Documentation/userspace-api/netlink/intro-specs.rst

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

=====================================
Using Netlink protocol specifications
=====================================

This document is a quick starting guide for using Netlink protocol
specifications. For more detailed description of the specs see :doc:`specs`.

Simple CLI
==========

Kernel comes with a simple CLI tool which should be useful when
developing Netlink related code. The tool is implemented in Python
and can use a YAML specification to issue Netlink requests
to the kernel. Only Generic Netlink is supported.

The tool is located at ``tools/net/ynl/pyynl/cli.py``. It accepts
a handul of arguments, the most important ones are:

 - ``--spec`` - point to the spec file
 - ``--do $name`` / ``--dump $name`` - issue request ``$name``
 - ``--json $attrs`` - provide attributes for the request
 - ``--subscribe $group`` - receive notifications from ``$group``

YAML specs can be found under ``Documentation/netlink/specs/``.

Example use::

  $ ./tools/net/ynl/pyynl/cli.py --spec Documentation/netlink/specs/ethtool.yaml \
        --do rings-get \
	--json '{"header":{"dev-index": 18}}'
  {'header': {'dev-index': 18, 'dev-name': 'eni1np1'},
   'rx': 0,
   'rx-jumbo': 0,
   'rx-jumbo-max': 4096,
   'rx-max': 4096,
   'rx-mini': 0,
   'rx-mini-max': 4096,
   'tx': 0,
   'tx-max': 4096,
   'tx-push': 0}

The input arguments are parsed as JSON, while the output is only
Python-pretty-printed. This is because some Netlink types can't
be expressed as JSON directly. If such attributes are needed in
the input some hacking of the script will be necessary.

The spec and Netlink internals are factored out as a standalone
library - it should be easy to write Python tools / tests reusing
code from ``cli.py``.

Generating kernel code
======================

``tools/net/ynl/ynl-regen.sh`` scans the kernel tree in search of
auto-generated files which need to be updated. Using this tool is the easiest
way to generate / update auto-generated code.

By default code is re-generated only if spec is newer than the source,
to force regeneration use ``-f``.

``ynl-regen.sh`` searches for ``YNL-GEN`` in the contents of files
(note that it only scans files in the git index, that is only files
tracked by git!) For instance the ``fou_nl.c`` kernel source contains::

  /*	Documentation/netlink/specs/fou.yaml */
  /* YNL-GEN kernel source */

``ynl-regen.sh`` will find this marker and replace the file with
kernel source based on fou.yaml.

The simplest way to generate a new file based on a spec is to add
the two marker lines like above to a file, add that file to git,
and run the regeneration tool. Grep the tree for ``YNL-GEN``
to see other examples.

The code generation itself is performed by ``tools/net/ynl/pyynl/ynl_gen_c.py``
but it takes a few arguments so calling it directly for each file
quickly becomes tedious.

YNL lib
=======

``tools/net/ynl/lib/`` contains an implementation of a C library
(based on libmnl) which integrates with code generated by
``tools/net/ynl/pyynl/ynl_gen_c.py`` to create easy to use netlink wrappers.

YNL basics
----------

The YNL library consists of two parts - the generic code (functions
prefix by ``ynl_``) and per-family auto-generated code (prefixed
with the name of the family).

To create a YNL socket call ynl_sock_create() passing the family
struct (family structs are exported by the auto-generated code).
ynl_sock_destroy() closes the socket.

YNL requests
------------

Steps for issuing YNL requests are best explained on an example.
All the functions and types in this example come from the auto-generated
code (for the netdev family in this case):

.. code-block:: c

   // 0. Request and response pointers
   struct netdev_dev_get_req *req;
   struct netdev_dev_get_rsp *d;

   // 1. Allocate a request
   req = netdev_dev_get_req_alloc();
   // 2. Set request parameters (as needed)
   netdev_dev_get_req_set_ifindex(req, ifindex);

   // 3. Issues the request
   d = netdev_dev_get(ys, req);
   // 4. Free the request arguments
   netdev_dev_get_req_free(req);
   // 5. Error check (the return value from step 3)
   if (!d) {
	// 6. Print the YNL-generated error
	fprintf(stderr, "YNL: %s\n", ys->err.msg);
        return -1;
   }

   // ... do stuff with the response @d

   // 7. Free response
   netdev_dev_get_rsp_free(d);

YNL dumps
---------

Performing dumps follows similar pattern as requests.
Dumps return a list of objects terminated by a special marker,
or NULL on error. Use ``ynl_dump_foreach()`` to iterate over
the result.

YNL notifications
-----------------

YNL lib supports using the same socket for notifications and
requests. In case notifications arrive during processing of a request
they are queued internally and can be retrieved at a later time.

To subscribed to notifications use ``ynl_subscribe()``.
The notifications have to be read out from the socket,
``ynl_socket_get_fd()`` returns the underlying socket fd which can
be plugged into appropriate asynchronous IO API like ``poll``,
or ``select``.

Notifications can be retrieved using ``ynl_ntf_dequeue()`` and have
to be freed using ``ynl_ntf_free()``. Since we don't know the notification
type upfront the notifications are returned as ``struct ynl_ntf_base_type *``
and user is expected to cast them to the appropriate full type based
on the ``cmd`` member.