linux/Documentation/userspace-api/netlink/genetlink-legacy.rst

.. SPDX-License-Identifier: BSD-3-Clause

=================================================================
Netlink specification support for legacy Generic Netlink families
=================================================================

This document describes the many additional quirks and properties
required to describe older Generic Netlink families which form
the ``genetlink-legacy`` protocol level.

The spec is a work in progress, some of the quirks are just documented
for future reference.

Specification (defined)
=======================

Attribute type nests
--------------------

New Netlink families should use ``multi-attr`` to define arrays.
Older families (e.g. ``genetlink`` control family) attempted to
define array types reusing attribute type to carry information.

For reference the ``multi-attr`` array may look like this::

  [ARRAY-ATTR]
    [INDEX (optionally)]
    [MEMBER1]
    [MEMBER2]
  [SOME-OTHER-ATTR]
  [ARRAY-ATTR]
    [INDEX (optionally)]
    [MEMBER1]
    [MEMBER2]

where ``ARRAY-ATTR`` is the array entry type.

array-nest
~~~~~~~~~~

``array-nest`` creates the following structure::

  [SOME-OTHER-ATTR]
  [ARRAY-ATTR]
    [ENTRY]
      [MEMBER1]
      [MEMBER2]
    [ENTRY]
      [MEMBER1]
      [MEMBER2]

It wraps the entire array in an extra attribute (hence limiting its size
to 64kB). The ``ENTRY`` nests are special and have the index of the entry
as their type instead of normal attribute type.

type-value
~~~~~~~~~~

``type-value`` is a construct which uses attribute types to carry
information about a single object (often used when array is dumped
entry-by-entry).

``type-value`` can have multiple levels of nesting, for example
genetlink's policy dumps create the following structures::

  [POLICY-IDX]
    [ATTR-IDX]
      [POLICY-INFO-ATTR1]
      [POLICY-INFO-ATTR2]

Where the first level of nest has the policy index as it's attribute
type, it contains a single nest which has the attribute index as its
type. Inside the attr-index nest are the policy attributes. Modern
Netlink families should have instead defined this as a flat structure,
the nesting serves no good purpose here.

Operations
==========

Enum (message ID) model
-----------------------

unified
~~~~~~~

Modern families use the ``unified`` message ID model, which uses
a single enumeration for all messages within family. Requests and
responses share the same message ID. Notifications have separate
IDs from the same space. For example given the following list
of operations:

.. code-block:: yaml

  -
    name: a
    value: 1
    do: ...
  -
    name: b
    do: ...
  -
    name: c
    value: 4
    notify: a
  -
    name: d
    do: ...

Requests and responses for operation ``a`` will have the ID of 1,
the requests and responses of ``b`` - 2 (since there is no explicit
``value`` it's previous operation ``+ 1``). Notification ``c`` will
use the ID of 4, operation ``d`` 5 etc.

directional
~~~~~~~~~~~

The ``directional`` model splits the ID assignment by the direction of
the message. Messages from and to the kernel can't be confused with
each other so this conserves the ID space (at the cost of making
the programming more cumbersome).

In this case ``value`` attribute should be specified in the ``request``
``reply`` sections of the operations (if an operation has both ``do``
and ``dump`` the IDs are shared, ``value`` should be set in ``do``).
For notifications the ``value`` is provided at the op level but it
only allocates a ``reply`` (i.e. a "from-kernel" ID). Let's look
at an example:

.. code-block:: yaml

  -
    name: a
    do:
      request:
        value: 2
        attributes: ...
      reply:
        value: 1
        attributes: ...
  -
    name: b
    notify: a
  -
    name: c
    notify: a
    value: 7
  -
    name: d
    do: ...

In this case ``a`` will use 2 when sending the message to the kernel
and expects message with ID 1 in response. Notification ``b`` allocates
a "from-kernel" ID which is 2. ``c`` allocates "from-kernel" ID of 7.
If operation ``d`` does not set ``values`` explicitly in the spec
it will be allocated 3 for the request (``a`` is the previous operation
with a request section and the value of 2) and 8 for response (``c`` is
the previous operation in the "from-kernel" direction).

Other quirks (todo)
===================

Structures
----------

Legacy families can define C structures both to be used as the contents of
an attribute and as a fixed message header. Structures are defined in
``definitions``  and referenced in operations or attributes. Note that
structures defined in YAML are implicitly packed according to C
conventions. For example, the following struct is 4 bytes, not 6 bytes:

.. code-block:: c

  struct {
          u8 a;
          u16 b;
          u8 c;
  }

Any padding must be explicitly added and C-like languages should infer the
need for explicit padding from whether the members are naturally aligned.

Here is the struct definition from above, declared in YAML:

.. code-block:: yaml

  definitions:
    -
      name: message-header
      type: struct
      members:
        -
          name: a
          type: u8
        -
          name: b
          type: u16
        -
          name: c
          type: u8

Fixed Headers
~~~~~~~~~~~~~

Fixed message headers can be added to operations using ``fixed-header``.
The default ``fixed-header`` can be set in ``operations`` and it can be set
or overridden for each operation.

.. code-block:: yaml

  operations:
    fixed-header: message-header
    list:
      -
        name: get
        fixed-header: custom-header
        attribute-set: message-attrs

Attributes
~~~~~~~~~~

A ``binary`` attribute can be interpreted as a C structure using a
``struct`` property with the name of the structure definition. The
``struct`` property implies ``sub-type: struct`` so it is not necessary to
specify a sub-type.

.. code-block:: yaml

  attribute-sets:
    -
      name: stats-attrs
      attributes:
        -
          name: stats
          type: binary
          struct: vport-stats

C Arrays
--------

Legacy families also use ``binary`` attributes to encapsulate C arrays. The
``sub-type`` is used to identify the type of scalar to extract.

.. code-block:: yaml

  attributes:
    -
      name: ports
      type: binary
      sub-type: u32

Multi-message DO
----------------

New Netlink families should never respond to a DO operation with multiple
replies, with ``NLM_F_MULTI`` set. Use a filtered dump instead.

At the spec level we can define a ``dumps`` property for the ``do``,
perhaps with values of ``combine`` and ``multi-object`` depending
on how the parsing should be implemented (parse into a single reply
vs list of objects i.e. pretty much a dump).