tools: ynl: Add fixed-header support to ynl

Add support for netlink families that add an optional fixed header structure
after the genetlink header and before any attributes. The fixed-header can be
specified on a per op basis, or once for all operations, which serves as a
default value that can be overridden.

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

Documentation/netlink/genetlink-legacy.yaml

···
       async-enum:
         description: Name for the enum type with notifications/events.
         type: string
+      # Start genetlink-legacy
+      fixed-header: &fixed-header
+        description: |
+          Name of the structure defining the optional fixed-length protocol
+          header. This header is placed in a message after the netlink and
+          genetlink headers and before any attributes.
+        type: string
+      # End genetlink-legacy
       list:
         description: List of commands
         type: array
···
               type: array
               items:
                 enum: [ strict, dump ]
+            # Start genetlink-legacy
+            fixed-header: *fixed-header
+            # End genetlink-legacy
             do: &subop-type
               description: Main command handler.
               type: object

tools/net/ynl/lib/nlspec.py

···
     Information about a single Netlink operation.
 
     Attributes:
-        value       numerical ID when serialized, None if req/rsp values differ
+        value           numerical ID when serialized, None if req/rsp values differ
 
-        req_value   numerical ID when serialized, user -> kernel
-        rsp_value   numerical ID when serialized, user <- kernel
-        is_call     bool, whether the operation is a call
-        is_async    bool, whether the operation is a notification
-        is_resv     bool, whether the operation does not exist (it's just a reserved ID)
-        attr_set    attribute set name
+        req_value       numerical ID when serialized, user -> kernel
+        rsp_value       numerical ID when serialized, user <- kernel
+        is_call         bool, whether the operation is a call
+        is_async        bool, whether the operation is a notification
+        is_resv         bool, whether the operation does not exist (it's just a reserved ID)
+        attr_set        attribute set name
+        fixed_header    string, optional name of fixed header struct
 
-        yaml        raw spec as loaded from the spec file
+        yaml            raw spec as loaded from the spec file
     """
     def __init__(self, family, yaml, req_value, rsp_value):
         super().__init__(family, yaml)
···
         self.is_call = 'do' in yaml or 'dump' in yaml
         self.is_async = 'notify' in yaml or 'event' in yaml
         self.is_resv = not self.is_async and not self.is_call
+        self.fixed_header = self.yaml.get('fixed-header', family.fixed_header)
 
         # Added by resolve:
         self.attr_set = None
···
         msgs_by_value  dict of all messages (indexed by name)
         ops        dict of all valid requests / responses
         consts     dict of all constants/enums
+        fixed_header  string, optional name of family default fixed header struct
     """
     def __init__(self, spec_path, schema_path=None):
         with open(spec_path, "r") as stream:
···
         self._resolution_list.append(elem)
 
     def _dictify_ops_unified(self):
+        self.fixed_header = self.yaml['operations'].get('fixed-header')
         val = 1
         for elem in self.yaml['operations']['list']:
             if 'value' in elem:
···
             self.msgs[op.name] = op
 
     def _dictify_ops_directional(self):
+        self.fixed_header = self.yaml['operations'].get('fixed-header')
         req_val = rsp_val = 1
         for elem in self.yaml['operations']['list']:
             if 'notify' in elem:

tools/net/ynl/lib/ynl.py

···
 
 
 class GenlMsg:
-    def __init__(self, nl_msg):
+    def __init__(self, nl_msg, fixed_header_members=[]):
         self.nl = nl_msg
 
         self.hdr = nl_msg.raw[0:4]
-        self.raw = nl_msg.raw[4:]
+        offset = 4
 
         self.genl_cmd, self.genl_version, _ = struct.unpack("BBH", self.hdr)
 
+        self.fixed_header_attrs = dict()
+        for m in fixed_header_members:
+            format, size = NlAttr.type_formats[m.type]
+            decoded = struct.unpack_from(format, nl_msg.raw, offset)
+            offset += size
+            self.fixed_header_attrs[m.name] = decoded[0]
+
+        self.raw = nl_msg.raw[offset:]
         self.raw_attrs = NlAttrs(self.raw)
 
     def __repr__(self):
···
 
         req_seq = random.randint(1024, 65535)
         msg = _genl_msg(self.family.family_id, nl_flags, op.req_value, 1, req_seq)
+        fixed_header_members = []
+        if op.fixed_header:
+            fixed_header_members = self.consts[op.fixed_header].members
+            for m in fixed_header_members:
+                value = vals.pop(m.name)
+                format, _ = NlAttr.type_formats[m.type]
+                msg += struct.pack(format, value)
         for name, value in vals.items():
             msg += self._add_attr(op.attr_set.name, name, value)
         msg = _genl_msg_finalize(msg)
···
                     done = True
                     break
 
-                gm = GenlMsg(nl_msg)
+                gm = GenlMsg(nl_msg, fixed_header_members)
                 # Check if this is a reply to our request
                 if nl_msg.nl_seq != req_seq or gm.genl_cmd != op.rsp_value:
                     if gm.genl_cmd in self.async_msg_ids:
···
                         print('Unexpected message: ' + repr(gm))
                         continue
 
-                rsp.append(self._decode(gm.raw_attrs, op.attr_set.name))
+                rsp.append(self._decode(gm.raw_attrs, op.attr_set.name)
+                           | gm.fixed_header_attrs)
 
         if not rsp:
             return None