tools: ynl: Add struct attr decoding to ynl

Add support for decoding attributes that contain C structs.

Signed-off-by: Donald Hunter <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/netlink/genetlink-legacy.yaml

···
                     description: Max length for a string or a binary attribute.
                     $ref: '#/$defs/len-or-define'
               sub-type: *attr-type
+              # Start genetlink-legacy
+              struct:
+                description: Name of the struct type used for the attribute.
+                type: string
+              # End genetlink-legacy
 
       # Make sure name-prefix does not appear in subsets (subsets inherit naming)
       dependencies:

tools/net/ynl/lib/nlspec.py

···
         value         numerical ID when serialized
         attr_set      Attribute Set containing this attr
         is_multi      bool, attr may repeat multiple times
+        struct_name   string, name of struct definition
         sub_type      string, name of sub type
     """
     def __init__(self, family, attr_set, yaml, value):
···
         self.value = value
         self.attr_set = attr_set
         self.is_multi = yaml.get('multi-attr', False)
+        self.struct_name = yaml.get('struct')
         self.sub_type = yaml.get('sub-type')
 
 

tools/net/ynl/lib/ynl.py

···
         format, _ = self.type_formats[type]
         return list({ x[0] for x in struct.iter_unpack(format, self.raw) })
 
+    def as_struct(self, members):
+        value = dict()
+        offset = 0
+        for m in members:
+            # TODO: handle non-scalar members
+            format, size = self.type_formats[m.type]
+            decoded = struct.unpack_from(format, self.raw, offset)
+            offset += size
+            value[m.name] = decoded[0]
+        return value
+
     def __repr__(self):
         return f"[type:{self.type} len:{self._len}] {self.raw}"
 
···
         rsp[attr_spec['name']] = value
 
     def _decode_binary(self, attr, attr_spec):
-        if attr_spec.sub_type:
+        if attr_spec.struct_name:
+            decoded = attr.as_struct(self.consts[attr_spec.struct_name])
+        elif attr_spec.sub_type:
             decoded = attr.as_c_array(attr_spec.sub_type)
         else:
             decoded = attr.as_bin()