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 via ioctls on files exposed through a filesystem called 'kdbusfs'. The default mount point of kdbusfs is /sys/fs/kdbus. This patch adds detailed documentation about the kernel level API design. Signed-off-by: Daniel Mack <daniel@xxxxxxxxxx> Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxx> Signed-off-by: Djalal Harouni <tixxdz@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- Documentation/kdbus.txt | 2107 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 2107 insertions(+) create mode 100644 Documentation/kdbus.txt diff --git a/Documentation/kdbus.txt b/Documentation/kdbus.txt new file mode 100644 index 000000000000..2592a7e37079 --- /dev/null +++ b/Documentation/kdbus.txt @@ -0,0 +1,2107 @@ +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 exposes its functionality via files in a +filesystem called 'kdbusfs'. All communication between processes takes place +via ioctls on files exposed through the mount point of a kdbusfs. The default +mount point of kdbusfs is /sys/fs/kdbus. + +Please note that kdbus was designed as transport layer for D-Bus, but is in no +way limited, nor controlled by the D-Bus protocol specification. The D-Bus +protocol is one possible application layer on top of 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 created each time a kdbusfs is mounted. Each process that is + capable to mount a new instance of a kdbusfs will have its own kdbus + hierarchy. Each domain (ie, each mount point) offers its own "control" + file to create new buses. 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 endpoint of + a bus, where clients establish the connection to, is the "bus" file + /sys/fs/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 buses. See section 5 for more details. + + Endpoint: + An endpoint provides a file to talk to a bus. Opening an endpoint + creates a new connection to the bus to which the endpoint belongs. All + endpoints have unique names and are accessible as files underneath the + directory of a bus, e.g., /sys/fs/kdbus/<bus>/<endpoint> + Every bus has a default endpoint called "bus". A bus can optionally offer + additional endpoints with custom names to provide restricted access to the + bus. Custom endpoints carry additional policy which can be used to create + sandboxes with locked-down, limited, filtered access to a bus. See + section 5 for more details. + + Connection: + A connection to a bus is created by opening an endpoint file of a bus and + becoming an active client with the HELLO exchange. Every ordinary 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 commands 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 address space. + 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 connections or owners of + custom endpoints. See section 11 for more details. + 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. Control Files Layout +=============================================================================== + +The kdbus interface is exposed through files in its kdbusfs mount point +(defaults to /sys/fs/kdbus): + + /sys/fs/kdbus (mount point of kdbusfs) + |-- control (domain control-file) + |-- 0-system (bus of user uid=0) + | |-- bus (default endpoint of bus '0-system') + | `-- ep.apache (custom endpoint of bus '0-system') + |-- 1000-user (bus of user uid=1000) + | `-- bus (default endpoint of bus '1000-user') + `-- 2702-user (bus of user uid=2702) + |-- bus (default endpoint of bus '2702-user') + `-- ep.app (custom endpoint of bus '2702-user') + + +3. Data Structures and flags +=============================================================================== + +3.1 Data structures and interconnections +---------------------------------------- + + +--------------------------------------------------------------------------+ + | Domain (Mount Point) | + | /sys/fs/kdbus/control | + | +----------------------------------------------------------------------+ | + | | Bus (System Bus) | | + | | /sys/fs/kdbus/0-system/ | | + | | +-------------------------------+ +--------------------------------+ | | + | | | Endpoint | | Endpoint | | | + | | | /sys/fs/kdbus/0-system/bus | | /sys/fs/kdbus/0-system/ep.app | | | + | | +-------------------------------+ +--------------------------------+ | | + | | +--------------+ +--------------+ +--------------+ +---------------+ | | + | | | Connection | | Connection | | Connection | | Connection | | | + | | | :1.22 | | :1.25 | | :1.55 | | :1.81 | | | + | | +--------------+ +--------------+ +--------------+ +---------------+ | | + | +----------------------------------------------------------------------+ | + | | + | +----------------------------------------------------------------------+ | + | | Bus (User Bus for UID 2702) | | + | | /sys/fs/kdbus/2702-user/ | | + | | +-------------------------------+ +--------------------------------+ | | + | | | Endpoint | | Endpoint | | | + | | | /sys/fs/kdbus/2702-user/bus | | /sys/fs/kdbus/2702-user/ep.app | | | + | | +-------------------------------+ +--------------------------------+ | | + | | +--------------+ +--------------+ +--------------+ +---------------+ | | + | | | Connection | | Connection | | Connection | | Connection | | | + | | | :1.22 | | :1.25 | | :1.55 | | :1.81 | | | + | | +--------------+ +--------------+ +--------------------------------+ | | + | +----------------------------------------------------------------------+ | + +--------------------------------------------------------------------------+ + +The above description uses the D-Bus notation of unique connection names that +adds a ":1." prefix to the connection's unique ID. kdbus itself doesn't +use that notation, neither internally nor externally. However, libraries and +other userspace code that aims for compatibility to D-Bus might. + +3.2 Flags +--------- + +All ioctls used in the communication with the driver contain three 64-bit +fields: 'flags', 'kernel_flags' and 'return_flags'. All of them are specific +to the ioctl used. + +In 'flags', the behavior of the command can be tweaked. All bits that are not +recognized by the kernel in this field are rejected, and the ioctl fails with +-EINVAL. + +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 userspace code forward and backward +compatible. + +In 'return_flags', the kernel can return results of the command, in addition +to the actual return value. This is mostly to inform userspace about non-fatal +conditions that occurred during the execution of the command. + + +4. Items +=============================================================================== + +To flexibly augment transport structures, data blobs of type struct kdbus_item +can be attached to the structs passed into the ioctls. Some ioctls make items +of certain types mandatory, others are optional. Unsupported items will cause +the ioctl to fail -EINVAL. + +The total size of an item 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). + +Items are also used for information stored in a connection's pool, such as +received messages, name lists or requested connection or bus owner information. + +Whenever 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 multiple items +per ioctl. + +The kernel expects all items to be aligned to 8-byte boundaries. Unaligned +items or such that are unsupported by the ioctl are rejected. + +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 +=============================================================================== + + +5.1 Domains +----------- + +A domain is a container of buses. Domains themselves do not provide any IPC +functionality. Their sole purpose is to manage buses allocated in their +domain. Each time kdbusfs is mounted, a new kdbus domain is created, with its +own 'control' file. The lifetime of the domain ends once the user has unmounted +the kdbusfs. If you mount kdbusfs multiple times, each will have its own kdbus +domain internally. Operations performed on one domain do not affect any +other domain. + +The full kdbusfs hierarchy, any sub-directory, or file can be bind-mounted to +an external mount point and will remain fully functional. The kdbus domain and +any linked resources stay available until the original mount and all subsequent +bind-mounts have been unmounted. + +During creation, domains pin the user-namespace of the creator and use +it as controlling user-namespace for this domain. Any user accounting is done +relative to that user-namespace. + +Newly created kdbus domains do not have any bus pre-created. The only resource +available is a 'control' file, which is used to manage kdbus domains. +Currently, 'control' files are exclusively used to create buses via +KDBUS_CMD_BUS_MAKE, but further ioctls might be added in the future. + + +5.2 Buses +--------- + +A bus is a shared resource between connections to transmit messages. Each bus +is independent and operations on the bus will not have any effect on other +buses. A bus is a management entity, that controls the addresses of its +connections, their policies and message transactions performed via this bus. + +Each bus is bound to the domain it was created on. It has a custom name that is +unique across all buses of a domain. In kdbusfs, a bus is presented as a +directory. No operations can be performed on the bus itself, instead you need +to perform those on an endpoint associated with the bus. Endpoints are +accessible as files underneath the bus directory. A default endpoint called +"bus" is provided on each bus. + +Bus names may be chosen freely except for one restriction: the name +must be prefixed with the numeric UID of the creator and a dash. This +is required to avoid namespace clashes between different users. When +creating a bus the name must be passed in properly formatted, or the +kernel will refuse creation of the bus. Example: "1047-foobar" is an +OK name for a bus registered by a user with UID 1047. However, +"1024-foobar" is not, and neither is "foobar". +The UID must be provided in the user-namespace of the parent domain. + +To create a new bus, you need to open the control file of a domain and run the +KDBUS_CMD_BUS_MAKE ioctl. The control file descriptor that was used to issue +KDBUS_CMD_BUS_MAKE must not have been used for any other control-ioctl before +and needs to be kept open for the entire life-time of the created bus. Closing +it will immediately cleanup the entire bus and all its associated resources and +endpoints. Every control file descriptor can only be used to create a single +new bus; from that point on, 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 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 variant bits set to 'DCE', and denote +version 4 (random). + +When creating buses, a variable list of items that must be passed in +the items array is expected otherwise bus creation will fail. + + +5.3 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). +An endpoint is presented as file underneath the directory of the parent bus. + +To create a custom endpoint, open the default endpoint ('bus') and use the +KDBUS_CMD_ENDPOINT_MAKE ioctl with "struct kdbus_cmd_make". Custom endpoints +always have a policy database that, by default, forbids any operation. You have +to explicitly install policy entries to allow any operation on this endpoint. +Once KDBUS_CMD_ENDPOINT_MAKE succeeded, this file descriptor will manage the +newly created endpoint resource. It cannot be used to manage further resources. + +Endpoint names may be chosen freely except for one restriction: the name +must be prefixed with the numeric UID of the creator and a dash. This +is required to avoid namespace clashes between different users. When +creating an endpoint the name must be passed in properly formatted, or the +kernel will refuse creation of the endpoint. Example: "1047-foobar" is an +OK name for an endpoint registered by a user with UID 1047. However, +"1024-foobar" is not, and neither is "foobar". +The UID must be provided in the user-namespace of the parent domain. + +To create connections to a bus, you use KDBUS_CMD_HELLO. See section 6 for +details. Note that once KDBUS_CMD_HELLO succeeded, this file descriptor manages +the newly created connection resource. It cannot be used to manage further +resources. + + +5.4 Creating buses and endpoints +-------------------------------- + +KDBUS_CMD_BUS_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 bus or endpoint file group-accessible + + KDBUS_MAKE_ACCESS_WORLD + Make the bus or endpoint file world-accessible + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + struct kdbus_item items[0]; + A list of items that has specific meanings for KDBUS_CMD_BUS_MAKE + and KDBUS_CMD_ENDPOINT_MAKE (see above). + + Following items are expected for KDBUS_CMD_BUS_MAKE: + KDBUS_ITEM_MAKE_NAME + Contains a string to identify the bus name. + + KDBUS_ITEM_BLOOM_PARAMETER + Bus-wide bloom parameters passed in a dbus_bloom_parameter struct + + KDBUS_ITEM_ATTACH_FLAGS_RECV + An optional item that contains a set of required attach flags + that connections must allow. This item is used as a negotiation + measure during connection creation. If connections do not satisfy + the bus requirements, they are not allowed on the bus. + If not set, the bus does not require any metadata to be attached, + in this case connections are free to set their own attach flags. + + KDBUS_ITEM_ATTACH_FLAGS_SEND + An optional item that contains a set of attach flags that are + returned to connections when they query the bus creator metadata. + If not set, no metadata is returned. + + Unrecognized items are rejected, and the ioctl will fail with -EINVAL. +}; + + +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 file 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 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. See section 6.8 for + more information. + +To also receive broadcast messages, + the connection has to upload appropriate matches as well. + This flag is only valid for privileged bus connections. + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + __u64 attach_flags_send; + Set the bits for metadata this connection permits to be sent to the + receiving peer. Only metadata items that are both allowed to be sent by + the sender and that are requested by the receiver will effectively be + attached to the message eventually. Note, however, that the bus may + optionally enforce some of those bits to be set. If the match fails, + -ECONNREFUSED will be returned. In either case, this field will be set + to the mask of metadata items that are enforced by the bus. The + KDBUS_FLAGS_KERNEL bit will as well be set. + + __u64 attach_flags_recv; + 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. + + __u64 offset; + The kernel will return the offset in the pool where returned details + will be stored. + + __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_DESCRIPTION + 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). + + KDBUS_ITEM_CREDS + KDBUS_ITEM_PIDS + KDBUS_ITEM_SECLABEL + Privileged bus users may submit these types in order to create + connections with faked credentials. This information will be returned + when peer information is queried by KDBUS_CMD_CONN_INFO. See section + 13 for more information. + + Items of other types are rejected, and the ioctl will fail with -EINVAL. +}; + +At the offset returned in the 'offset' field of struct kdbus_cmd_hello, the +kernel will store items of the following types: + + KDBUS_ITEM_BLOOM_PARAMETER + Bloom filter parameter as defined by the bus creator (see below). + +The offset in the pool has to be freed with the KDBUS_CMD_FREE ioctl. + +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 implementer +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 metadata items should be attached to the answer. + See section 13 for more details. + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + __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. + + __u64 info_size; + The kernel will return the size of the returned information, so applications + can optionally mmap specific parts of the pool. + + struct kdbus_item items[0]; + The optional item list, containing the well-known name to look up as + a KDBUS_ITEM_OWNED_NAME. Only required if the 'id' field is set to 0. + Items of other types are rejected, and the ioctl will fail with -EINVAL. +}; + +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. + + struct kdbus_item items[0]; + Depending on the 'flags' field in struct kdbus_cmd_info, items of + types KDBUS_ITEM_OWNED_NAME and KDBUS_ITEM_CONN_DESCRIPTION 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. The item list contains + a KDBUS_ITEM_MAKE_NAME item that indicates the bus name of the + calling connection. +}; + +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. + + __u64 flags; + Currently no flags are supported. Reserved for future use. + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + struct kdbus_item items[0]; + Items to describe the connection details to be updated. The following item + types are supported: + + KDBUS_ITEM_ATTACH_FLAGS_SEND + Supply a new set of items that this connection permits to be sent along + with messages. + + KDBUS_ITEM_ATTACH_FLAGS_RECV + 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. + + Items of other types are rejected, and the ioctl will fail with -EINVAL. +}; + + +6.7 Termination +--------------- + +A connection can be terminated by simply closing its file descriptor. All +pending incoming messages will be discarded, and the memory in the pool will +be freed. + +An alternative 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. + + +6.8 Monitor connections ('eavesdropper') +---------------------------------------- + +Eavesdropping connections are created by setting the KDBUS_HELLO_MONITOR flag +in struct kdbus_hello.flags. Such connections have all properties of any other, +regular connection, except for the following details: + + * They will get every message sent over the bus, both unicasts and broadcasts + + * Installing matches for broadcast messages is neither necessary nor allowed + + * They cannot send messages or be directly addressed as receiver + + * Their creation and destruction will not cause KDBUS_ITEM_ID_{ADD,REMOVE} + notifications to be generated, so other connections cannot detect the + presence of an eavesdropper. + + +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_SEND ioctl. Depending +on 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_SEND ioctl uses struct kdbus_cmd_send to describe the message +transfer. + +struct kdbus_cmd_send { + __u64 size; + The overall size of the struct, including the attached items. + + __u64 flags; + Flags for message delivery: + + KDBUS_SEND_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_SYNC_REPLY controls. The KDBUS_CMD_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_RECV ioctl or anything else to receive + the reply. + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call of + KDBUS_CMD_SEND. + + __u64 kernel_msg_flags; + Valid bits for message flags, returned by the kernel upon each call of + KDBUS_CMD_SEND. + + __u64 return_flags; + Kernel-provided flags, returning non-fatal errors that occurred during + send. Currently unused. + + __u64 msg_address; + Userspace has to provide a pointer to a message (struct kdbus_msg) to send. + + struct kdbus_msg_info reply; + Only used for synchronous replies. See description of struct kdbus_cmd_recv + for more details. + + struct kdbus_item items[0]; + The following items are currently recognized: + + KDBUS_ITEM_CANCEL_FD + When this optional item is passed in, and the call is executed as SYNC + call, the passed in file descriptor can be used as alternative + cancellation point. The kernel will call poll() on this file descriptor, + and if it reports any incoming bytes, the blocking send operation will + be canceled, and the call will return -ECANCELED. Any type of file + descriptor that implements poll() can be used as payload to this item. + For asynchronous message sending, this item is accepted but ignored. + + All other items are rejected, and the ioctl will fail with -EINVAL. +}; + +The message referenced by 'msg_address' above has the following layout. + +struct kdbus_msg { + __u64 size; + The overall size of the struct, including the attached items. + + __u64 flags; + KDBUS_MSG_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_NO_AUTO_START + By default, when a message is sent to an activator connection, the + activator notified and will start an implementer. This flag inhibits + that behavior. With this bit set, and the remote being an activator, + -EADDRNOTAVAIL is returned from the ioctl. + + __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. + + 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 13.2 ("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 | | + | | ... | | + | +---------------------------------------------------------------------+ | + | ... further data records ... | + +-------------------------------------------------------------------------+ + + +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 memfds that have (F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_SEAL) set + 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. + +The sender must not make any assumptions on the type how data is received by the +remote peer. The kernel is free to re-pack multiple VEC and MEMFD payloads. For +instance, the kernel may decide to merge multiple VECs into a single VEC, inline +MEMFD payloads into memory or merge all passed VECs into a single MEMFD. +However, the kernel preserves the order of passed data. This means, the order of +all VEC and MEMFD items is not changed in respect to each other. + +In other words: All passed VEC and MEMFD data payloads are treated as a single +stream of data that may be received by the remote peer in a different set of +hunks than it was sent as. + + +7.4 Receiving messages +---------------------- + +Messages are received by the client with the KDBUS_CMD_RECV ioctl. The endpoint +file of the bus supports poll() to wake up the receiving process when new +messages are queued up to be received. + +With the KDBUS_CMD_RECV ioctl, a struct kdbus_cmd_recv is used. + +struct kdbus_cmd_recv { + __u64 size; + The overall size of the struct, including the attached items. + + __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. + + __u64 return_flags; + Kernel-provided flags, returning non-fatal errors that occurred during + send. Currently unused. + + __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 dropped_msgs; + If the CMD_RECV ioctl fails with EOVERFLOW, this field is filled by + the kernel with the number of messages that couldn't be transmitted to + this connection. In that case, the @offset member must not be accessed. + + struct kdbus_msg_info msg; + Embedded struct to be filled when the command succeeded (see below). + + struct kdbus_item items[0]; + Items to specify further details for the receive command. Currently unused. +}; + +Both 'struct kdbus_cmd_recv' and 'struct kdbus_cmd_send' embed 'struct +kdbus_msg_info'. For the SEND ioctl, it is used to catch synchronous replies, +if one was requested, and is unused otherwise. + +struct kdbus_msg_info { + __u64 offset; + Upon return of the ioctl, this field contains the offset in the receiver's + memory pool. The memory must be freed with KDBUS_CMD_FREE. + + __u64 msg_size; + Upon successful return of the ioctl, this field contains the size of the + allocated slice at offset @offset. It is the combination of the size of + the stored kdbus_msg object plus all appended VECs. You can use it in + combination with @offset to map a single message, instead of mapping the + whole pool. + + __u64 return_flags; + Kernel-provided return flags. Currently, the following flags are defined. + + KDBUS_RECV_RETURN_INCOMPLETE_FDS + The message contained file descriptors which couldn't be installed + into the receiver's task. Most probably that happened because the + maximum number of file descriptors for that task were exceeded. + The message is still delivered, so this is not a fatal condition. + File descriptors inside the KDBUS_ITEM_FDS item that could not be + installed will be set to -1. +}; + +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_RECV ioctl returns. +The receiving task is obliged to close all of them appropriately. If +KDBUS_RECV_PEEK is set, no file descriptors are installed. This allows for +peeking at a message and dropping it via KDBUS_RECV_DROP, without installing +the passed file descriptors into the receiving process. + +The caller is obliged to call KDBUS_CMD_FREE with the returned offset when +the memory is no longer needed. + + +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. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + struct kdbus_item items[0]; + Items to submit the name. Currently, one item of type KDBUS_ITEM_NAME is + expected and allowed, and the contained string must be a valid bus name. + Items of other types are rejected, and the ioctl will fail with -EINVAL. +}; + + +8.3 Releasing a name +-------------------- + +A connection may release a name explicitly with the KDBUS_CMD_NAME_RELEASE +ioctl. If the connection was an implementer 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; + Flags to the command. Currently unused. + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + struct kdbus_item items[0]; + Items to submit the name. Currently, 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 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + __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 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 item of type + KDBUS_ITEM_OWNED_NAME will be attached, including the name's flags. In that + item, the flags field of the name may carry the following bits: + + KDBUS_NAME_ALLOW_REPLACEMENT + Other connections are allowed to take over this name from the + connection that owns it. + + 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 implementer, + 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 + implementer 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. +}; + +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 an ordinary connection (not a monitor) 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. Eavesdroppers (monitor + connections) do not cause such notifications to be sent. They are invisible + on the bus. + + * When a connection gains or loses ownership of a name, 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 + +Kernel notifications have an item of type KDBUS_ITEM_TIMESTAMP 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. + + __u64 kernel_flags; + Valid flags for this command, returned by the kernel upon each call. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + 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. + + Items of other types are rejected, and the ioctl will fail with -EINVAL. +}; + + +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 signal 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 signals 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 +signals, with an item of type KDBUS_ITEM_BLOOM_FILTER. + +For a filter to match, all its bits have to be set in the 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 signal 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. + + __u64 return_flags; + Flags returned by the kernel. Currently unused. + + struct kdbus_item items[0]; + Unused und not allowed 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 wild card suffix +(".*"). That way, a policy can be uploaded that is effective for every +well-known 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 ID 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. + +For broadcast messages, the receiver needs TALK permissions to the sender to +receive the broadcast. + +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 that +override installed policies might be enforced. + +On default endpoints, the following set is enforced and checked before +any user-supplied policy is checked. + + * 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. + +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 the information stored in the +KDBUS_ITEM_BLOOM_PARAMETER item that is returned by KDBUS_CMD_HELLO. + +The pool is written to by the kernel when one of the following ioctls is issued: + + * KDBUS_CMD_HELLO, to receive details about the bus the connection was made to + * KDBUS_CMD_RECV, to receive a message + * KDBUS_CMD_NAME_LIST, to dump the name registry + * KDBUS_CMD_CONN_INFO, to retrieve information on a connection + +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 returned through the KDBUS_ITEM_BLOOM_PARAMETER + * item field when the KDBUS_CMD_HELLO ioctl returned. + */ + + buf = mmap(NULL, POOL_SIZE, PROT_READ, MAP_SHARED, conn_fd, 0); + +Alternatively, instead of mapping the entire pool buffer, only parts of it can +be mapped. The length of the response is returned by the kernel along with the +offset for each of the ioctls listed above. + + +13. Metadata +=============================================================================== + +kdbus records data about the system in certain situations. Such metadata can +refer to the currently active process (creds, PIDs, current user groups, process +names and its executable path, cgroup membership, capabilities, security label +and audit information), connection information (description string, currently +owned names) and the timestamp. + +Metadata is collected in the following circumstances: + + * When a bus is created (KDBUS_CMD_MAKE), information about the calling task + is collected. This data is returned by the kernel via the + KDBUS_CMD_BUS_CREATOR_INFO call- + + * When a connection is created (KDBUS_CMD_HELLO), information about the + calling task is collected. Alternatively, a privileged connection may + provide 'faked' information about credentials, PIDs and a security labels + which will be taken instead. This data is returned by the kernel as + information on a connection (KDBUS_CMD_CONN_INFO). + + * When a message is sent (KDBUS_CMD_SEND), information about the sending task + and the sending connection are collected. This metadata will be attached + to the message when it arrives in the receiver's pool. If the connection + sending the message installed faked credentials (see above), the message + will not be augmented by any information about the currently sending task. + +Which metadata items are actually delivered depends on the following sets and +masks: + + (a) the system-wide kmod creds mask (module parameter 'attach_flags_mask') + (b) the per-connection send creds mask, set by the connecting client + (c) the per-connection receive creds mask, set by the connecting client + (d) the per-bus minimal creds mask, set by the bus creator + (e) the per-bus owner creds mask, set by the bus creator + (f) the mask specified when querying creds of a bus peer + (g) the mask specified when querying creds of a bus owner + +With the following rules: + + [1] The creds attached to messages are determined as (a & b & c). + [2] When connecting to a bus (KDBUS_CMD_HELLO), and (~b & d) != 0, the call + will fail, the connection is refused. + [3] When querying creds of a bus peer, the creds returned are (a & b & f) + [4] When querying creds of a bus owner, the creds returned are (a & e & g) + [5] When creating a new bus, and (d & ~a) != 0, then bus creation will fail + +Hence, userspace might not always get all requested metadata items that it +requested. Code must be written so that it can cope with this fact. + + +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 struct kdbus_creds: the user and group IDs in the usual four + flavors: real, effective, saved and file-system related. + + KDBUS_ATTACH_PIDS + Attaches an item of type KDBUS_ITEM_PIDS, containing information on the + process. In particular, the PID (process ID), TID (thread ID), and PPID + (PID of the parent process). + + 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_OWNED_NAME, one for each name the sending + connection currently owns. The name and flags are stored in kdbus_item.name + for each of them. + + KDBUS_ATTACH_TID_COMM [*] + Attaches an items of type KDBUS_ITEM_TID_COMM, transporting the sending + task's 'comm', for the tid. The string is stored in kdbus_item.str. + + KDBUS_ATTACH_PID_COMM [*] + Attaches an items of type KDBUS_ITEM_PID_COMM, transporting the sending + task's 'comm', for the pid. The string is 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. SELinux and other MACs might want to + do additional per-service security checks. For example, a service manager + might want to check the security label of a service file against the + security label of the client process checking the SELinux database before + allowing access. The label can be accessed via kdbus_item->str. + + KDBUS_ATTACH_AUDIT + Attaches an item of type KDBUS_ITEM_AUDIT, which contains the audit + sessionid and loginuid of the sending task. Access via kdbus_item->audit. + + KDBUS_ATTACH_CONN_DESCRIPTION + Attaches an item of type KDBUS_ITEM_CONN_DESCRIPTION that contains a + descriptive string of the sending peer. That string can be supplied + during HELLO by attaching an item of type KDBUS_ITEM_CONN_DESCRIPTION. + + +[*] Note that the content stored in these items can easily be tampered by + the sending tasks. Therefore, they should NOT be used for any sort of + security relevant assumptions. The only reason why they are transmitted is + to let receivers know about details that were set when metadata was + collected, even though the task they were collected from is not active any + longer when the items are received. + + +13.2 Metadata and namespaces +---------------------------- + +Metadata such as PIDs, UIDs or GIDs are automatically translated to the +namespaces of the task that receives them. + + +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 + -EINVAL Unsupported item attached to command + +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_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 + -ECONNREFUSED The attach_flags_send field did not satisfy the requirements of + the bus + -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 + -EOPNOTSUPP The endpoint does not support the connection flags + supplied in the kdbus_cmd_hello struct + +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_SEND: + + -EOPNOTSUPP The connection is not an ordinary connection, or the passed + file descriptors are either kdbus handles or unix domain + sockets. Both are currently unsupported + -EINVAL The submitted payload type is KDBUS_PAYLOAD_KERNEL, + KDBUS_MSG_EXPECT_REPLY was set without timeout or cookie + values, KDBUS_MSG_SYNC_REPLY was set without + KDBUS_MSG_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 + null-terminated + -ENOTUNIQ The supplied destination is KDBUS_DST_ID_BROADCAST, a file + descriptor was passed, KDBUS_MSG_EXPECT_REPLY was set, + or a timeout was given for a broadcast message + -E2BIG Too many items + -EMSGSIZE The size of the message header and items or the payload vector + is too big. + -EEXIST Multiple KDBUS_ITEM_FDS, KDBUS_ITEM_BLOOM_FILTER or + KDBUS_ITEM_DST_NAME items were supplied + -EBADF The supplied KDBUS_ITEM_FDS or KDBUS_MSG_PAYLOAD_MEMFD items + contained an illegal file descriptor + -EMEDIUMTYPE The supplied memfd is not a sealed kdbus 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 The supplied kdbus memfd file cannot be sealed or the seal + was 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_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 + -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. + -EXFULL The memory pool of the receiver is full + -EREMOTEIO While synchronously waiting for a reply, the remote peer + failed with an I/O error. + +For KDBUS_CMD_RECV: + + -EINVAL Invalid flags or offset + -EAGAIN No message found in the queue + -ENOMSG No message of the requested priority found + -EOVERFLOW Broadcast messages have been lost + +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 + -E2BIG The maximum number of well-known names per connection + is exhausted + -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 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. + + +15. Internal object relations +=============================================================================== + +This is a simplified outline of the internal kdbus object relations, for +those interested in the inner life of the driver implementation. + +From the a mount point's (domain's) perspective: + +struct kdbus_domain + |» struct kdbus_domain_user *user (many, owned) + '» struct kdbus_node node (embedded) + |» struct kdbus_node children (many, referenced) + |» struct kdbus_node *parent (pinned) + '» struct kdbus_bus (many, pinned) + |» struct kdbus_node node (embedded) + '» struct kdbus_ep (many, pinned) + |» struct kdbus_node node (embedded) + |» struct kdbus_bus *bus (pinned) + |» struct kdbus_conn conn_list (many, pinned) + | |» struct kdbus_ep *ep (pinned) + | |» struct kdbus_name_entry *activator_of (owned) + | |» struct kdbus_match_db *match_db (owned) + | |» struct kdbus_meta *meta (owned) + | |» struct kdbus_match_db *match_db (owned) + | | '» struct kdbus_match_entry (many, owned) + | | + | |» struct kdbus_pool *pool (owned) + | | '» struct kdbus_pool_slice *slices (many, owned) + | | '» struct kdbus_pool *pool (pinned) + | | + | |» struct kdbus_domain_user *user (pinned) + | `» struct kdbus_queue_entry entries (many, embedded) + | |» struct kdbus_pool_slice *slice (pinned) + | |» struct kdbus_conn_reply *reply (owned) + | '» struct kdbus_domain_user *user (pinned) + | + '» struct kdbus_domain_user *user (pinned) + '» struct kdbus_policy_db policy_db (embedded) + |» struct kdbus_policy_db_entry (many, owned) + | |» struct kdbus_conn (pinned) + | '» struct kdbus_ep (pinned) + | + '» struct kdbus_policy_db_cache_entry (many, owned) + '» struct kdbus_conn (pinned) + + +For the life-time of a file descriptor derived from calling open() on a file +inside the mount point: + +struct kdbus_handle + |» struct kdbus_meta *meta (owned) + |» struct kdbus_ep *ep (pinned) + |» struct kdbus_conn *conn (owned) + '» struct kdbus_ep *ep (owned) -- 2.2.1 -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
