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

[David Aldrich <david.aldrich.ntml () gmail com>]

Hi Chris



Thanks very much for your reply and suggested code.  I have a couple of
questions:



1) I need to declare RuPortId_F somehow.  Previously I had:



RuPortId_F = ProtoField.uint16("my_protocol.RuPortId", "RU Port ID",
base.HEX, NULL, 0x000F)



but now the masking is done in your function. So how should I declare it?



2) What does this line do?



table.insert(t, (bit.band(bit.rshift(val, i), 1) == 1 and '1') or '0')



3) Your function makes things much tidier but, as I think Jeff points out,
the preference still gets read in the header code. This means that if the
user changes the value of the preference, your function does not reflect
the change until Wireshark is restarted (and the dissector reloaded).  Do
you agree? Is there any way around this?



Best regards



David

On Tue, Sep 4, 2018 at 6:28 PM Jeff Morriss <jeff.morriss.ws () gmail com>
wrote:

> 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?)
>
> ___________________________________________________________________________
> Sent via:    Wireshark-dev mailing list <wireshark-dev () wireshark org>
> Archives:    https://www.wireshark.org/lists/wireshark-dev
> Unsubscribe: https://www.wireshark.org/mailman/options/wireshark-dev
>              mailto:wireshark-dev-request () wireshark org
> ?subject=unsubscribe
___________________________________________________________________________
Sent via:    Wireshark-dev mailing list <wireshark-dev () wireshark org>
Archives:    https://www.wireshark.org/lists/wireshark-dev
Unsubscribe: https://www.wireshark.org/mailman/options/wireshark-dev
             mailto:wireshark-dev-request () wireshark org?subject=unsubscribe