
function?  The /potential/ problem I see in the above code is that you're
reading the value in the header code which IIRC is only executed at
startup.  (Ideally you'd actually only read the preference once after
they're changed; here Lua has a disadvantage over C dissectors who just
reference the variable that holds the value of the preference; maybe the
Lua API should have a "preferences changed" callback to let Lua dissectors
know that one or more preferences have changed?)

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