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diff --git a/Documentation/userspace-api/netlink/specs.rst b/Documentation/userspace-api/netlink/specs.rst new file mode 100644 index 0000000000..cc4e243099 --- /dev/null +++ b/Documentation/userspace-api/netlink/specs.rst @@ -0,0 +1,458 @@ +.. SPDX-License-Identifier: BSD-3-Clause + +========================================= +Netlink protocol specifications (in YAML) +========================================= + +Netlink protocol specifications are complete, machine readable descriptions of +Netlink protocols written in YAML. The goal of the specifications is to allow +separating Netlink parsing from user space logic and minimize the amount of +hand written Netlink code for each new family, command, attribute. +Netlink specs should be complete and not depend on any other spec +or C header file, making it easy to use in languages which can't include +kernel headers directly. + +Internally kernel uses the YAML specs to generate: + + - the C uAPI header + - documentation of the protocol as a ReST file + - policy tables for input attribute validation + - operation tables + +YAML specifications can be found under ``Documentation/netlink/specs/`` + +This document describes details of the schema. +See :doc:`intro-specs` for a practical starting guide. + +All specs must be licensed under +``((GPL-2.0 WITH Linux-syscall-note) OR BSD-3-Clause)`` +to allow for easy adoption in user space code. + +Compatibility levels +==================== + +There are four schema levels for Netlink specs, from the simplest used +by new families to the most complex covering all the quirks of the old ones. +Each next level inherits the attributes of the previous level, meaning that +user capable of parsing more complex ``genetlink`` schemas is also compatible +with simpler ones. The levels are: + + - ``genetlink`` - most streamlined, should be used by all new families + - ``genetlink-c`` - superset of ``genetlink`` with extra attributes allowing + customization of define and enum type and value names; this schema should + be equivalent to ``genetlink`` for all implementations which don't interact + directly with C uAPI headers + - ``genetlink-legacy`` - Generic Netlink catch all schema supporting quirks of + all old genetlink families, strange attribute formats, binary structures etc. + - ``netlink-raw`` - catch all schema supporting pre-Generic Netlink protocols + such as ``NETLINK_ROUTE`` + +The definition of the schemas (in ``jsonschema``) can be found +under ``Documentation/netlink/``. + +Schema structure +================ + +YAML schema has the following conceptual sections: + + - globals + - definitions + - attributes + - operations + - multicast groups + +Most properties in the schema accept (or in fact require) a ``doc`` +sub-property documenting the defined object. + +The following sections describe the properties of the most modern ``genetlink`` +schema. See the documentation of :doc:`genetlink-c <c-code-gen>` +for information on how C names are derived from name properties. + +See also :ref:`Documentation/core-api/netlink.rst <kernel_netlink>` for +information on the Netlink specification properties that are only relevant to +the kernel space and not part of the user space API. + +genetlink +========= + +Globals +------- + +Attributes listed directly at the root level of the spec file. + +name +~~~~ + +Name of the family. Name identifies the family in a unique way, since +the Family IDs are allocated dynamically. + +version +~~~~~~~ + +Generic Netlink family version, default is 1. + +protocol +~~~~~~~~ + +The schema level, default is ``genetlink``, which is the only value +allowed for new ``genetlink`` families. + +definitions +----------- + +Array of type and constant definitions. + +name +~~~~ + +Name of the type / constant. + +type +~~~~ + +One of the following types: + + - const - a single, standalone constant + - enum - defines an integer enumeration, with values for each entry + incrementing by 1, (e.g. 0, 1, 2, 3) + - flags - defines an integer enumeration, with values for each entry + occupying a bit, starting from bit 0, (e.g. 1, 2, 4, 8) + +value +~~~~~ + +The value for the ``const``. + +value-start +~~~~~~~~~~~ + +The first value for ``enum`` and ``flags``, allows overriding the default +start value of ``0`` (for ``enum``) and starting bit (for ``flags``). +For ``flags`` ``value-start`` selects the starting bit, not the shifted value. + +Sparse enumerations are not supported. + +entries +~~~~~~~ + +Array of names of the entries for ``enum`` and ``flags``. + +header +~~~~~~ + +For C-compatible languages, header which already defines this value. +In case the definition is shared by multiple families (e.g. ``IFNAMSIZ``) +code generators for C-compatible languages may prefer to add an appropriate +include instead of rendering a new definition. + +attribute-sets +-------------- + +This property contains information about netlink attributes of the family. +All families have at least one attribute set, most have multiple. +``attribute-sets`` is an array, with each entry describing a single set. + +Note that the spec is "flattened" and is not meant to visually resemble +the format of the netlink messages (unlike certain ad-hoc documentation +formats seen in kernel comments). In the spec subordinate attribute sets +are not defined inline as a nest, but defined in a separate attribute set +referred to with a ``nested-attributes`` property of the container. + +Spec may also contain fractional sets - sets which contain a ``subset-of`` +property. Such sets describe a section of a full set, allowing narrowing down +which attributes are allowed in a nest or refining the validation criteria. +Fractional sets can only be used in nests. They are not rendered to the uAPI +in any fashion. + +name +~~~~ + +Uniquely identifies the attribute set, operations and nested attributes +refer to the sets by the ``name``. + +subset-of +~~~~~~~~~ + +Re-defines a portion of another set (a fractional set). +Allows narrowing down fields and changing validation criteria +or even types of attributes depending on the nest in which they +are contained. The ``value`` of each attribute in the fractional +set is implicitly the same as in the main set. + +attributes +~~~~~~~~~~ + +List of attributes in the set. + +.. _attribute_properties: + +Attribute properties +-------------------- + +name +~~~~ + +Identifies the attribute, unique within the set. + +type +~~~~ + +Netlink attribute type, see :ref:`attr_types`. + +.. _assign_val: + +value +~~~~~ + +Numerical attribute ID, used in serialized Netlink messages. +The ``value`` property can be skipped, in which case the attribute ID +will be the value of the previous attribute plus one (recursively) +and ``1`` for the first attribute in the attribute set. + +Attributes (and operations) use ``1`` as the default value for the first +entry (unlike enums in definitions which start from ``0``) because +entry ``0`` is almost always reserved as undefined. Spec can explicitly +set value to ``0`` if needed. + +Note that the ``value`` of an attribute is defined only in its main set +(not in subsets). + +enum +~~~~ + +For integer types specifies that values in the attribute belong +to an ``enum`` or ``flags`` from the ``definitions`` section. + +enum-as-flags +~~~~~~~~~~~~~ + +Treat ``enum`` as ``flags`` regardless of its type in ``definitions``. +When both ``enum`` and ``flags`` forms are needed ``definitions`` should +contain an ``enum`` and attributes which need the ``flags`` form should +use this attribute. + +nested-attributes +~~~~~~~~~~~~~~~~~ + +Identifies the attribute space for attributes nested within given attribute. +Only valid for complex attributes which may have sub-attributes. + +multi-attr (arrays) +~~~~~~~~~~~~~~~~~~~ + +Boolean property signifying that the attribute may be present multiple times. +Allowing an attribute to repeat is the recommended way of implementing arrays +(no extra nesting). + +byte-order +~~~~~~~~~~ + +For integer types specifies attribute byte order - ``little-endian`` +or ``big-endian``. + +checks +~~~~~~ + +Input validation constraints used by the kernel. User space should query +the policy of the running kernel using Generic Netlink introspection, +rather than depend on what is specified in the spec file. + +The validation policy in the kernel is formed by combining the type +definition (``type`` and ``nested-attributes``) and the ``checks``. + +sub-type +~~~~~~~~ + +Legacy families have special ways of expressing arrays. ``sub-type`` can be +used to define the type of array members in case array members are not +fully defined as attributes (in a bona fide attribute space). For instance +a C array of u32 values can be specified with ``type: binary`` and +``sub-type: u32``. Binary types and legacy array formats are described in +more detail in :doc:`genetlink-legacy`. + +display-hint +~~~~~~~~~~~~ + +Optional format indicator that is intended only for choosing the right +formatting mechanism when displaying values of this type. Currently supported +hints are ``hex``, ``mac``, ``fddi``, ``ipv4``, ``ipv6`` and ``uuid``. + +operations +---------- + +This section describes messages passed between the kernel and the user space. +There are three types of entries in this section - operations, notifications +and events. + +Operations describe the most common request - response communication. User +sends a request and kernel replies. Each operation may contain any combination +of the two modes familiar to netlink users - ``do`` and ``dump``. +``do`` and ``dump`` in turn contain a combination of ``request`` and +``response`` properties. If no explicit message with attributes is passed +in a given direction (e.g. a ``dump`` which does not accept filter, or a ``do`` +of a SET operation to which the kernel responds with just the netlink error +code) ``request`` or ``response`` section can be skipped. +``request`` and ``response`` sections list the attributes allowed in a message. +The list contains only the names of attributes from a set referred +to by the ``attribute-set`` property. + +Notifications and events both refer to the asynchronous messages sent by +the kernel to members of a multicast group. The difference between the +two is that a notification shares its contents with a GET operation +(the name of the GET operation is specified in the ``notify`` property). +This arrangement is commonly used for notifications about +objects where the notification carries the full object definition. + +Events are more focused and carry only a subset of information rather than full +object state (a made up example would be a link state change event with just +the interface name and the new link state). Events contain the ``event`` +property. Events are considered less idiomatic for netlink and notifications +should be preferred. + +list +~~~~ + +The only property of ``operations`` for ``genetlink``, holds the list of +operations, notifications etc. + +Operation properties +-------------------- + +name +~~~~ + +Identifies the operation. + +value +~~~~~ + +Numerical message ID, used in serialized Netlink messages. +The same enumeration rules are applied as to +:ref:`attribute values<assign_val>`. + +attribute-set +~~~~~~~~~~~~~ + +Specifies the attribute set contained within the message. + +do +~~~ + +Specification for the ``doit`` request. Should contain ``request``, ``reply`` +or both of these properties, each holding a :ref:`attr_list`. + +dump +~~~~ + +Specification for the ``dumpit`` request. Should contain ``request``, ``reply`` +or both of these properties, each holding a :ref:`attr_list`. + +notify +~~~~~~ + +Designates the message as a notification. Contains the name of the operation +(possibly the same as the operation holding this property) which shares +the contents with the notification (``do``). + +event +~~~~~ + +Specification of attributes in the event, holds a :ref:`attr_list`. +``event`` property is mutually exclusive with ``notify``. + +mcgrp +~~~~~ + +Used with ``event`` and ``notify``, specifies which multicast group +message belongs to. + +.. _attr_list: + +Message attribute list +---------------------- + +``request``, ``reply`` and ``event`` properties have a single ``attributes`` +property which holds the list of attribute names. + +Messages can also define ``pre`` and ``post`` properties which will be rendered +as ``pre_doit`` and ``post_doit`` calls in the kernel (these properties should +be ignored by user space). + +mcast-groups +------------ + +This section lists the multicast groups of the family. + +list +~~~~ + +The only property of ``mcast-groups`` for ``genetlink``, holds the list +of groups. + +Multicast group properties +-------------------------- + +name +~~~~ + +Uniquely identifies the multicast group in the family. Similarly to +Family ID, Multicast Group ID needs to be resolved at runtime, based +on the name. + +.. _attr_types: + +Attribute types +=============== + +This section describes the attribute types supported by the ``genetlink`` +compatibility level. Refer to documentation of different levels for additional +attribute types. + +Scalar integer types +-------------------- + +Fixed-width integer types: +``u8``, ``u16``, ``u32``, ``u64``, ``s8``, ``s16``, ``s32``, ``s64``. + +Note that types smaller than 32 bit should be avoided as using them +does not save any memory in Netlink messages (due to alignment). +See :ref:`pad_type` for padding of 64 bit attributes. + +The payload of the attribute is the integer in host order unless ``byte-order`` +specifies otherwise. + +.. _pad_type: + +pad +--- + +Special attribute type used for padding attributes which require alignment +bigger than standard 4B alignment required by netlink (e.g. 64 bit integers). +There can only be a single attribute of the ``pad`` type in any attribute set +and it should be automatically used for padding when needed. + +flag +---- + +Attribute with no payload, its presence is the entire information. + +binary +------ + +Raw binary data attribute, the contents are opaque to generic code. + +string +------ + +Character string. Unless ``checks`` has ``unterminated-ok`` set to ``true`` +the string is required to be null terminated. +``max-len`` in ``checks`` indicates the longest possible string, +if not present the length of the string is unbounded. + +Note that ``max-len`` does not count the terminating character. + +nest +---- + +Attribute containing other (nested) attributes. +``nested-attributes`` specifies which attribute set is used inside. |