diff options
author | Donald Hunter <donald.hunter@gmail.com> | 2023-12-15 09:37:10 +0000 |
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committer | Jakub Kicinski <kuba@kernel.org> | 2023-12-18 14:39:43 -0800 |
commit | 17ed5c1a9e3674a7e6b3e7bd66824ecd79ecce02 (patch) | |
tree | b725a75e19177b18afaaf3d52c57161861109f65 | |
parent | de2d98743b83cf970f75b7cbf650c08a659121a5 (diff) |
doc/netlink: Document the sub-message format for netlink-raw
Document the spec format used by netlink-raw families like rt and tc.
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231215093720.18774-4-donald.hunter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
-rw-r--r-- | Documentation/userspace-api/netlink/netlink-raw.rst | 96 |
1 files changed, 95 insertions, 1 deletions
diff --git a/Documentation/userspace-api/netlink/netlink-raw.rst b/Documentation/userspace-api/netlink/netlink-raw.rst index f07fb9b9c101..1e14f5f22b8e 100644 --- a/Documentation/userspace-api/netlink/netlink-raw.rst +++ b/Documentation/userspace-api/netlink/netlink-raw.rst @@ -14,7 +14,8 @@ Specification The netlink-raw schema extends the :doc:`genetlink-legacy <genetlink-legacy>` schema with properties that are needed to specify the protocol numbers and multicast IDs used by raw netlink families. See :ref:`classic_netlink` for more -information. +information. The raw netlink families also make use of type-specific +sub-messages. Globals ------- @@ -56,3 +57,96 @@ group registration. - name: rtnlgrp-mctp-ifaddr value: 34 + +Sub-messages +------------ + +Several raw netlink families such as +:doc:`rt_link<../../networking/netlink_spec/rt_link>` and +:doc:`tc<../../networking/netlink_spec/tc>` use attribute nesting as an +abstraction to carry module specific information. + +Conceptually it looks as follows:: + + [OUTER NEST OR MESSAGE LEVEL] + [GENERIC ATTR 1] + [GENERIC ATTR 2] + [GENERIC ATTR 3] + [GENERIC ATTR - wrapper] + [MODULE SPECIFIC ATTR 1] + [MODULE SPECIFIC ATTR 2] + +The ``GENERIC ATTRs`` at the outer level are defined in the core (or rt_link or +core TC), while specific drivers, TC classifiers, qdiscs etc. can carry their +own information wrapped in the ``GENERIC ATTR - wrapper``. Even though the +example above shows attributes nesting inside the wrapper, the modules generally +have full freedom to define the format of the nest. In practice the payload of +the wrapper attr has very similar characteristics to a netlink message. It may +contain a fixed header / structure, netlink attributes, or both. Because of +those shared characteristics we refer to the payload of the wrapper attribute as +a sub-message. + +A sub-message attribute uses the value of another attribute as a selector key to +choose the right sub-message format. For example if the following attribute has +already been decoded: + +.. code-block:: json + + { "kind": "gre" } + +and we encounter the following attribute spec: + +.. code-block:: yaml + + - + name: data + type: sub-message + sub-message: linkinfo-data-msg + selector: kind + +Then we look for a sub-message definition called ``linkinfo-data-msg`` and use +the value of the ``kind`` attribute i.e. ``gre`` as the key to choose the +correct format for the sub-message: + +.. code-block:: yaml + + sub-messages: + name: linkinfo-data-msg + formats: + - + value: bridge + attribute-set: linkinfo-bridge-attrs + - + value: gre + attribute-set: linkinfo-gre-attrs + - + value: geneve + attribute-set: linkinfo-geneve-attrs + +This would decode the attribute value as a sub-message with the attribute-set +called ``linkinfo-gre-attrs`` as the attribute space. + +A sub-message can have an optional ``fixed-header`` followed by zero or more +attributes from an ``attribute-set``. For example the following +``tc-options-msg`` sub-message defines message formats that use a mixture of +``fixed-header``, ``attribute-set`` or both together: + +.. code-block:: yaml + + sub-messages: + - + name: tc-options-msg + formats: + - + value: bfifo + fixed-header: tc-fifo-qopt + - + value: cake + attribute-set: tc-cake-attrs + - + value: netem + fixed-header: tc-netem-qopt + attribute-set: tc-netem-attrs + +Note that a selector attribute must appear in a netlink message before any +sub-message attributes that depend on it. |