Question: Usage of NLA_SLOT objects

[Vic063]

Hello,

I'm trying to implement a new ematch decoder which uses different types of values depending on the selected ematch kind.

The value can either be a string value or an uint32_t value.
As a workaroud, I set the value type to 'hex' in the nla_map so I can manipulate the hex data as I want to decode it as an integer or a string.

However, when I want to replace the original hex value with the one just decoded, I thought it was a tuple object but it's a nla_slot object (and the nldecap decoder does not print the tree correctly).

As an essay, I tried to directly create a nla_slot but it fails. I looked at the code (class nla_slot in pyroute2/netlink/init.py) but it seems there is no way to edit/replace the value of a nla_slot.

Do you think it can be acceptable to add a 'set_value' method to override nla_slot value?
Or is there a way to do it without any modification in the pyroute2 engine?

Thank you.

[svinota]

As a generic solution you can do this way:

from pyroute2.netlink import nla

class some_normal_msg(nlmsg):
    nla_map = (('NORMAL_UNSPEC', 'none'),
               ('NORMAL_EXAMPLE_UINT32', 'uint32'),
               ('NORMAL_EXAMPLE_CUSTOM', 'my_nla'))

    class my_nla(nla):
        # the parser interacts with self.fields, in this case it will be self['value']
        fields = [('value', 's')]
        # in the case of a simple nla with the only value the user will get/set
        # the self.value attribute implicitly, see also encode()/decode() below

        def decode(self):
            # first run generic decode into a string data
            nla.decode(self)
            # now the data is saved into self['value'], manipulate it
            # self.value will be returned as a simple value of the nla
            self.value = my_transform_out(self['value'])

        def encode(self):
            # the assigned value is stored in self.value, transform it
            self['value'] = my_transform_in(self.value)
            # now run the generic encoding
            nla.encode(self)

The API is a bit weird and badly needs redesign, but by historical reasons it's like that right now.

The nla_slot is a proxy object that has only one goal: it defers nla decoding until it will be referenced directly. Thus not referenced data doesn't get decoded, saving some CPU resources.

[svinota]

Or maybe I got your question wrong. Then ping me pls.

[Vic063]

Thank you for your answer. You can look at PR #671 to see exactly what I'm talking about (from lines 87 to 96) :)

I wanted to try your advice however it breaks the netlink packet structure.

[svinota]

Ok, let me play a bit with the code.

[svinota]

from lines 87 to 96

Yep, I've got it. I believe we can solve this.