kdbus: add documentation

[Greg Kroah-Hartman]

From: Daniel Mack <daniel@xxxxxxxxxx>

kdbus is a system for low-latency, low-overhead, easy to use
interprocess communication (IPC).

The interface to all functions in this driver is implemented through ioctls
on /dev nodes.  This patch adds detailed documentation about the kernel
level API design.

Signed-off-by: Daniel Mack <daniel@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 Documentation/kdbus.txt | 1815 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 1815 insertions(+)
 create mode 100644 Documentation/kdbus.txt

diff --git a/Documentation/kdbus.txt b/Documentation/kdbus.txt
new file mode 100644
index 000000000000..ac1a18908976
--- /dev/null
+++ b/Documentation/kdbus.txt
@@ -0,0 +1,1815 @@
+D-Bus is a system for powerful, easy to use interprocess communication (IPC).
+
+The focus of this document is an overview of the low-level, native kernel D-Bus
+transport called kdbus. Kdbus in the kernel acts similar to a device driver,
+all communication between processes take place over special character device
+nodes in /dev/kdbus/.
+
+For the general D-Bus protocol specification, the payload format, the
+marshaling, and the communication semantics, please refer to:
+  http://dbus.freedesktop.org/doc/dbus-specification.html
+
+For a kdbus specific userspace library implementation please refer to:
+  http://cgit.freedesktop.org/systemd/systemd/tree/src/systemd/sd-bus.h
+
+Articles about D-Bus and kdbus:
+  http://lwn.net/Articles/580194/
+
+
+1. Terminology
+===============================================================================
+
+  Domain:
+    A domain is a named object containing a number of buses. A system
+    container that contains its own init system and users usually also
+    runs in its own kdbus domain. The /dev/kdbus/domain/<container-name>/
+    directory shows up inside the domain as /dev/kdbus/. Every domain offers
+    its own "control" device node to create new buses or new sub-domains.
+    Domains have no connection to each other and cannot see nor talk to
+    each other. See section 5 for more details.
+
+  Bus:
+    A bus is a named object inside a domain. Clients exchange messages
+    over a bus. Multiple buses themselves have no connection to each other;
+    messages can only be exchanged on the same bus. The default entry point to
+    a bus, where clients establish the connection to, is the "bus" device node
+    /dev/kdbus/<bus name>/bus.
+    Common operating system setups create one "system bus" per system, and one
+    "user bus" for every logged-in user. Applications or services may create
+    their own private named buses. See section 5 for more details.
+
+  Endpoint:
+    An endpoint provides the device node to talk to a bus. Opening an
+    endpoint creates a new connection to the bus to which the endpoint belongs.
+    Every bus has a default endpoint called "bus".
+    A bus can optionally offer additional endpoints with custom names to
+    provide a restricted access to the same bus. Custom endpoints carry
+    additional policy which can be used to give sandboxed processes only
+    a locked-down, limited, filtered access to the same bus.
+    See section 5 for more details.
+
+  Connection:
+    A connection to a bus is created by opening an endpoint device node of
+    a bus and becoming an active client with the HELLO exchange. Every
+    connected client connection has a unique identifier on the bus and can
+    address messages to every other connection on the same bus by using
+    the peer's connection id as the destination.
+    See section 6 for more details.
+
+  Pool:
+    Each connection allocates a piece of shmem-backed memory that is used
+    to receive messages and answers to ioctl command from the kernel. It is
+    never used to send anything to the kernel. In order to access that memory,
+    userspace must mmap() it into its task.
+    See section 12 for more details.
+
+  Well-known Name:
+    A connection can, in addition to its implicit unique connection id, request
+    the ownership of a textual well-known name. Well-known names are noted in
+    reverse-domain notation, such as com.example.service1. Connections offering
+    a service on a bus are usually reached by its well-known name. The analogy
+    of connection id and well-known name is an IP address and a DNS name
+    associated with that address.
+
+  Message:
+    Connections can exchange messages with other connections by addressing
+    the peers with their connection id or well-known name. A message consists
+    of a message header with kernel-specific information on how to route the
+    message, and the message payload, which is a logical byte stream of
+    arbitrary size. Messages can carry additional file descriptors to be passed
+    from one connection to another. Every connection can specify which set of
+    metadata the kernel should attach to the message when it is delivered
+    to the receiving connection. Metadata contains information like: system
+    timestamps, uid, gid, tid, proc-starttime, well-known-names, process comm,
+    process exe, process argv, cgroup, capabilities, seclabel, audit session,
+    loginuid and the connection's human-readable name.
+    See section 7 and 13 for more details.
+
+  Item:
+    The API of kdbus implements a notion of items, submitted through and
+    returned by most ioctls, and stored inside data structures in the
+    connection's pool. See section 4 for more details.
+
+  Broadcast and Match:
+    Broadcast messages are potentially sent to all connections of a bus. By
+    default, the connections will not actually receive any of the sent
+    broadcast messages; only after installing a match for specific message
+    properties, a broadcast message passes this filter.
+    See section 10 for more details.
+
+  Policy:
+    A policy is a set of rules that define which connections can see, talk to,
+    or register a well-know name on the bus. A policy is attached to buses and
+    custom endpoints, and modified by policy holder connection or owners of
+    custom endpoints. See section 11 for more details.
+
+    Access rules to allow who can see a name on the bus are only checked on
+    custom endpoints. Policies may be defined with names that end with '.*'.
+    When matching a well-known name against such a wildcard entry, the last
+    part of the name is ignored and checked against the wildcard name without
+    the trailing '.*'. See section 11 for more details.
+
+  Privileged bus users:
+    A user connecting to the bus is considered privileged if it is either the
+    creator of the bus, or if it has the CAP_IPC_OWNER capability flag set.
+
+
+2. Device Node Layout
+===============================================================================
+
+The kdbus interface is exposed through device nodes in /dev.
+
+  /sys/bus/kdbus
+  `-- devices
+    |-- kdbus!0-system!bus -> ../../../devices/virtual/kdbus/kdbus!0-system!bus
+    |-- kdbus!2702-user!bus -> ../../../devices/virtual/kdbus/kdbus!2702-user!bus
+    |-- kdbus!2702-user!ep.app -> ../../../devices/virtual/kdbus/kdbus!2702-user!ep.app
+    `-- kdbus!control -> ../../../devices/kdbus!control
+
+  /dev/kdbus
+  |-- control
+  |-- 0-system
+  |   |-- bus
+  |   `-- ep.apache
+  |-- 1000-user
+  |   `-- bus
+  |-- 2702-user
+  |   |-- bus
+  |   `-- ep.app
+  `-- domain
+      |-- fedoracontainer
+      |   |-- control
+      |   |-- 0-system
+      |   |   `-- bus
+      |   `-- 1000-user
+      |       `-- bus
+      `-- mydebiancontainer
+          |-- control
+          `-- 0-system
+              `-- bus
+
+Note:
+  The device node subdirectory layout is arranged that a future version of
+  kdbus could be implemented as a file system with a separate instance mounted
+  for each domain. For any future changes, this always needs to be kept
+  in mind. Also the dependency on udev's userspace hookups or sysfs attribute
+  use should be limited to the absolute minimum for the same reason.
+
+
+3. Data Structures and flags
+===============================================================================
+
+3.1 Data structures and interconnections
+----------------------------------------
+
+  +-------------------------------------------------------------------------+
+  | Domain (Init Domain)                                                    |
+  | /dev/kdbus/control                                                      |
+  | +---------------------------------------------------------------------+ |
+  | | Bus (System Bus)                                                    | |
+  | | /dev/kdbus/0-system/                                                | |
+  | | +-------------------------------+ +-------------------------------+ | |
+  | | | Endpoint                      | | Endpoint                      | | |
+  | | | /dev/kdbus/0-system/bus       | | /dev/kdbus/0-system/ep.app    | | |
+  | | +-------------------------------+ +-------------------------------+ | |
+  | | +--------------+ +--------------+ +--------------+ +--------------+ | |
+  | | | Connection   | | Connection   | | Connection   | | Connection   | | |
+  | | | :1.22        | | :1.25        | | :1.55        | | :1.81        | | |
+  | | +--------------+ +--------------+ +--------------+ +--------------+ | |
+  | +---------------------------------------------------------------------+ |
+  |                                                                         |
+  | +---------------------------------------------------------------------+ |
+  | | Bus (User Bus for UID 2702)                                         | |
+  | | /dev/kdbus/2702-user/                                               | |
+  | | +-------------------------------+ +-------------------------------+ | |
+  | | | Endpoint                      | | Endpoint                      | | |
+  | | | /dev/kdbus/2702-user/bus      | | /dev/kdbus/2702-user/ep.app   | | |
+  | | +-------------------------------+ +-------------------------------+ | |
+  | | +--------------+ +--------------+ +--------------+ +--------------+ | |
+  | | | Connection   | | Connection   | | Connection   | | Connection   | | |
+  | | | :1.22        | | :1.25        | | :1.55        | | :1.81        | | |
+  | | +--------------+ +--------------+ +-------------------------------+ | |
+  | +---------------------------------------------------------------------+ |
+  |                                                                         |
+  | +---------------------------------------------------------------------+ |
+  | | Domain (Container; inside it, fedoracontainer/ becomes /dev/kdbus/) | |
+  | | /dev/kdbus/domain/fedoracontainer/control                           | |
+  | | +-----------------------------------------------------------------+ | |
+  | | | Bus (System Bus of "fedoracontainer")                           | | |
+  | | | /dev/kdbus/domain/fedoracontainer/0-system/                     | | |
+  | | | +-----------------------------+                                 | | |
+  | | | | Endpoint                    |                                 | | |
+  | | | | /dev/.../0-system/bus       |                                 | | |
+  | | | +-----------------------------+                                 | | |
+  | | | +-------------+ +-------------+                                 | | |
+  | | | | Connection  | | Connection  |                                 | | |
+  | | | | :1.22       | | :1.25       |                                 | | |
+  | | | +-------------+ +-------------+                                 | | |
+  | | +-----------------------------------------------------------------+ | |
+  | |                                                                     | |
+  | | +-----------------------------------------------------------------+ | |
+  | | | Bus (User Bus for UID 270 of "fedoracontainer")                 | | |
+  | | | /dev/kdbus/domain/fedoracontainer/2702-user/                    | | |
+  | | | +-----------------------------+                                 | | |
+  | | | | Endpoint                    |                                 | | |
+  | | | | /dev/.../2702-user/bus      |                                 | | |
+  | | | +-----------------------------+                                 | | |
+  | | | +-------------+ +-------------+                                 | | |
+  | | | | Connection  | | Connection  |                                 | | |
+  | | | | :1.22       | | :1.25       |                                 | | |
+  | | | +-------------+ +-------------+                                 | | |
+  | | +-----------------------------------------------------------------+ | |
+  | +---------------------------------------------------------------------+ |
+  +-------------------------------------------------------------------------+
+
+The above description uses the D-Bus notation of unique connection names that
+adds a ":1." prefix to the connection's unique ID. kbus itself doesn't
+use that notation, neither internally nor externally. However, libraries and
+other usespace code that aims for compatibility to D-Bus might.
+
+3.2 Flags
+---------
+
+All ioctls used in the communication with the driver contain two 64-bit fields,
+'flags' and 'kernel_flags'. In 'flags', the behavior of the command can be
+tweaked, whereas in 'kernel_flags', the kernel driver writes back the mask of
+supported bits upon each call, and sets the KDBUS_FLAGS_KERNEL bit. This is a
+way to probe possible kernel features and make code forward and backward
+compatible.
+
+All bits that are not recognized by the kernel in 'flags' are rejected, and the
+ioctl fails with -EINVAL.
+
+
+4. Items
+===============================================================================
+
+To flexibly augment transport structures used by kdbus, data blobs of type
+struct kdbus_item are used. An item has a fixed-sized header that only stores
+the type of the item and the overall size. The total size is variable and is
+in some cases defined by the item type, in other cases, they can be of
+arbitrary length (for instance, a string).
+
+In the external kernel API, items are used for many ioctls to transport
+optional information from userspace to kernelspace. They are also used for
+information stored in a connection's pool, such as messages, name lists or
+requested connection information.
+
+In all such occasions where items are used as part of the kdbus kernel API,
+they are embedded in structs that have an overall size of their own, so there
+can be many of them.
+
+The kernel expects all items to be aligned to 8-byte boundaries.
+
+A simple iterator in userspace would iterate over the items until the items
+have reached the embedding structure's overall size. An example implementation
+of such an iterator can be found in tools/testing/selftests/kdbus/kdbus-util.h.
+
+
+5. Creation of new domains, buses and endpoints
+===============================================================================
+
+The initial kdbus domain is unconditionally created by the kernel module. A
+domain contains a "control" device node which allows to create a new bus or
+domain. New domains do not have any buses created by default.
+
+
+5.1 Domains and buses
+---------------------
+
+Opening the control device node returns a file descriptor, it accepts the
+ioctls KDBUS_CMD_BUS_MAKE and KDBUS_CMD_DOMAIN_MAKE which specify the name of
+the new bus or domain to create. The control file descriptor needs to be kept
+open for the entire life-time of the created bus or domain, closing it will
+immediately cleanup the entire bus or domain and all its associated
+resources and connections. Every control file descriptor can only be used once
+to create a new bus or domain; from that point, it is not used for any
+further communication until the final close().
+
+Each bus will generate a random, 128-bit UUID upon creation. It will be
+returned to the creators of connections through kdbus_cmd_hello.id128 and can
+be used by userspace to uniquely identify buses, even across different machines
+or containers. The UUID will have its its variant bits set to 'DCE', and denote
+version 4 (random).
+
+When a new domain is created, its structure in /dev/kdbus/<name>/ is a
+replication of what's initially created in /dev/kdbus. In fact, internally,
+a dummy default domain is set up when the driver is loaded. This allows
+userspace to bind-mount domain subtrees of /dev/kdbus into a container's
+filesystem view, and hence achieve complete isolation from the host's domain
+and those of other containers.
+
+
+5.2 Endpoints
+-------------
+
+Endpoints are entry points to a bus. By default, each bus has a default
+endpoint called 'bus'. The bus owner has the ability to create custom
+endpoints with specific names, permissions, and policy databases (see below).
+
+To create a custom endpoint, use the KDBUS_CMD_ENDPOINT_MAKE ioctl with struct
+kdbus_cmd_make. Custom endpoints always have a policy db that, by default,
+does not allow anything. Everything that users of this new endpoint should be
+able to do has to be explicitly specified through KDBUS_ITEM_NAME and
+KDBUS_ITEM_POLICY_ACCESS items.
+
+5.3 Creating domains, buses and endpoints
+-----------------------------------------
+
+KDBUS_CMD_BUS_MAKE, KDBUS_CMD_DOMAIN_MAKE and KDBUS_CMD_ENDPOINT_MAKE take a
+struct kdbus_cmd_make argument.
+
+struct kdbus_cmd_make {
+  __u64 size;
+    The overall size of the struct, including its items.
+
+  __u64 flags;
+    The flags for creation.
+
+    KDBUS_MAKE_ACCESS_GROUP
+      Make the device node group-accessible
+
+    KDBUS_MAKE_ACCESS_WORLD
+      Make the device node world-accessible
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  struct kdbus_item items[0];
+    A list of items, only used for creating custom endpoints. Ignored for
+    buses and domains.
+};
+
+
+6. Connections
+===============================================================================
+
+
+6.1 Connection IDs and well-known connection names
+--------------------------------------------------
+
+Connections are identified by their connection id, internally implemented as a
+uint64_t counter. The IDs of every newly created bus start at 1, and every new
+connection will increment the counter by 1. The ids are not reused.
+
+In higher level tools, the user visible representation of a connection is
+defined by the D-Bus protocol specification as ":1.<id>".
+
+Messages with a specific uint64_t destination id are directly delivered to
+the connection with the corresponding id. Messages with the special destination
+id KDBUS_DST_ID_BROADCAST are broadcast messages and are potentially delivered
+to all known connections on the bus; clients interested in broadcast messages
+need to subscribe to the specific messages they are interested though, before
+any broadcast message reaches them.
+
+Messages synthesized and sent directly by the kernel will carry the special
+source id KDBUS_SRC_ID_KERNEL (0).
+
+In addition to the unique uint64_t connection id, established connections can
+request the ownership of well-known names, under which they can be found and
+addressed by other bus clients. A well-known name is associated with one and
+only one connection at a time. See section 8 on name acquisition and the
+name registry, and the validity of names.
+
+Messages can specify the special destination id 0 and carry a well-known name
+in the message data. Such a message is delivered to the destination connection
+which owns that well-known name.
+
+  +-------------------------------------------------------------------------+
+  | +---------------+     +---------------------------+                     |
+  | | Connection    |     | Message                   | -----------------+  |
+  | | :1.22         | --> | src: 22                   |                  |  |
+  | |               |     | dst: 25                   |                  |  |
+  | |               |     |                           |                  |  |
+  | |               |     |                           |                  |  |
+  | |               |     +---------------------------+                  |  |
+  | |               |                                                    |  |
+  | |               | <--------------------------------------+           |  |
+  | +---------------+                                        |           |  |
+  |                                                          |           |  |
+  | +---------------+     +---------------------------+      |           |  |
+  | | Connection    |     | Message                   | -----+           |  |
+  | | :1.25         | --> | src: 25                   |                  |  |
+  | |               |     | dst: 0xffffffffffffffff   | -------------+   |  |
+  | |               |     |  (KDBUS_DST_ID_BROADCAST) |              |   |  |
+  | |               |     |                           | ---------+   |   |  |
+  | |               |     +---------------------------+          |   |   |  |
+  | |               |                                            |   |   |  |
+  | |               | <--------------------------------------------------+  |
+  | +---------------+                                            |   |      |
+  |                                                              |   |      |
+  | +---------------+     +---------------------------+          |   |      |
+  | | Connection    |     | Message                   | --+      |   |      |
+  | | :1.55         | --> | src: 55                   |   |      |   |      |
+  | |               |     | dst: 0 / org.foo.bar      |   |      |   |      |
+  | |               |     |                           |   |      |   |      |
+  | |               |     |                           |   |      |   |      |
+  | |               |     +---------------------------+   |      |   |      |
+  | |               |                                     |      |   |      |
+  | |               | <------------------------------------------+   |      |
+  | +---------------+                                     |          |      |
+  |                                                       |          |      |
+  | +---------------+                                     |          |      |
+  | | Connection    |                                     |          |      |
+  | | :1.81         |                                     |          |      |
+  | | org.foo.bar   |                                     |          |      |
+  | |               |                                     |          |      |
+  | |               |                                     |          |      |
+  | |               | <-----------------------------------+          |      |
+  | |               |                                                |      |
+  | |               | <----------------------------------------------+      |
+  | +---------------+                                                       |
+  +-------------------------------------------------------------------------+
+
+
+6.2 Creating connections
+------------------------
+
+A connection to a bus is created by opening an endpoint device node of
+a bus and becoming an active client with the KDBUS_CMD_HELLO ioctl. Every
+connected client connection has a unique identifier on the bus and can
+address messages to every other connection on the same bus by using
+the peer's connection id as the destination.
+
+The KDBUS_CMD_HELLO ioctl takes the following struct as argument.
+
+struct kdbus_cmd_hello {
+  __u64 size;
+    The overall size of the struct, including all attached items.
+
+  __u64 conn_flags;
+    Flags to apply to this connection:
+
+    KDBUS_HELLO_ACCEPT_FD
+      When this flag is set, the connection can be sent file descriptors
+      as message payload. If it's not set, any attempt of doing so will
+      result in -ECOMM on the sender's side.
+
+    KDBUS_HELLO_ACTIVATOR
+      Make this connection an activator (see below). With this bit set,
+      an item of type KDBUS_ITEM_NAME has to be attached which describes
+      the well-known name this connection should be an activator for.
+
+    KDBUS_HELLO_POLICY_HOLDER
+      Make this connection a policy holder (see below). With this bit set,
+      an item of type KDBUS_ITEM_NAME has to be attached which describes
+      the well-known name this connection should hold a policy for.
+
+    KDBUS_HELLO_MONITOR
+      Make this connection an eaves-dropping connection that receives all
+      unicast messages sent on the bus. To also receive broadcast messages,
+      the connection has to upload appropriate matches as well.
+      This flag is only valid for privileged bus connections.
+
+  __u64 attach_flags;
+      Request the attachment of metadata for each message received by this
+      connection. The metadata actually attached may actually augment the list
+      of requested items. See section 13 for more details.
+
+  __u64 bus_flags;
+      Upon successful completion of the ioctl, this member will contain the
+      flags of the bus it connected to.
+
+  __u64 id;
+      Upon successful completion of the ioctl, this member will contain the
+      id of the new connection.
+
+  __u64 pool_size;
+      The size of the communication pool, in bytes. The pool can be accessed
+      by calling mmap() on the file descriptor that was used to issue the
+      KDBUS_CMD_HELLO ioctl.
+
+  struct kdbus_bloom_parameter bloom;
+      Bloom filter parameter (see below).
+
+  __u8 id128[16];
+      Upon successful completion of the ioctl, this member will contain the
+      128 bit wide UUID of the connected bus.
+
+  struct kdbus_item items[0];
+      Variable list of items to add optional additional information. The
+      following items are currently expected/valid:
+
+      KDBUS_ITEM_CONN_NAME
+        Contains a string to describes this connection's name, so it can be
+        identified later.
+
+      KDBUS_ITEM_NAME
+      KDBUS_ITEM_POLICY_ACCESS
+        For activators and policy holders only, combinations of these two
+        items describe policy access entries (see section about policy db).
+
+      KDBUS_ITEM_CREDS
+      KDBUS_ITEM_SECLABEL
+        Privileged bus users may submit these types in order to create
+        connections with faked credentials. The only real use case for this
+        is a proxy service which acts on behalf of some other tasks. For a
+        connection that runs in that mode, the message's metadata items will
+        be limited to what's specified here. See section 13 for more
+        information.
+
+      Items of other types are silently ignored.
+};
+
+
+6.3 Activator and policy holder connection
+------------------------------------------
+
+An activator connection is a placeholder for a well-known name. Messages sent
+to such a connection can be used by userspace to start an implementor
+connection, which will then get all the messages from the activator copied
+over. An activator connection cannot be used to send any message.
+
+A policy holder connection only installs a policy for one or more names.
+These policy entries are kept active as long as the connection is alive, and
+are removed once it terminates. Such a policy connection type can be used to
+deploy restrictions for names that are not yet active on the bus. A policy
+holder connection cannot be used to send any message.
+
+The creation of activator, policy holder or monitor connections is an operation
+restricted to privileged users on the bus (see section "Terminology").
+
+
+6.4 Retrieving information on a connection
+------------------------------------------
+
+The KDBUS_CMD_CONN_INFO ioctl can be used to retrieve credentials and
+properties of the initial creator of a connection. This ioctl uses the
+following struct:
+
+struct kdbus_cmd_info {
+  __u64 size;
+    The overall size of the struct, including the name with its 0-byte string
+    terminator.
+
+  __u64 flags;
+    Specify which items should be attached to the answer.
+    The following flags can be used:
+
+    KDBUS_ATTACH_NAMES
+      Add an item to the answer containing all the names the connection
+      currently owns.
+
+    KDBUS_ATTACH_CONN_NAME
+      Add an item to the answer containing the connection's name.
+
+    After the ioctl returns, this field will contain the current metadata
+    attach flags of the connection.
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  __u64 id;
+    The connection's numerical ID to retrieve information for. If set to
+    non-zero value, the 'name' field is ignored.
+
+  __u64 offset;
+    When the ioctl returns, this value will yield the offset of the connection
+    information inside the caller's pool.
+
+  struct kdbus_item items[0];
+    The optional item list, containing the well-known name to look up as
+    a KDBUS_ITEM_NAME. Only required if the 'id' field is set to 0.
+    All other items are currently ignored.
+};
+
+After the ioctl returns, the following struct  will be stored in the caller's
+pool at 'offset'.
+
+struct kdbus_info {
+  __u64 size;
+    The overall size of the struct, including all its items.
+
+  __u64 id;
+    The connection's unique ID.
+
+  __u64 flags;
+    The connection's flags as specified when it was created.
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  struct kdbus_item items[0];
+    Depending on the 'flags' field in struct kdbus_cmd_info, items of
+    types KDBUS_ITEM_NAME and KDBUS_ITEM_CONN_NAME are followed here.
+};
+
+Once the caller is finished with parsing the return buffer, it needs to call
+KDBUS_CMD_FREE for the offset.
+
+
+6.5 Getting information about a connection's bus creator
+--------------------------------------------------------
+
+The KDBUS_CMD_BUS_CREATOR_INFO ioctl takes the same struct as
+KDBUS_CMD_CONN_INFO but is used to retrieve information about the creator of
+the bus the connection is attached to. The metadata returned by this call is
+collected during the creation of the bus and is never altered afterwards, so
+it provides pristine information on the task that created the bus, at the
+moment when it did so.
+
+In response to this call, a slice in the connection's pool is allocated and
+filled with an object of type struct kdbus_info, pointed to by the ioctl's
+'offset' field.
+
+struct kdbus_info {
+  __u64 size;
+    The overall size of the struct, including all its items.
+
+  __u64 id;
+    The bus' ID
+
+  __u64 flags;
+    The bus' flags as specified when it was created.
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  struct kdbus_item items[0];
+    Metadata information is stored in items here.
+};
+
+Once the caller is finished with parsing the return buffer, it needs to call
+KDBUS_CMD_FREE for the offset.
+
+
+6.6 Updating connection details
+-------------------------------
+
+Some of a connection's details can be updated with the KDBUS_CMD_CONN_UPDATE
+ioctl, using the file descriptor that was used to create the connection.
+The update command uses the following struct.
+
+struct kdbus_cmd_update {
+  __u64 size;
+    The overall size of the struct, including all its items.
+
+  struct kdbus_item items[0];
+    Items to describe the connection details to be updated. The following item
+    types are supported:
+
+    KDBUS_ITEM_ATTACH_FLAGS
+      Supply a new set of items to be attached to each message.
+
+    KDBUS_ITEM_NAME
+    KDBUS_ITEM_POLICY_ACCESS
+      Policy holder connections may supply a new set of policy information
+      with these items. For other connection types, -EOPNOTSUPP is returned.
+};
+
+
+6.6 Termination
+---------------
+
+A connection can be terminated by simply closing the file descriptor that was
+used to start the connection. All pending incoming messages will be discarded,
+and the memory in the pool will be freed.
+
+An alternative way of way of closing down a connection is calling the
+KDBUS_CMD_BYEBYE ioctl on it, which will only succeed if the message queue
+of the connection is empty at the time of closing, otherwise, -EBUSY is
+returned.
+
+When this ioctl returns successfully, the connection has been terminated and
+won't accept any new messages from remote peers. This way, a connection can
+be terminated race-free, without losing any messages.
+
+
+7. Messages
+===============================================================================
+
+Messages consist of a fixed-size header followed directly by a list of
+variable-sized data 'items'. The overall message size is specified in the
+header of the message. The chain of data items can contain well-defined
+message metadata fields, raw data, references to data, or file descriptors.
+
+
+7.1 Sending messages
+--------------------
+
+Messages are passed to the kernel with the KDBUS_CMD_MSG_SEND ioctl. Depending
+on the the destination address of the message, the kernel delivers the message
+to the specific destination connection or to all connections on the same bus.
+Sending messages across buses is not possible. Messages are always queued in
+the memory pool of the destination connection (see below).
+
+The KDBUS_CMD_MSG_SEND ioctl uses struct kdbus_msg to describe the message to
+be sent.
+
+struct kdbus_msg {
+  __u64 size;
+    The over all size of the struct, including the attached items.
+
+  __u64 flags;
+    Flags for message delivery:
+
+    KDBUS_MSG_FLAGS_EXPECT_REPLY
+      Expect a reply from the remote peer to this message. With this bit set,
+      the timeout_ns field must be set to a non-zero number of nanoseconds in
+      which the receiving peer is expected to reply. If such a reply is not
+      received in time, the sender will be notified with a timeout message
+      (see below). The value must be an absolute value, in nanoseconds and
+      based on CLOCK_MONOTONIC.
+
+      For a message to be accepted as reply, it must be a direct message to
+      the original sender (not a broadcast), and its kdbus_msg.reply_cookie
+      must match the previous message's kdbus_msg.cookie.
+
+      Expected replies also temporarily open the policy of the sending
+      connection, so the other peer is allowed to respond within the given
+      time window.
+
+    KDBUS_MSG_FLAGS_SYNC_REPLY
+      By default, all calls to kdbus are considered asynchronous,
+      non-blocking. However, as there are many use cases that need to wait
+      for a remote peer to answer a method call, there's a way to send a
+      message and wait for a reply in a synchronous fashion. This is what
+      the KDBUS_MSG_FLAGS_SYNC_REPLY controls. The KDBUS_CMD_MSG_SEND ioctl
+      will block until the reply has arrived, the timeout limit is reached,
+      in case the remote connection was shut down, or if interrupted by
+      a signal before any reply; see signal(7).
+
+      The offset of the reply message in the sender's pool is stored in
+      in 'offset_reply' when the ioctl has returned without error. Hence,
+      there is no need for another KDBUS_CMD_MSG_RECV ioctl or anything else
+      to receive the reply.
+
+    KDBUS_MSG_FLAGS_NO_AUTO_START
+      By default, when a message is sent to an activator connection, the
+      activator notified and will start an implementor. This flag inhibits
+      that behavior. With this bit set, and the remote being an activator,
+      -EADDRNOTAVAIL is returned from the ioctl.
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call of
+    KDBUS_MSG_SEND.
+
+  __s64 priority;
+    The priority of this message. Receiving messages (see below) may
+    optionally be constrained to messages of a minimal priority. This
+    allows for use cases where timing critical data is interleaved with
+    control data on the same connection. If unused, the priority should be
+    set to zero.
+
+  __u64 dst_id;
+    The numeric ID of the destination connection, or KDBUS_DST_ID_BROADCAST
+    (~0ULL) to address every peer on the bus, or KDBUS_DST_ID_NAME (0) to look
+    it up dynamically from the bus' name registry. In the latter case, an item
+    of type KDBUS_ITEM_DST_NAME is mandatory.
+
+  __u64 src_id;
+    Upon return of the ioctl, this member will contain the sending
+    connection's numerical ID. Should be 0 at send time.
+
+  __u64 payload_type;
+    Type of the payload in the actual data records. Currently, only
+    KDBUS_PAYLOAD_DBUS is accepted as input value of this field. When
+    receiving messages that are generated by the kernel (notifications),
+    this field will yield KDBUS_PAYLOAD_KERNEL.
+
+  __u64 cookie;
+    Cookie of this message, for later recognition. Also, when replying
+    to a message (see above), the cookie_reply field must match this value.
+
+  __u64 timeout_ns;
+    If the message sent requires a reply from the remote peer (see above),
+    this field contains the timeout in absolute nanoseconds based on
+    CLOCK_MONOTONIC.
+
+  __u64 cookie_reply;
+    If the message sent is a reply to another message, this field must
+    match the cookie of the formerly received message.
+
+  __u64 offset_reply;
+    If the message successfully got a synchronous reply (see above), this
+    field will yield the offset of the reply message in the sender's pool.
+    Is is what KDBUS_CMD_MSG_RECV usually does for asynchronous messages.
+
+  struct kdbus_item items[0];
+    A dynamically sized list of items to contain additional information.
+    The following items are expected/valid:
+
+    KDBUS_ITEM_PAYLOAD_VEC
+    KDBUS_ITEM_PAYLOAD_MEMFD
+    KDBUS_ITEM_FDS
+      Actual data records containing the payload. See section "Passing of
+      Payload Data".
+
+    KDBUS_ITEM_BLOOM_FILTER
+      Bloom filter for matches (see below).
+
+    KDBUS_ITEM_DST_NAME
+      Well-known name to send this message to. Required if dst_id is set
+      to KDBUS_DST_ID_NAME. If a connection holding the given name can't
+      be found, -ESRCH is returned.
+      For messages to a unique name (ID), this item is optional. If present,
+      the kernel will make sure the name owner matches the given unique name.
+      This allows userspace tie the message sending to the condition that a
+      name is currently owned by a certain unique name.
+};
+
+The message will be augmented by the requested metadata items when queued into
+the receiver's pool. See also section 13.1 ("Metadata and namespaces").
+
+
+7.2 Message layout
+------------------
+
+The layout of a message is shown below.
+
+  +-------------------------------------------------------------------------+
+  | Message                                                                 |
+  | +---------------------------------------------------------------------+ |
+  | | Header                                                              | |
+  | | size: overall message size, including the data records              | |
+  | | destination: connection id of the receiver                          | |
+  | | source: connection id of the sender (set by kernel)                 | |
+  | | payload_type: "DBusDBus" textual identifier stored as uint64_t      | |
+  | +---------------------------------------------------------------------+ |
+  | +---------------------------------------------------------------------+ |
+  | | Data Record                                                         | |
+  | | size: overall record size (without padding)                         | |
+  | | type: type of data                                                  | |
+  | | data: reference to data (address or file descriptor)                | |
+  | +---------------------------------------------------------------------+ |
+  | +---------------------------------------------------------------------+ |
+  | | padding bytes to the next 8 byte alignment                          | |
+  | +---------------------------------------------------------------------+ |
+  | +---------------------------------------------------------------------+ |
+  | | Data Record                                                         | |
+  | | size: overall record size (without padding)                         | |
+  | | ...                                                                 | |
+  | +---------------------------------------------------------------------+ |
+  | +---------------------------------------------------------------------+ |
+  | | padding bytes to the next 8 byte alignment                          | |
+  | +---------------------------------------------------------------------+ |
+  | +---------------------------------------------------------------------+ |
+  | | Data Record                                                         | |
+  | | size: overall record size                                           | |
+  | | ...                                                                 | |
+  | +---------------------------------------------------------------------+ |
+  | +---------------------------------------------------------------------+ |
+  | | padding bytes to the next 8 byte alignment                          | |
+  | +---------------------------------------------------------------------+ |
+  +-------------------------------------------------------------------------+
+
+
+7.3 Passing of Payload Data
+---------------------------
+
+When connecting to the bus, receivers request a memory pool of a given size,
+large enough to carry all backlog of data enqueued for the connection. The
+pool is internally backed by a shared memory file which can be mmap()ed by
+the receiver.
+
+KDBUS_MSG_PAYLOAD_VEC:
+  Messages are directly copied by the sending process into the receiver's pool,
+  that way two peers can exchange data by effectively doing a single-copy from
+  one process to another, the kernel will not buffer the data anywhere else.
+
+KDBUS_MSG_PAYLOAD_MEMFD:
+  Messages can reference memfd files which contain the data.
+  memfd files are tmpfs-backed files that allow sealing of the content of the
+  file, which prevents all writable access to the file content.
+  Only sealed memfd files are accepted as payload data, which enforces
+  reliable passing of data; the receiver can assume that neither the sender nor
+  anyone else can alter the content after the message is sent.
+
+Apart from the sender filling-in the content into memfd files, the data will
+be passed as zero-copy from one process to another, read-only, shared between
+the peers.
+
+
+7.4 Receiving messages
+----------------------
+
+Messages are received by the client with the KDBUS_CMD_MSG_RECV ioctl. The
+endpoint device node of the bus supports poll() to wake up the receiving
+process when new messages are queued up to be received.
+
+With the KDBUS_CMD_MSG_RECV ioctl, a struct kdbus_cmd_recv is used.
+
+struct kdbus_cmd_recv {
+  __u64 flags;
+    Flags to control the receive command.
+
+    KDBUS_RECV_PEEK
+      Just return the location of the next message. Do not install file
+      descriptors or anything else. This is usually used to determine the
+      sender of the next queued message.
+
+    KDBUS_RECV_DROP
+      Drop the next message without doing anything else with it, and free the
+      pool slice. This a short-cut for KDBUS_RECV_PEEK and KDBUS_CMD_FREE.
+
+    KDBUS_RECV_USE_PRIORITY
+      Use the priority field (see below).
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  __s64 priority;
+      With KDBUS_RECV_USE_PRIORITY set in flags, receive the next message in
+      the queue with at least the given priority. If no such message is waiting
+      in the queue, -ENOMSG is returned.
+
+  __u64 offset;
+      Upon return of the ioctl, this field contains the offset in the
+      receiver's memory pool.
+};
+
+Unless KDBUS_RECV_DROP was passed, and given that the ioctl succeeded, the
+offset field contains the location of the new message inside the receiver's
+pool. The message is stored as struct kdbus_msg at this offset, and can be
+interpreted with the semantics described above.
+
+Also, if the connection allowed for file descriptor to be passed
+(KDBUS_HELLO_ACCEPT_FD), and if the message contained any, they will be
+installed into the receiving process after the KDBUS_CMD_MSG_RECV ioctl
+returns. The receiving task is obliged to close all of them appropriately.
+
+The caller is obliged to call KDBUS_CMD_FREE with the returned offset when
+the memory is no longer needed.
+
+
+7.5 Canceling messages synchronously waiting for replies
+--------------------------------------------------------
+
+When a connection sends a message with KDBUS_MSG_FLAGS_SYNC_REPLY and
+blocks while waiting for the reply, the KDBUS_CMD_MSG_CANCEL ioctl can be
+used on the same file descriptor to cancel the message, based on its cookie.
+If there are multiple messages with the same cookie that are all synchronously
+waiting for a reply, all of them will be canceled. Obviously, this is only
+possible in multi-threaded applications.
+
+
+8. Name registry
+===============================================================================
+
+Each bus instantiates a name registry to resolve well-known names into unique
+connection IDs for message delivery. The registry will be queried when a
+message is sent with kdbus_msg.dst_id set to KDBUS_DST_ID_NAME, or when a
+registry dump is requested.
+
+All of the below is subject to policy rules for SEE and OWN permissions.
+
+
+8.1 Name validity
+-----------------
+
+A name has to comply to the following rules to be considered valid:
+
+ - The name has two or more elements separated by a period ('.') character
+ - All elements must contain at least one character
+ - Each element must only contain the ASCII characters "[A-Z][a-z][0-9]_"
+   and must not begin with a digit
+ - The name must contain at least one '.' (period) character
+   (and thus at least two elements)
+ - The name must not begin with a '.' (period) character
+ - The name must not exceed KDBUS_NAME_MAX_LEN (255)
+
+
+8.2 Acquiring a name
+--------------------
+
+To acquire a name, a client uses the KDBUS_CMD_NAME_ACQUIRE ioctl with the
+following data structure.
+
+struct kdbus_cmd_name {
+  __u64 size;
+    The overall size of this struct, including the name with its 0-byte string
+    terminator.
+
+  __u64 flags;
+    Flags to control details in the name acquisition.
+
+    KDBUS_NAME_REPLACE_EXISTING
+      Acquiring a name that is already present usually fails, unless this flag
+      is set in the call, and KDBUS_NAME_ALLOW_REPLACEMENT or (see below) was
+      set when the current owner of the name acquired it, or if the current
+      owner is an activator connection (see below).
+
+    KDBUS_NAME_ALLOW_REPLACEMENT
+      Allow other connections to take over this name. When this happens, the
+      former owner of the connection will be notified of the name loss.
+
+    KDBUS_NAME_QUEUE (acquire)
+      A name that is already acquired by a connection, and which wasn't
+      requested with the KDBUS_NAME_ALLOW_REPLACEMENT flag set can not be
+      acquired again. However, a connection can put itself in a queue of
+      connections waiting for the name to be released. Once that happens, the
+      first connection in that queue becomes the new owner and is notified
+      accordingly.
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  struct kdbus_item items[0];
+    Items to submit the name. Currently, one one item of type KDBUS_ITEM_NAME
+    is expected and allowed, and the contained string must be a valid bus name.
+};
+
+
+8.3 Releasing a name
+--------------------
+
+A connection may release a name explicitly with the KDBUS_CMD_NAME_RELEASE
+ioctl. If the connection was an implementor of an activatable name, its
+pending messages are moved back to the activator. If there are any connections
+queued up as waiters for the name, the oldest one of them will become the new
+owner. The same happens implicitly for all names once a connection terminates.
+
+The KDBUS_CMD_NAME_RELEASE ioctl uses the same data structure as the
+acquisition call, but with slightly different field usage.
+
+struct kdbus_cmd_name {
+  __u64 size;
+    The overall size of this struct, including the name with its 0-byte string
+    terminator.
+
+  __u64 flags;
+
+  struct kdbus_item items[0];
+    Items to submit the name. Currently, one one item of type KDBUS_ITEM_NAME
+    is expected and allowed, and the contained string must be a valid bus name.
+};
+
+
+8.4 Dumping the name registry
+-----------------------------
+
+A connection may request a complete or filtered dump of currently active bus
+names with the KDBUS_CMD_NAME_LIST ioctl, which takes a struct
+kdbus_cmd_name_list as argument.
+
+struct kdbus_cmd_name_list {
+  __u64 flags;
+    Any combination of flags to specify which names should be dumped.
+
+    KDBUS_NAME_LIST_UNIQUE
+      List the unique (numeric) IDs of the connection, whether it owns a name
+      or not.
+
+    KDBUS_NAME_LIST_NAMES
+      List well-known names stored in the database which are actively owned by
+      a real connection (not an activator).
+
+    KDBUS_NAME_LIST_ACTIVATORS
+      List names that are owned by an activator.
+
+    KDBUS_NAME_LIST_QUEUED
+      List connections that are not yet owning a name but are waiting for it
+      to become available.
+
+  __u64 offset;
+    When the ioctl returns successfully, the offset to the name registry dump
+    inside the connection's pool will be stored in this field.
+};
+
+The returned list of names is stored in a struct kdbus_name_list that in turn
+contains a dynamic number of struct kdbus_cmd_name that carry the actual
+information. The fields inside that struct kdbus_cmd_name is described next.
+
+struct kdbus_name_info {
+  __u64 size;
+    The overall size of this struct, including the name with its 0-byte string
+    terminator.
+
+  __u64 flags;
+    The current flags for this name. Can be any combination of
+
+    KDBUS_NAME_ALLOW_REPLACEMENT
+
+    KDBUS_NAME_IN_QUEUE (list)
+      When retrieving a list of currently acquired name in the registry, this
+      flag indicates whether the connection actually owns the name or is
+      currently waiting for it to become available.
+
+    KDBUS_NAME_ACTIVATOR (list)
+      An activator connection owns a name as a placeholder for an implementor,
+      which is started on demand as soon as the first message arrives. There's
+      some more information on this topic below. In contrast to
+      KDBUS_NAME_REPLACE_EXISTING, when a name is taken over from an activator
+      connection, all the messages that have been queued in the activator
+      connection will be moved over to the new owner. The activator connection
+      will still be tracked for the name and will take control again if the
+      implementor connection terminates.
+      This flag can not be used when acquiring a name, but is implicitly set
+      through KDBUS_CMD_HELLO with KDBUS_HELLO_ACTIVATOR set in
+      kdbus_cmd_hello.conn_flags.
+
+  __u64 owner_id;
+    The owning connection's unique ID.
+
+  __u64 conn_flags;
+    The flags of the owning connection.
+
+  struct kdbus_item items[0];
+    Items containing the actual name. Currently, one one item of type
+    KDBUS_ITEM_NAME will be attached.
+};
+
+The returned buffer must be freed with the KDBUS_CMD_FREE ioctl when the user
+is finished with it.
+
+
+9. Notifications
+===============================================================================
+
+The kernel will notify its users of the following events.
+
+  * When connection A is terminated while connection B is waiting for a reply
+    from it, connection B is notified with a message with an item of type
+    KDBUS_ITEM_REPLY_DEAD.
+
+  * When connection A does not receive a reply from connection B within the
+    specified timeout window, connection A will receive a message with an item
+    of type KDBUS_ITEM_REPLY_TIMEOUT.
+
+  * When a connection is created on or removed from a bus, messages with an
+    item of type KDBUS_ITEM_ID_ADD or KDBUS_ITEM_ID_REMOVE, respectively, are
+    sent to all bus members that match these messages through their match
+    database.
+
+  * When a connection owns or loses a name, or a name is moved from one
+    connection to another, messages with an item of type KDBUS_ITEM_NAME_ADD,
+    KDBUS_ITEM_NAME_REMOVE or KDBUS_ITEM_NAME_CHANGE are sent to all bus
+    members that match these messages through their match database.
+
+A kernel notification is a regular kdbus message with the following details.
+
+  * kdbus_msg.src_id == KDBUS_SRC_ID_KERNEL
+  * kdbus_msg.dst_id == KDBUS_DST_ID_BROADCAST
+  * kdbus_msg.payload_type == KDBUS_PAYLOAD_KERNEL
+  * Has exactly one of the aforementioned items attached
+
+
+10. Message Matching, Bloom filters
+===============================================================================
+
+10.1 Matches for broadcast messages from other connections
+----------------------------------------------------------
+
+A message addressed at the connection ID KDBUS_DST_ID_BROADCAST (~0ULL) is a
+broadcast message, delivered to all connected peers which installed a rule to
+match certain properties of the message. Without any rules installed in the
+connection, no broadcast message or kernel-side notifications will be delivered
+to the connection. Broadcast messages are subject to policy rules and TALK
+access checks.
+
+See section 11 for details on policies, and section 11.5 for more
+details on implicit policies.
+
+Matches for messages from other connections (not kernel notifications) are
+implemented as bloom filters. The sender adds certain properties of the message
+as elements to a bloom filter bit field, and sends that along with the
+broadcast message.
+
+The connection adds the message properties it is interested as elements to a
+bloom mask bit field, and uploads the mask to the match rules of the
+connection.
+
+The kernel will match the broadcast message's bloom filter against the
+connections bloom mask (simply by &-ing it), and decide whether the message
+should be delivered to the connection.
+
+The kernel has no notion of any specific properties of the message, all it
+sees are the bit fields of the bloom filter and mask to match against. The
+use of bloom filters allows simple and efficient matching, without exposing
+any message properties or internals to the kernel side. Clients need to deal
+with the fact that they might receive broadcasts which they did not subscribe
+to, as the bloom filter might allow false-positives to pass the filter.
+
+To allow the future extension of the set of elements in the bloom filter, the
+filter specifies a "generation" number. A later generation must always contain
+all elements of the set of the previous generation, but can add new elements
+to the set. The match rules mask can carry an array with all previous
+generations of masks individually stored. When the filter and mask are matched
+by the kernel, the mask with the closest matching "generation" is selected
+as the index into the mask array.
+
+
+10.2 Matches for kernel notifications
+------------------------------------
+
+To receive kernel generated notifications (see section 9), a connection must
+install special match rules that are different from the bloom filter matches
+described in the section above. They can be filtered by a sender connection's
+ID, by one of the name the sender connection owns at the time of sending the
+message, or by type of the notification (id/name add/remove/change).
+
+10.3 Adding a match
+-------------------
+
+To add a match, the KDBUS_CMD_MATCH_ADD ioctl is used, which takes a struct
+of the struct described below.
+
+Note that each of the items attached to this command will internally create
+one match 'rule', and the collection of them, which is submitted as one block
+via the ioctl is called a 'match'. To allow a message to pass, all rules of a
+match have to be satisfied. Hence, adding more items to the command will only
+narrow the possibility of a match to effectively let the message pass, and will
+cause the connection's user space process to wake up less likely.
+
+Multiple matches can be installed per connection. As long as one of it has a
+set of rules which allows the message to pass, this one will be decisive.
+
+struct kdbus_cmd_match {
+  __u64 size;
+    The overall size of the struct, including its items.
+
+  __u64 cookie;
+    A cookie which identifies the match, so it can be referred to at removal
+    time.
+
+  __u64 flags;
+    Flags to control the behavior of the ioctl.
+
+    KDBUS_MATCH_REPLACE:
+      Remove all entries with the given cookie before installing the new one.
+      This allows for race-free replacement of matches.
+
+  struct kdbus_item items[0];
+    Items to define the actual rules of the matches. The following item types
+    are expected. Each item will cause one new match rule to be created.
+
+    KDBUS_ITEM_BLOOM_MASK
+      An item that carries the bloom filter mask to match against in its
+      data field. The payload size must match the bloom filter size that
+      was specified when the bus was created.
+      See section 10.4 for more information.
+
+    KDBUS_ITEM_NAME
+      Specify a name that a sending connection must own at a time of sending
+      a broadcast message in order to match this rule.
+
+    KDBUS_ITEM_ID
+      Specify a sender connection's ID that will match this rule.
+
+    KDBUS_ITEM_NAME_ADD
+    KDBUS_ITEM_NAME_REMOVE
+    KDBUS_ITEM_NAME_CHANGE
+      These items request delivery of broadcast messages that describe a name
+      acquisition, loss, or change. The details are stored in the item's
+      kdbus_notify_name_change member. All information specified must be
+      matched in order to make the message pass. Use KDBUS_MATCH_ID_ANY to
+      match against any unique connection ID.
+
+    KDBUS_ITEM_ID_ADD
+    KDBUS_ITEM_ID_REMOVE
+      These items request delivery of broadcast messages that are generated
+      when a connection is created or terminated. struct kdbus_notify_id_change
+      is used to store the actual match information. This item can be used to
+      monitor one particular connection ID, or, when the id field is set to
+      KDBUS_MATCH_ID_ANY, all of them.
+
+    Other item types are ignored.
+};
+
+
+10.4 Bloom filters
+------------------
+
+Bloom filters allow checking whether a given word is present in a dictionary.
+This allows connections to set up a mask for information it is interested in,
+and will be delivered broadcast messages that have a matching filter.
+
+For general information on bloom filters, see
+
+  https://en.wikipedia.org/wiki/Bloom_filter
+
+The size of the bloom filter is defined per bus when it is created, in
+kdbus_bloom_parameter.size. All bloom filters attached to broadcast messages
+on the bus must match this size, and all bloom filter matches uploaded by
+connections must also match the size, or a multiple thereof (see below).
+
+The calculation of the mask has to be done on the userspace side. The kernel
+just checks the bitmasks to decide whether or not to let the message pass. All
+bits in the mask must match the filter in and bit-wise AND logic, but the
+mask may have more bits set than the filter. Consequently, false positive
+matches are expected to happen, and userspace must deal with that fact.
+
+Masks are entities that are always passed to the kernel as part of a match
+(with an item of type KDBUS_ITEM_BLOOM_MASK), and filters can be attached to
+broadcast messages (with an item of type KDBUS_ITEM_BLOOM_FILTER).
+
+For a broadcast to match, all set bits in the filter have to be set in the
+installed match mask as well. For example, consider a bus has a bloom size
+of 8 bytes, and the following mask/filter combinations:
+
+    filter  0x0101010101010101
+    mask    0x0101010101010101
+            -> matches
+
+    filter  0x0303030303030303
+    mask    0x0101010101010101
+            -> doesn't match
+
+    filter  0x0101010101010101
+    mask    0x0303030303030303
+            -> matches
+
+Hence, in order to catch all messages, a mask filled with 0xff bytes can be
+installed as a wildcard match rule.
+
+Uploaded matches may contain multiple masks, each of which in the size of the
+bloom size defined by the bus. Each block of a mask is called a 'generation',
+starting at index 0.
+
+At match time, when a broadcast message is about to be delivered, a bloom
+mask generation is passed, which denotes which of the bloom masks the filter
+should be matched against. This allows userspace to provide backward compatible
+masks at upload time, while older clients can still match against older
+versions of filters.
+
+
+10.5 Removing a match
+--------------------
+
+Matches can be removed through the KDBUS_CMD_MATCH_REMOVE ioctl, which again
+takes struct kdbus_cmd_match as argument, but its fields are used slightly
+differently.
+
+struct kdbus_cmd_match {
+  __u64 size;
+    The overall size of the struct. As it has no items in this use case, the
+    value should yield 16.
+
+  __u64 cookie;
+    The cookie of the match, as it was passed when the match was added.
+    All matches that have this cookie will be removed.
+
+  __u64 flags;
+    Unused for this use case,
+
+  __u64 kernel_flags;
+    Valid flags for this command, returned by the kernel upon each call.
+
+  struct kdbus_item items[0];
+    Unused for this use case.
+};
+
+
+11. Policy
+===============================================================================
+
+A policy databases restrict the possibilities of connections to own, see and
+talk to well-known names. It can be associated with a bus (through a policy
+holder connection) or a custom endpoint.
+
+See section 8.1 for more details on the validity of well-known names.
+
+Default endpoints of buses always have a policy database. The default
+policy is to deny all operations except for operations that are covered by
+implicit policies. Custom endpoints always have a policy, and by default,
+a policy database is empty. Therefore, unless policy rules are added, all
+operations will also be denied by default.
+
+See section 11.5 for more details on implicit policies.
+
+A set of policy rules is described by a name and multiple access rules, defined
+by the following struct.
+
+struct kdbus_policy_access {
+  __u64 type;	/* USER, GROUP, WORLD */
+    One of the following.
+
+    KDBUS_POLICY_ACCESS_USER
+      Grant access to a user with the uid stored in the 'id' field.
+
+    KDBUS_POLICY_ACCESS_GROUP
+      Grant access to a user with the gid stored in the 'id' field.
+
+    KDBUS_POLICY_ACCESS_WORLD
+      Grant access to everyone. The 'id' field is ignored.
+
+  __u64 access;	/* OWN, TALK, SEE */
+    The access to grant.
+
+    KDBUS_POLICY_SEE
+      Allow the name to be seen.
+
+    KDBUS_POLICY_TALK
+      Allow the name to be talked to.
+
+    KDBUS_POLICY_OWN
+      Allow the name to be owned.
+
+  __u64 id;
+    For KDBUS_POLICY_ACCESS_USER, stores the uid.
+    For KDBUS_POLICY_ACCESS_GROUP, stores the gid.
+};
+
+Policies are set through KDBUS_CMD_HELLO (when creating a policy holder
+connection), KDBUS_CMD_CONN_UPDATE (when updating a policy holder connection),
+KDBUS_CMD_ENDPOINT_MAKE (creating a custom endpoint) or
+KDBUS_CMD_ENDPOINT_UPDATE (updating a custom endpoint). In all cases, the name
+and policy access information is stored in items of type KDBUS_ITEM_NAME and
+KDBUS_ITEM_POLICY_ACCESS. For this transport, the following rules apply.
+
+  * An item of type KDBUS_ITEM_NAME must be followed by at least one
+    KDBUS_ITEM_POLICY_ACCESS item
+  * An item of type KDBUS_ITEM_NAME can be followed by an arbitrary number of
+    KDBUS_ITEM_POLICY_ACCESS items
+  * An arbitrary number of groups of names and access levels can be passed
+
+uids and gids are internally always stored in the kernel's view of global ids,
+and are translated back and forth on the ioctl level accordingly.
+
+
+11.2 Wildcard names
+-------------------
+
+Policy holder connections may upload names that contain the wildcard suffix
+(".*"). That way, a policy can be uploaded that is effective for every
+well-kwown name that extends the provided name by exactly one more level.
+
+For example, if an item of a set up uploaded policy rules contains the name
+"foo.bar.*", both "foo.bar.baz" and "foo.bar.bazbaz" are valid, but
+"foo.bar.baz.baz" is not.
+
+This allows connections to take control over multiple names that the policy
+holder doesn't need to know about when uploading the policy.
+
+Such wildcard entries are not allowed for custom endpoints.
+
+
+11.3 Policy example
+-------------------
+
+For example, a set of policy rules may look like this:
+
+  KDBUS_ITEM_NAME: str='org.foo.bar'
+  KDBUS_ITEM_POLICY_ACCESS: type=USER, access=OWN, id=1000
+  KDBUS_ITEM_POLICY_ACCESS: type=USER, access=TALK, id=1001
+  KDBUS_ITEM_POLICY_ACCESS: type=WORLD, access=SEE
+  KDBUS_ITEM_NAME: str='org.blah.baz'
+  KDBUS_ITEM_POLICY_ACCESS: type=USER, access=OWN, id=0
+  KDBUS_ITEM_POLICY_ACCESS: type=WORLD, access=TALK
+
+That means that 'org.foo.bar' may only be owned by uid 1000, but every user on
+the bus is allowed to see the name. However, only uid 1001 may actually send
+a message to the connection and receive a reply from it.
+
+The second rule allows 'org.blah.baz' to be owned by uid 0 only, but every user
+may talk to it.
+
+
+11.4 TALK access and multiple well-known names per connection
+-------------------------------------------------------------
+
+Note that TALK access is checked against all names of a connection.
+For example, if a connection owns both 'org.foo.bar' and 'org.blah.baz', and
+the policy database allows 'org.blah.baz' to be talked to by WORLD, then this
+permission is also granted to 'org.foo.bar'. That might sound illogical, but
+after all, we allow messages to be directed to either the name or a well-known
+name, and policy is applied to the connection, not the name. In other words,
+the effective TALK policy for a connection is the most permissive of all names
+the connection owns.
+
+If a policy database exists for a bus (because a policy holder created one on
+demand) or for a custom endpoint (which always has one), each one is consulted
+during name registry listing, name owning or message delivery. If either one
+fails, the operation is failed with -EPERM.
+
+For best practices, connections that own names with a restricted TALK
+access should not install matches. This avoids cases where the sent
+message may pass the bloom filter due to false-positives and may also
+satisfy the policy rules.
+
+11.5 Implicit policies
+----------------------
+
+Depending on the type of the endpoint, a set of implicit rules might be
+enforced. On default endpoints, the following set is enforced:
+
+  * Privileged connections always override any installed policy. Those
+    connections could easily install their own policies, so there is no
+    reason to enforce installed policies.
+  * Connections can always talk to connections of the same user. This
+    includes broadcast messages.
+  * Connections that own names might send broadcast messages to other
+    connections that belong to a different user, but only if that
+    destination connection does not own any name.
+
+Custom endpoints have stricter policies. The following rules apply:
+
+  * Policy rules are always enforced, even if the connection is a privileged
+    connection.
+  * Policy rules are always enforced for TALK access, even if both ends are
+    running under the same user. This includes broadcast messages.
+  * To restrict the set of names that can be seen, endpoint policies can
+    install "SEE" policies.
+
+
+12. Pool
+===============================================================================
+
+A pool for data received from the kernel is installed for every connection of
+the bus, and is sized according to kdbus_cmd_hello.pool_size. It is accessed
+when one of the following ioctls is issued:
+
+  * KDBUS_CMD_MSG_RECV, to receive a message
+  * KDBUS_CMD_NAME_LIST, to dump the name registry
+  * KDBUS_CMD_CONN_INFO, to retrieve information on a connection
+
+Internally, the pool is organized in slices, stored in an rb-tree. The offsets
+returned by either one of the aforementioned ioctls describe offsets inside the
+pool. In order to make the slice available for subsequent calls, KDBUS_CMD_FREE
+has to be called on the offset.
+
+To access the memory, the caller is expected to mmap() it to its task, like
+this:
+
+  /*
+   * POOL_SIZE has to be a multiple of PAGE_SIZE, and it must match the
+   * value that was previously passed in the .pool_size field of struct
+   * kdbus_cmd_hello.
+   */
+
+  buf = mmap(NULL, POOL_SIZE, PROT_READ, MAP_PRIVATE, conn_fd, 0);
+
+
+13. Metadata
+===============================================================================
+
+When a message is delivered to a receiver connection, it is augmented by
+metadata items in accordance to the destination's current attach flags. The
+information stored in those metadata items refer to the sender task at the
+time of sending the message, so even if any detail of the sender task has
+already changed upon message reception (or if the sender task does not exist
+anymore), the information is still preserved and won't be modfied until the
+message is freed.
+
+Note that there are two exceptions to the above rules:
+
+  a) Kernel generated messages don't have a source connection, so they won't be
+     augmented.
+
+  b) If a connection was created with faked credentials (see section 6.2),
+     the only attached metadata items are the ones provided by the connection
+     itself. The destination's attach_flags won't be looked at in such cases.
+
+Also, there are two things to be considered by userspace programs regarding
+those metadata items:
+
+  a) Userspace must cope with the fact that it might get more metadata than
+     they requested. That happens, for example, when a broadcast message is
+     sent and receivers have different attach flags. Items that haven't been
+     requested should hence be silently ignored.
+
+  b) Userspace might not always get all requested metadata items that it
+     requested. That is because some of those items are only added if a
+     corresponding kernel feature has been enabled. Also, the two exceptions
+     described above will as well lead to less items be attached than
+     requested.
+
+
+13.1 Known item types
+---------------------
+
+The following attach flags are currently supported.
+
+  KDBUS_ATTACH_TIMESTAMP
+    Attaches an item of type KDBUS_ITEM_TIMESTAMP which contains both the
+    monotonic and the realtime timestamp, taken when the message was
+    processed on the kernel side.
+
+  KDBUS_ATTACH_CREDS
+    Attaches an item of type KDBUS_ITEM_CREDS, containing credentials as
+    described in kdbus_creds: the uid, gid, pid, tid and starttime of the task.
+
+  KDBUS_ATTACH_AUXGROUPS
+    Attaches an item of type KDBUS_ITEM_AUXGROUPS, containing a dynamic
+    number of auxiliary groups the sending task was a member of.
+
+  KDBUS_ATTACH_NAMES
+    Attaches items of type KDBUS_ITEM_NAME, one for each name the sending
+    connection currently owns. The name is stored in kdbus_item.str for each
+    of them.
+
+  KDBUS_ATTACH_COMM
+    Attaches an items of type KDBUS_ITEM_PID_COMM and KDBUS_ITEM_TID_COMM,
+    both transporting the sending task's 'comm', for both the pid and the tid.
+    The strings are stored in kdbus_item.str.
+
+  KDBUS_ATTACH_EXE
+    Attaches an item of type KDBUS_ITEM_EXE, containing the path to the
+    executable of the sending task, stored in kdbus_item.str.
+
+  KDBUS_ATTACH_CMDLINE
+    Attaches an item of type KDBUS_ITEM_CMDLINE, containing the command line
+    arguments of the sending task, as an array of strings, stored in
+    kdbus_item.str.
+
+  KDBUS_ATTACH_CGROUP
+    Attaches an item of type KDBUS_ITEM_CGROUP with the task's cgroup path.
+
+  KDBUS_ATTACH_CAPS
+    Attaches an item of type KDBUS_ITEM_CAPS, carrying sets of capabilities
+    that should be accessed via kdbus_item.caps.caps. Also, userspace should
+    be written in a way that it takes kdbus_item.caps.last_cap into account,
+    and derive the number of sets and rows from the item size and the reported
+    number of valid capability bits.
+
+  KDBUS_ATTACH_SECLABEL
+    Attaches an item of type KDBUS_ITEM_SECLABEL, which contains the SELinux
+    security label of the sending task. Access via kdbus_item->str.
+
+  KDBUS_ATTACH_AUDIT
+    Attaches an item of type KDBUS_ITEM_AUDIT, which contains the audio label
+    of the sending taskj. Access via kdbus_item->str.
+
+  KDBUS_ATTACH_CONN_NAME
+    Attaches an item of type KDBUS_ITEM_CONN_NAME that contain's the
+    sending's connection current name in kdbus_item.str.
+
+
+13.1 Metadata and namespaces
+----------------------------
+Note that if the user or PID namespaces of a connection at the time of sending
+differ from those that were active then the connection was created
+(KDBUS_CMD_HELLO), data structures such as messages will not have any metadata
+attached to prevent leaking security-relevant information.
+
+
+14. Error codes
+===============================================================================
+
+Below is a list of error codes that might be returned by the individual
+ioctl commands. The list focuses on the return values from kdbus code itself,
+and might not cover those of all kernel internal functions.
+
+For all ioctls:
+
+  -ENOMEM	The kernel memory is exhausted
+  -ENOTTY	Illegal ioctl command issued for the file descriptor
+  -ENOSYS	The requested functionality is not available
+
+For all ioctls that carry a struct as payload:
+
+  -EFAULT	The supplied data pointer was not 64-bit aligned, or was
+		inaccessible from the kernel side.
+  -EINVAL	The size inside the supplied struct was smaller than expected
+  -EMSGSIZE	The size inside the supplied struct was bigger than expected
+  -ENAMETOOLONG	A supplied name is larger than the allowed maximum size
+
+For KDBUS_CMD_BUS_MAKE:
+
+  -EINVAL	The flags supplied in the kdbus_cmd_make struct are invalid or
+		the supplied name does not start with the current uid and a '-'
+  -EEXIST	A bus of that name already exists
+  -ESHUTDOWN	The domain for the bus is already shut down
+  -EMFILE	The maximum number of buses for the current user is exhausted
+
+For KDBUS_CMD_DOMAIN_MAKE:
+
+  -EPERM	The calling user does not have CAP_IPC_OWNER set, or
+  -EINVAL	The flags supplied in the kdbus_cmd_make struct are invalid, or
+		no name supplied for top-level domain
+  -EEXIST	A domain of that name already exists
+
+For KDBUS_CMD_ENDPOINT_MAKE:
+
+  -EPERM	The calling user is not privileged (see Terminology)
+  -EINVAL	The flags supplied in the kdbus_cmd_make struct are invalid
+  -EEXIST	An endpoint of that name already exists
+
+For KDBUS_CMD_HELLO:
+
+  -EFAULT	The supplied pool size was 0 or not a multiple of the page size
+  -EINVAL	The flags supplied in the kdbus_cmd_make struct are invalid, or
+		an illegal combination of KDBUS_HELLO_MONITOR,
+		KDBUS_HELLO_ACTIVATOR and KDBUS_HELLO_POLICY_HOLDER was passed
+		in the flags, or an invalid set of items was supplied
+  -EPERM	An KDBUS_ITEM_CREDS items was supplied, but the current user is
+		not privileged
+  -ESHUTDOWN	The bus has already been shut down
+  -EMFILE	The maximum number of connection on the bus has been reached
+
+For KDBUS_CMD_BYEBYE:
+
+  -EALREADY	The connection has already been shut down
+  -EBUSY	There are still messages queued up in the connection's pool
+
+For KDBUS_CMD_MSG_SEND:
+
+  -EOPNOTSUPP	The connection is unconnected, or a fd was passed that is
+		either a kdbus handle itself or a unix domain socket. Both is
+		currently unsupported.
+  -EINVAL	The submitted payload type is KDBUS_PAYLOAD_KERNEL,
+		KDBUS_MSG_FLAGS_EXPECT_REPLY was set without a timeout value,
+		KDBUS_MSG_FLAGS_SYNC_REPLY was set without
+		KDBUS_MSG_FLAGS_EXPECT_REPLY, an invalid item was supplied,
+		src_id was != 0 and different from the current connection's ID,
+		a supplied memfd had a size of 0, a string was not properly
+		nul-terminated
+  -ENOTUNIQ	KDBUS_MSG_FLAGS_EXPECT_REPLY was set, but the dst_id is set
+		to KDBUS_DST_ID_BROADCAST
+  -E2BIG	Too many items
+  -EMSGSIZE	A payload vector was too big, and the current user is
+		unprivileged.
+  -ENOTUNIQ	A fd or memfd payload was passed in a broadcast message, or
+		a timeout was given for a broadcast message
+  -EEXIST	Multiple KDBUS_ITEM_FDS or KDBUS_ITEM_BLOOM_FILTER,
+		KDBUS_ITEM_DST_NAME were supplied
+  -EBADF	A memfd item contained an illegal fd
+  -EMEDIUMTYPE	A file descriptor which is not a kdbus memfd was
+		refused to send as KDBUS_MSG_PAYLOAD_MEMFD.
+  -EMFILE	Too many file descriptors inside a KDBUS_ITEM_FDS
+  -EBADMSG	An item had illegal size, both a dst_id and a
+		KDBUS_ITEM_DST_NAME was given, or both a name and a bloom
+		filter was given
+  -ETXTBSY	A kdbus memfd file cannot be sealed or the seal removed,
+		because it is shared with other processes or still mmap()ed
+  -ECOMM	A peer does not accept the file descriptors addressed to it
+  -EFAULT	The supplied bloom filter size was not 64-bit aligned
+  -EDOM		The supplied bloom filter size did not match the bloom filter
+		size of the bus
+  -EDESTADDRREQ	dst_id was set to KDBUS_DST_ID_NAME, but no KDBUS_ITEM_DST_NAME
+		was attached
+  -ESRCH	The name to look up was not found in the name registry
+  -EADDRNOTAVAIL KDBUS_MSG_FLAGS_NO_AUTO_START was given but the destination
+		 connection is an activator.
+  -ENXIO	The passed numeric destination connection ID couldn't be found,
+		or is not connected
+  -ECONNRESET	The destination connection is no longer active
+  -ETIMEDOUT	Timeout while synchronously waiting for a reply
+  -EINTR	System call interrupted while synchronously waiting for a reply
+  -EPIPE	When sending a message, a synchronous reply from the receiving
+		connection was expected but the connection died before
+		answering
+  -ECANCELED	A synchronous message sending was cancelled
+  -ENOBUFS	Too many pending messages on the receiver side
+  -EREMCHG	Both a well-known name and a unique name (ID) was given, but
+		the name is not currently owned by that connection.
+
+For KDBUS_CMD_MSG_RECV:
+
+  -EINVAL	Invalid flags or offset
+  -EAGAIN	No message found in the queue
+  -ENOMSG	No message of the requested priority found
+
+For KDBUS_CMD_MSG_CANCEL:
+
+  -EINVAL	Invalid flags
+  -ENOENT	Pending message with the supplied cookie not found
+
+For KDBUS_CMD_FREE:
+
+  -ENXIO	No pool slice found at given offset
+  -EINVAL	Invalid flags provided, the offset is valid, but the user is
+		not allowed to free the slice. This happens, for example, if
+		the offset was retrieved with KDBUS_RECV_PEEK.
+
+For KDBUS_CMD_NAME_ACQUIRE:
+
+  -EINVAL	Illegal command flags, illegal name provided, or an activator
+		tried to acquire a second name
+  -EPERM	Policy prohibited name ownership
+  -EALREADY	Connection already owns that name
+  -EEXIST	The name already exists and can not be taken over
+  -ECONNRESET	The connection was reset during the call
+
+For KDBUS_CMD_NAME_RELEASE:
+
+  -EINVAL	Invalid command flags, or invalid name provided
+  -ESRCH	Name is not found found in the registry
+  -EADDRINUSE	Name is owned by a different connection and can't be released
+
+For KDBUS_CMD_NAME_LIST:
+
+  -EINVAL	Invalid flags
+  -ENOBUFS	No available memory in the connection's pool.
+
+For KDBUS_CMD_CONN_INFO:
+
+  -EINVAL	Invalid flags, or neither an ID nor a name was provided,
+		or the name is invalid.
+  -ESRCH	Connection lookup by name failed
+  -ENXIO	No connection with the provided number connection ID found
+
+For KDBUS_CMD_CONN_UPDATE:
+
+  -EINVAL	Illegal flags or items
+  -EOPNOTSUPP	Operation not supported by connection.
+  -E2BIG	Too many policy items attached
+  -EINVAL	Wildcards submitted in policy entries, or illegal sequence
+		of policy items
+
+For KDBUS_CMD_ENDPOINT_UPDATE:
+
+  -E2BIG	Too many policy items attached
+  -EINVAL	Invalid flags, or wildcards submitted in policy entries,
+		or illegal sequence of policy items
+
+For KDBUS_CMD_MATCH_ADD:
+
+  -EINVAL	Illegal flags or items
+  -EDOM		Illegal bloom filter size
+  -EMFILE	Too many matches for this connection
+
+For KDBUS_CMD_MATCH_REMOVE:
+
+  -EINVAL	Illegal flags
+  -ENOENT	A match entry with the given cookie could not be found.
-- 
2.1.2

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