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    Subjectkdbus: add documentation
    Date
    From: Daniel Mack <daniel@zonque.org>

    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@zonque.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    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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