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.
Specification¶
Globals¶
Attributes listed directly at the root level of the spec file.
version¶
Generic Netlink family version, default is 1.
version has historically been used to introduce family changes
which may break backwards compatibility. Since compatibility breaking changes
are generally not allowed version is very rarely used.
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.
indexed-array¶
indexed-array 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.
A sub-type is needed to describe what type in the ENTRY. A nest
sub-type means there are nest arrays in the ENTRY, with the structure
looks like:
[SOME-OTHER-ATTR]
[ARRAY-ATTR]
[ENTRY]
[MEMBER1]
[MEMBER2]
[ENTRY]
[MEMBER1]
[MEMBER2]
Other sub-type like u32 means there is only one member as described
in sub-type in the ENTRY. The structure looks like:
[SOME-OTHER-ATTR]
[ARRAY-ATTR]
[ENTRY u32]
[ENTRY u32]
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.
Operations¶
Enum (message ID) model¶
unified¶
Modern families use the unified message ID model, which uses
a single enumeration for all messages within family. Requests and
responses share the same message ID. Notifications have separate
IDs from the same space. For example given the following list
of operations:
-
name: a
value: 1
do: ...
-
name: b
do: ...
-
name: c
value: 4
notify: a
-
name: d
do: ...
Requests and responses for operation a will have the ID of 1,
the requests and responses of b - 2 (since there is no explicit
value it’s previous operation + 1). Notification c will
use the ID of 4, operation d 5 etc.
directional¶
The directional model splits the ID assignment by the direction of
the message. Messages from and to the kernel can’t be confused with
each other so this conserves the ID space (at the cost of making
the programming more cumbersome).
In this case value attribute should be specified in the request
reply sections of the operations (if an operation has both do
and dump the IDs are shared, value should be set in do).
For notifications the value is provided at the op level but it
only allocates a reply (i.e. a “from-kernel” ID). Let’s look
at an example:
-
name: a
do:
request:
value: 2
attributes: ...
reply:
value: 1
attributes: ...
-
name: b
notify: a
-
name: c
notify: a
value: 7
-
name: d
do: ...
In this case a will use 2 when sending the message to the kernel
and expects message with ID 1 in response. Notification b allocates
a “from-kernel” ID which is 2. c allocates “from-kernel” ID of 7.
If operation d does not set values explicitly in the spec
it will be allocated 3 for the request (a is the previous operation
with a request section and the value of 2) and 8 for response (c is
the previous operation in the “from-kernel” direction).
Other quirks¶
Structures¶
Legacy families can define C structures both to be used as the contents of
an attribute and as a fixed message header. Structures are defined in
definitions and referenced in operations or attributes.
members¶
name- The attribute name of the struct member
type- One of the scalar typesu8,u16,u32,u64,s8,s16,s32,s64,string,binaryorbitfield32.
byte-order-big-endianorlittle-endian
doc,enum,enum-as-flags,display-hint- Same as for attribute definitions
Note that structures defined in YAML are implicitly packed according to C conventions. For example, the following struct is 4 bytes, not 6 bytes:
struct {
u8 a;
u16 b;
u8 c;
}
Any padding must be explicitly added and C-like languages should infer the need for explicit padding from whether the members are naturally aligned.
Here is the struct definition from above, declared in YAML:
definitions:
-
name: message-header
type: struct
members:
-
name: a
type: u8
-
name: b
type: u16
-
name: c
type: u8
Fixed Headers¶
Fixed message headers can be added to operations using fixed-header.
The default fixed-header can be set in operations and it can be set
or overridden for each operation.
operations:
fixed-header: message-header
list:
-
name: get
fixed-header: custom-header
attribute-set: message-attrs
Attributes¶
A binary attribute can be interpreted as a C structure using a
struct property with the name of the structure definition. The
struct property implies sub-type: struct so it is not necessary to
specify a sub-type.
attribute-sets:
-
name: stats-attrs
attributes:
-
name: stats
type: binary
struct: vport-stats
C Arrays¶
Legacy families also use binary attributes to encapsulate C arrays. The
sub-type is used to identify the type of scalar to extract.
attributes:
-
name: ports
type: binary
sub-type: u32
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).