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

["Maynard, Chris" <Christopher.Maynard () IGT com>]

You might want to implement a function to handle this, something like so?

local function dissect_RuPortId_F(tree, buffer)

    local t = {}
    local mask = 2^my_protocol.prefs.ru_port_id_width - 1
    local val = bit.band(buffer(4, 2):uint(), mask)

    for i = 15, 0, -1 do
        if bit.band(bit.rshift(mask, i), 1) == 1 then
            table.insert(t, (bit.band(bit.rshift(val, i), 1) == 1 and '1') or '0')
        else
            table.insert(t, '.')
        end

        if (i % 4) == 0 then
            table.insert(t, ' ')
        end
    end
    tree:add(RuPortId_F, buffer(4, 2)):set_text(table.concat(t) .. " = Ru Port ID: " .. val)

end -- dissect_RuPortId_F()

…

dissect_RuPortId_F(tree, buffer)

If the offset isn’t fixed at 4, then you can pass that as an argument too, and if you want to be able to add any 
information to the Info column, then pass pinfo also.

And you probably want to limit the bit width to only values between 1-16.  One way to do this is to use an enum 
preference instead of a uint preference, e.g.:

local ru_port_id_width_enums = {
{1, "1", 1}, {2, "2", 2}, {3, "3", 3}, {4, "4", 4},
{5, "5", 5}, {6, "6", 6}, {7, "7", 7}, {8, "8", 8},
{9, "9", 9}, {10, "10", 10}, {11, "11", 11}, {12, "12", 12},
{13, "13", 13}, {14, "14", 14}, {15, "15", 15}, {16, "16", 16}
}
my_protocol.prefs.ru_port_id_width = Pref.enum("RU Port ID width", 4, "The RU Port ID width, in bits (1-16)", 
ru_port_id_width_enums, false)

You *may* be able to still error check the preference if declared as a uint using a my_protocol.prefs_changed() 
function[1], but the enum type seems ok here, and anyway, I’m actually not entirely sure how to do it since the 
following does not work:

function my_protocol.prefs_changed()

    if my_protocol.prefs.ru_port_id_width < 1 then
        my_protocol.prefs.ru_port_id_width = 1
    end

    if my_protocol.prefs.ru_port_id_width > 16 then
        my_protocol.prefs.ru_port_id_width = 16
    end

end -- my_protocol.prefs_changed()

Here, you end up with an error of the sort, “Lua: Error During execution of prefs apply callback: [string 
“C:\Users\cmaynard\AppData\Roaming\Wireshark\p…”:51: bad argument #3 to '__newindex' (userdata expected, got number)”.  
As far as I can tell, there seems to be no way to limit preferences to a valid range in Lua.  If this is the case, then 
it seems to be a limitation worthy of an enhancement bug report.

Hope it helps.
- Chris
[1]: See 11.6.4.6. proto.prefs_changed at https://www.wireshark.org/docs/wsdg_html_chunked/lua_module_Proto.html


From: Wireshark-dev [mailto:wireshark-dev-bounces () wireshark org] On Behalf Of David Aldrich
Sent: Monday, September 3, 2018 11:32 AM
To: wireshark-dev () wireshark org
Subject: [Wireshark-dev] Lua dissector: How to set sub-field bit widths using preferences?

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.

Is there a better sequence of code that I code use to overcome this limitation?

Best regards

David






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