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