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
vocabulary. It knows no false negatives, but it does know false
positives.
-The bloom filter that needs to be included has the parameters m=512
-(bits in the filter), k=8 (nr of hash functions). The underlying hash
-function is SipHash-2-4. We calculate two hash values for an input
-strings, one with the hash key b9660bf0467047c18875c49c54b9bd15 (this
-is supposed to be read as a series of 16 hexadecimal formatted
-bytes), and one with the hash key
-aaa154a2e0714b39bfe1dd2e9fc54a3b. This results in two 64bit hash
-values, A and B. The 8 hash functions for the bloom filter require a 9
-bit output each (since m=512=2^9), to generate these we XOR combine
-the first 8 bit of A shifted to the left by 1, with the first 8 bit of
-B. Then, for the next hash function we use the second 8 bit pair, and
-so on.
+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 (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,
+ 63,fd,ae,be,cd,82,48,12,a1,6e,41,26,cb,fa,a0,c8,
+ 23,be,45,29,32,d2,46,2d,82,03,52,28,fe,37,17,f5,
+ 56,3b,bf,ee,5a,4f,43,39,af,aa,94,08,df,f0,fc,10,
+ 31,80,c8,73,c7,ea,46,d3,aa,25,75,0f,9e,4c,09,29,
+ 7d,f7,18,4b,7b,a4,44,d5,85,3c,06,e0,65,53,96,6d,
+ f2,77,e9,6f,93,b5,4e,71,9a,0c,34,88,39,25,bf,35
+
+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 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
+depleted it is recalculated with the next 16 byte hash key from the
+array above and the same input data.
For each message to send across the bus we populate the bloom filter
with all possible matchable strings. If a client then wants to
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
i.e. KDBUS_ITEM_NAME_ADD, KDBUS_ITEM_NAME_REMOVE,
KDBUS_ITEM_NAME_CHANGE, KDBUS_ITEM_ID_ADD, KDBUS_ITEM_ID_REMOVE and
fill them out. Note however, that you have some wildcards in this
-case, for example the .id field of KDBUS_ITEM_ADD/KDBUS_ITEM_REMOVE
+case, for example the .id field of KDBUS_ITEM_ID_ADD/KDBUS_ITEM_ID_REMOVE
structures may be set to 0 to match against any id addition/removal.
Note that dbus match strings do no map 1:1 to these ioctl() calls. In
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