docs: add more netlink docs (incl. spec docs)

Add documentation about the upcoming Netlink protocol specs.

Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/core-api/index.rst

@@ -127,6 +127,7 @@ Documents that don't fit elsewhere or which have yet to be categorized.
    :maxdepth: 1
 
    librs
+   netlink
 
 .. only:: subproject and html
 

Documentation/core-api/netlink.rst

@@ -0,0 +1,101 @@
+.. SPDX-License-Identifier: BSD-3-Clause
+
+.. _kernel_netlink:
+
+===================================
+Netlink notes for kernel developers
+===================================
+
+General guidance
+================
+
+Attribute enums
+---------------
+
+Older families often define "null" attributes and commands with value
+of ``0`` and named ``unspec``. This is supported (``type: unused``)
+but should be avoided in new families. The ``unspec`` enum values are
+not used in practice, so just set the value of the first attribute to ``1``.
+
+Message enums
+-------------
+
+Use the same command IDs for requests and replies. This makes it easier
+to match them up, and we have plenty of ID space.
+
+Use separate command IDs for notifications. This makes it easier to
+sort the notifications from replies (and present them to the user
+application via a different API than replies).
+
+Answer requests
+---------------
+
+Older families do not reply to all of the commands, especially NEW / ADD
+commands. User only gets information whether the operation succeeded or
+not via the ACK. Try to find useful data to return. Once the command is
+added whether it replies with a full message or only an ACK is uAPI and
+cannot be changed. It's better to err on the side of replying.
+
+Specifically NEW and ADD commands should reply with information identifying
+the created object such as the allocated object's ID (without having to
+resort to using ``NLM_F_ECHO``).
+
+NLM_F_ECHO
+----------
+
+Make sure to pass the request info to genl_notify() to allow ``NLM_F_ECHO``
+to take effect.  This is useful for programs that need precise feedback
+from the kernel (for example for logging purposes).
+
+Support dump consistency
+------------------------
+
+If iterating over objects during dump may skip over objects or repeat
+them - make sure to report dump inconsistency with ``NLM_F_DUMP_INTR``.
+This is usually implemented by maintaining a generation id for the
+structure and recording it in the ``seq`` member of struct netlink_callback.
+
+Netlink specification
+=====================
+
+Documentation of the Netlink specification parts which are only relevant
+to the kernel space.
+
+Globals
+-------
+
+kernel-policy
+~~~~~~~~~~~~~
+
+Defines if the kernel validation policy is per operation (``per-op``)
+or for the entire family (``global``). New families should use ``per-op``
+(default) to be able to narrow down the attributes accepted by a specific
+command.
+
+checks
+------
+
+Documentation for the ``checks`` sub-sections of attribute specs.
+
+unterminated-ok
+~~~~~~~~~~~~~~~
+
+Accept strings without the null-termination (for legacy families only).
+Switches from the ``NLA_NUL_STRING`` to ``NLA_STRING`` policy type.
+
+max-len
+~~~~~~~
+
+Defines max length for a binary or string attribute (corresponding
+to the ``len`` member of struct nla_policy). For string attributes terminating
+null character is not counted towards ``max-len``.
+
+The field may either be a literal integer value or a name of a defined
+constant. String types may reduce the constant by one
+(i.e. specify ``max-len: CONST - 1``) to reserve space for the terminating
+character so implementations should recognize such pattern.
+
+min-len
+~~~~~~~
+
+Similar to ``max-len`` but defines minimum length.

Documentation/userspace-api/netlink/c-code-gen.rst

@@ -0,0 +1,107 @@
+.. SPDX-License-Identifier: BSD-3-Clause
+
+==============================
+Netlink spec C code generation
+==============================
+
+This document describes how Netlink specifications are used to render
+C code (uAPI, policies etc.). It also defines the additional properties
+allowed in older families by the ``genetlink-c`` protocol level,
+to control the naming.
+
+For brevity this document refers to ``name`` properties of various
+objects by the object type. For example ``$attr`` is the value
+of ``name`` in an attribute, and ``$family`` is the name of the
+family (the global ``name`` property).
+
+The upper case is used to denote literal values, e.g. ``$family-CMD``
+means the concatenation of ``$family``, a dash character, and the literal
+``CMD``.
+
+The names of ``#defines`` and enum values are always converted to upper case,
+and with dashes (``-``) replaced by underscores (``_``).
+
+If the constructed name is a C keyword, an extra underscore is
+appended (``do`` -> ``do_``).
+
+Globals
+=======
+
+``c-family-name`` controls the name of the ``#define`` for the family
+name, default is ``$family-FAMILY-NAME``.
+
+``c-version-name`` controls the name of the ``#define`` for the version
+of the family, default is ``$family-FAMILY-VERSION``.
+
+``max-by-define`` selects if max values for enums are defined as a
+``#define`` rather than inside the enum.
+
+Definitions
+===========
+
+Constants
+---------
+
+Every constant is rendered as a ``#define``.
+The name of the constant is ``$family-$constant`` and the value
+is rendered as a string or integer according to its type in the spec.
+
+Enums and flags
+---------------
+
+Enums are named ``$family-$enum``. The full name can be set directly
+or suppressed by specifying the ``enum-name`` property.
+Default entry name is ``$family-$enum-$entry``.
+If ``name-prefix`` is specified it replaces the ``$family-$enum``
+portion of the entry name.
+
+Boolean ``render-max`` controls creation of the max values
+(which are enabled by default for attribute enums).
+
+Attributes
+==========
+
+Each attribute set (excluding fractional sets) is rendered as an enum.
+
+Attribute enums are traditionally unnamed in netlink headers.
+If naming is desired ``enum-name`` can be used to specify the name.
+
+The default attribute name prefix is ``$family-A`` if the name of the set
+is the same as the name of the family and ``$family-A-$set`` if the names
+differ. The prefix can be overridden by the ``name-prefix`` property of a set.
+The rest of the section will refer to the prefix as ``$pfx``.
+
+Attributes are named ``$pfx-$attribute``.
+
+Attribute enums end with two special values ``__$pfx-MAX`` and ``$pfx-MAX``
+which are used for sizing attribute tables.
+These two names can be specified directly with the ``attr-cnt-name``
+and ``attr-max-name`` properties respectively.
+
+If ``max-by-define`` is set to ``true`` at the global level ``attr-max-name``
+will be specified as a ``#define`` rather than an enum value.
+
+Operations
+==========
+
+Operations are named ``$family-CMD-$operation``.
+If ``name-prefix`` is specified it replaces the ``$family-CMD``
+portion of the name.
+
+Similarly to attribute enums operation enums end with special count and max
+attributes. For operations those attributes can be renamed with
+``cmd-cnt-name`` and ``cmd-max-name``. Max will be a define if ``max-by-define``
+is ``true``.
+
+Multicast groups
+================
+
+Each multicast group gets a define rendered into the kernel uAPI header.
+The name of the define is ``$family-MCGRP-$group``, and can be overwritten
+with the ``c-define-name`` property.
+
+Code generation
+===============
+
+uAPI header is assumed to come from ``<linux/$family.h>`` in the default header
+search path. It can be changed using the ``uapi-header`` global property.

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

@@ -0,0 +1,96 @@
+.. 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.
+
+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. The plan is to define
+the structs in ``definitions`` and link the appropriate attrs.
+
+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).

Documentation/userspace-api/netlink/index.rst

@@ -10,3 +10,8 @@ Netlink documentation for users.
    :maxdepth: 2
 
    intro
+   specs
+   c-code-gen
+   genetlink-legacy
+
+See also :ref:`Documentation/core-api/netlink.rst <kernel_netlink>`.