system, the lower 32bit indicate "compatible" feature additions. A
client that does not support a "compatible" feature addition can go on
communicating with the bus, however a client that does not support an
-"incompatible" feature must not proceed with the connection.
+"incompatible" feature must not proceed with the connection. When a
+client encountes such an "incompatible" feature it should immediately
+try the next bus address configured in the bus address string.
The hello structure also contains another flags field "attach_flags"
which indicates metadata that is optionally attached to all incoming
simple numeric value. For compatibility with classic dbus1 simply
format this as string and prefix ":0.".
-The kernel will also return the bloom filter size used for the signal
-broadcast bloom filter (see below).
+The kernel will also return the bloom filter size and bloom filter
+hash function number used for the signal broadcast bloom filter (see
+below).
The kernel will also return the bus ID of the bus in a 128bit field.
This pretty much explains the ioctl header. The actual payload of the
data is now referenced in additional items that are attached to this
ioctl header structure at the end. When sending a message, you attach
-items of the type PAYLOAD_VEC, PAYLOAD_MEMFD, FDS, BLOOM, DST_NAME to
-it:
+items of the type PAYLOAD_VEC, PAYLOAD_MEMFD, FDS, BLOOM_FILTER,
+DST_NAME to it:
KDBUS_ITEM_PAYLOAD_VEC: contains a pointer + length pair for
referencing arbitrary user memory. This is how you reference most
to send prepared "memfds" (see below) over. This item contains an
fd for a memfd plus a size.
- KDBUS_ITEM_PAYLOAD_FDS: for sending over fds attach an item of this
- type with an array of fds.
+ KDBUS_ITEM_FDS: for sending over fds attach an item of this type with
+ an array of fds.
- KDBUS_ITEM_BLOOM: the calculated bloom filter of this message, only
- for undirected (broadcast) message.
+ KDBUS_ITEM_BLOOM_FILTER: the calculated bloom filter of this message,
+ only for undirected (broadcast) message.
- KDBUS_DST_NAME: for messages that are directed to a well-known name
- (instead of a unique name), this item contains the well-known name
- field.
+ KDBUS_ITEM_DST_NAME: for messages that are directed to a well-known
+ name (instead of a unique name), this item contains the well-known
+ name field.
A single message may consists of no, one or more payload items of type
PAYLOAD_VEC or PAYLOAD_MEMFD. D-Bus protocol implementations should
The parameters for the bloom filters that need to be included in
broadcast message is communicated to userspace as part of the hello
-response structure. By default it has the parameters m=512 (bits in
-the filter), k=8 (nr of hash functions). Note however, that this is
-subject to change later on, and userspace implementations must be
-capable of handling m values between at least m=8 and m=2^32, and k
-values between at least k=1 and k=32. The underlying hash function is
-SipHash-2-4. It is used with a number of constant (yet originally
-randomly generated) 128bit hash keys, more specifically:
+response structure (see above). By default it has the parameters m=512
+(bits in the filter), k=8 (nr of hash functions). Note however, that
+this is subject to change in later versions, and userspace
+implementations must be capable of handling m values between at least
+m=8 and m=2^32, and k values between at least k=1 and k=32. The
+underlying hash function is SipHash-2-4. It is used with a number of
+constant (yet originally randomly generated) 128bit hash keys, more
+specifically:
b9,66,0b,f0,46,70,47,c1,88,75,c4,9c,54,b9,bd,15,
aa,a1,54,a2,e0,71,4b,39,bf,e1,dd,2e,9f,c5,4a,3b,
When calculating the first bit index into the bloom filter, the
SipHash-2-4 hash value is calculated for the input data and the first
16 bytes of the array above as hash key. Of the resulting 8 bytes of
-output, as many full bytes are taken for the index as necessary,
+output, as many full bytes are taken for the bit index as necessary,
starting from the output's first byte. For the second bit index the
same hash value is used, continuing with the next unused output byte,
and so on. Each time the bytes returned by the hash function are
relatively unrestricted on kdbus and people can send you all kinds of
non-sense).
+If a client connects to a bus whose bloom filter metrics (i.e. filter
+size and number of hash functions) are outside of the range the client
+supports it must immediately disconnect and continue connection with
+the next bus address of the bus connection string.
+
INSTALLING MATCHES
To install matches for broadcast messages, use the KDBUS_CMD_ADD_MATCH
different set of credentials to gather, than what was originally requested
for being attached to incoming messages.
-Credentials are always specific to the sender namespace that was
+Credentials are always specific to the sender's domain that was
current at the time of sending, and of the process that opened the
bus connection at the time of opening it. Note that this latter data
is cached!
Client libraries should use the following connection string when
connecting to the system bus:
- kernel:path=/dev/kdbus/0-system/bus;unix:path=/run/dbus/system_bus_socket
+ kernel:path=/dev/kdbus/0-system/bus;unix:path=/var/run/dbus/system_bus_socket
This will ensure that kdbus is preferred over the legacy AF_UNIX
socket, but compatibility is kept. For the user bus use:
~ianmdlvl / elogind.git / blobdiff