Wireshark Wlua does not dissect flags

[hamishwillee]

An error is shown if you try to inspect a message, such as HEARTBEAT, that has flags, and you can't inspect any fields after that flag. For example in HEARTBEAT this is the mode flag:

There are two issues. Firstly, the value returned by tree:add_le is nil, so what gets passed into dissect_flags_MAV_MODE_FLAG cannot be dissected. The docs say tree:add_le should return a value. My assumption is that this is failing for the reason given after the code. Note that this is the only place the value is actually used, which is why we don't see the error elsewhere.

    field_offset = offset + 6
    subtree, value = tree:add_le(f.HEARTBEAT_base_mode, padded(field_offset, 1))
    dissect_flags_MAV_MODE_FLAG(subtree, "HEARTBEAT_base_mode", value)

I "fixed" this error by passing in padded(field_offset, 1) to dissect_flags_MAV_MODE_FLAG instead of value. Wireshark now fails with the error that the data passed in is "userData" - i.e. an arbitrary C data that it can't interpret. That's correct, and probably why nil got returned from the tree:add_le.

The solution would be to cast the data to a number before passing it to tree:add_le. HOwever I don't know how to do this elegantly.

This is cross posted (and summarised) from mavlink/mavlink#1978

[hamishwillee]

FYI @auturgy can you seek expertise in the dev call. @peterbarker already pointed me to @rotu

[rotu]

I think I fixed this in https://github.com/rotu/pymavlink/tree/lua-bitflags but GitHub is being annoying and not letting me open a PR. Can you give this a shot?

[hamishwillee]

I certainly will. Maybe not until the weekend. Thank you very much

[rotu]

I have some tentative test code but I need a pcap to add to the testing script. got data?

[hamishwillee]

I don't know sorry. I planned to create your PR, try it out, and then see what else was required.