Documentation/userspace-api/netlink/genetlink-legacy.rst
.. SPDX-License-Identifier: BSD-3-Clause
This document describes the many additional quirks and properties
required to describe older Generic Netlink families which form
the genetlink-legacy protocol level.
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.
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
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).
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.
members
- ``name`` - The attribute name of the struct member
- ``type`` - One of the scalar types ``u8``, ``u16``, ``u32``, ``u64``, ``s8``,
``s16``, ``s32``, ``s64``, ``string`` or ``binary``.
- ``byte-order`` - ``big-endian`` or ``little-endian``
- ``doc``, ``enum``, ``enum-as-flags``, ``display-hint`` - Same as for
:ref:`attribute definitions <attribute_properties>`
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).