kdbus is a kernel-level IPC implementation that aims for resemblance to
the the protocol layer with the existing userspace D-Bus daemon while
enabling some features that couldn't be implemented before in userspace.
The documentation in the first patch in this series explains the
protocol and the API details.
Full details on what has changed from the v2 submission are at the
bottom of this email.
Reasons why this should be done in the kernel, instead of userspace as
it is currently done today include the following:
- performance: fewer process context switches, fewer copies, fewer
syscalls, larger memory chunks via memfd. This is really important
for a whole class of userspace programs that are ported from other
operating systems that are run on tiny ARM systems that rely on
hundreds of thousands of messages passed at boot time, and at
"critical" times in their user interaction loops.
- security: the peers which communicate do not have to trust each other,
as the only trustworthy compoenent in the game is the kernel which
adds metadata and ensures that all data passed as payload is either
copied or sealed, so that the receiver can parse the data without
having to protect against changing memory while parsing buffers. Also,
all the data transfer is controlled by the kernel, so that LSMs can
track and control what is going on, without involving userspace.
Because of the LSM issue, security people are much happier with this
model than the current scheme of having to hook into dbus to mediate
things.
- more metadata can be attached to messages than in userspace
- semantics for apps with heavy data payloads (media apps, for instance)
with optinal priority message dequeuing, and global message ordering.
Some "crazy" people are playing with using kdbus for audio data in the
system. I'm not saying that this is the best model for this, but
until now, there wasn't any other way to do this without having to
create custom "busses", one for each application library.
- being in the kernle closes a lot of races which can't be fixed with
the current userspace solutions. For example, with kdbus, there is a
way a client can disconnect from a bus, but do so only if no further
messages present in its queue, which is crucial for implementing
race-free "exit-on-idle" services
- eavesdropping on the kernel level, so privileged users can hook into
the message stream without hacking support for that into their
userspace processes
- a number of smaller benefits: for example kdbus learned a way to peek
full messages without dequeing them, which is really useful for
logging metadata when handling bus-activation requests.
Of course, some of the bits above could be implemented in userspace
alone, for example with more sophisticated memory management APIs, but
this is usually done by losing out on the other details. For example,
for many of the memory management APIs, it's hard to not require the
communicating peers to fully trust each other. And we _really_ don't
want peers to have to trust each other.
Another benefit of having this in the kernel, rather than as a userspace
daemon, is that you can now easily use the bus from the initrd, or up to
the very end when the system shuts down. On current userspace D-Bus,
this is not really possible, as this requires passing the bus instance
around between initrd and the "real" system. Such a transition of all
fds also requires keeping full state of what has already been read from
the connection fds. kdbus makes this much simpler, as we can change the
ownership of the bus, just by passing one fd over from one part to the
other.
Regarding binder: binder and kdbus follow very different design
concepts. Binder implies the use of thread-pools to dispatch incoming
method calls. This is a very efficient scheme, and completely natural
in programming languages like Java. On most Linux programs, however,
there's a much stronger focus on central poll() loops that dispatch all
sources a program cares about. kdbus is much more usable in such
environments, as it doesn't enforce a threading model, and it is happy
with serialized dispatching. In fact, this major difference had an
effect on much of the design decisions: binder does not guarantee global
message ordering due to the parallel dispatching in the thread-pools,
but kdbus does. Moreover, there's also a difference in the way message
handling. In kdbus, every message is basically taken and dispatched as
one blob, while in binder, continious connections to other peers are
created, which are then used to send messages on. Hence, the models are
quite different, and they serve different needs. I believe that the
D-Bus/kdbus model is more compatible and friendly with how Linux
programs are usually implemented.
This can also be found in a git tree, the kdbus branch of char-misc.git at:
https://git.kernel.org/cgit/linux/kernel/git/gregkh/char-misc.git/
Changes since v2:
* Add FS_USERNS_MOUNT to the file system flags, so users can mount
their own kdbusfs instances without being root in the parent
user-ns. Spotted by Andy Lutomirski.
* Rewrite major parts of the metadata implementation to allow for
per-recipient namespace translations. For this, namespaces are
now not pinned by domains anymore. Instead, metadata is recorded
in kernel scope, and exported into the currently active namespaces
at the time of message installing.
* Split PID and TID from KDBUS_ITEM_CREDS into KDBUS_ITEM_PIDS.
The starttime is there to detect re-used PIDs, so move it to that
new item type as well. Consequently, introduce struct kdbus_pids
to accommodate the information. Requested by Andy Lutomirski.
* Add {e,s,fs}{u,g}id to KDBUS_ITEM_CREDS, so users have a way to
get more fine-grained credential information.
* Removed KDBUS_CMD_CANCEL. The interface was not usable from
threaded userspace implementation due to inherent races. Instead,
add an item type CANCEL_FD which can be used to pass a file
descriptor to the CMD_SEND ioctl. When the SEND is done
synchronously, it will get cancelled as soon as the passed
FD signals POLLIN.
* Dropped startttime from KDBUS_ITEM_PIDS
* Restrict names of custom endpoints to names with a "<uid>-" prefix,
just like we do for buses.
* Provide module-parameter "kdbus.attach_flags_mask" to specify the
a mask of metadata items that is applied on all exported items.
* Monitors are now entirely invisible (IOW, there won't be any
notification when they are created) and they don't need to install
filters for broadcast messages anymore.
* All information exposed via a connection's pool now also reports
the length in addition to the offset. That way, userspace
applications can mmap() only parts of the pool on demand.
* Due to the metadata rework, KDBUS_ITEM_PAYLOAD_OFF items now
describe the offset relative to the pool, where they used to be
relative to the message header.
* Added return_flags bitmask to all kdbus_cmd_* structs, so the
kernel can report details of the command processing. This is
mostly reserved for future extensions.
* Some fixes in kdbus.txt and tests, spotted by Harald Hoyer, Andy
Lutomirski, Michele Curti, Sergei Zviagintsev, Sheng Yong, Torstein
Husebø and Hristo Venev.
* Fixed compiler warnings in test-message by Michele Curti
* Unexpected items are now rejected with -EINVAL
* Split signal and broadcast handling. Unicast signals are now
supported, and messages have a new KDBUS_MSG_SIGNAL flag.
* KDBUS_CMD_MSG_SEND was renamed to KDBUS_CMD_SEND, and now takes
a struct kdbus_cmd_send instead of a kdbus_msg.
* KDBUS_CMD_MSG_RECV was renamed to KDBUS_CMD_RECV.
* Test case memory leak plugged, and various other cleanups and
fixes, by Rui Miguel Silva.
* Build fix for s390
* Test case fix for 32bit archs
* The test framework now supports mount, pid and user namespaces.
* The test framework learned a --tap command line parameter to
format its output in the "Test Anything Protocol". This format
is chosen by default when "make kselftest" is invoked.
* Fixed buses and custom endpoints name validation, reported by
Andy Lutomirski.
* copy_from_user() return code issue fixed, reported by
Dan Carpenter.
* Avoid signed int overflow on archs without atomic_sub
* Avoid variable size stack items. Fixes a sparse warning in queue.c.
* New test case for kernel notification quota
* Switched back to enums for the list of ioctls. This has advantages
for userspace code as gdb, for instance, is able to resolve the
numbers into names. Added features can easily be detected with
autotools, and new iotcls can get #defines as well. Having #defines
for the initial set of ioctls is uncecessary.
Daniel Mack (13):
kdbus: add documentation
kdbus: add header file
kdbus: add driver skeleton, ioctl entry points and utility functions
kdbus: add connection pool implementation
kdbus: add connection, queue handling and message validation code
kdbus: add node and filesystem implementation
kdbus: add code to gather metadata
kdbus: add code for notifications and matches
kdbus: add code for buses, domains and endpoints
kdbus: add name registry implementation
kdbus: add policy database implementation
kdbus: add Makefile, Kconfig and MAINTAINERS entry
kdbus: add selftests
Documentation/ioctl/ioctl-number.txt | 1 +
Documentation/kdbus.txt | 2107 +++++++++++++++++++++
MAINTAINERS | 12 +
include/uapi/linux/Kbuild | 1 +
include/uapi/linux/kdbus.h | 1049 ++++++++++
include/uapi/linux/magic.h | 2 +
init/Kconfig | 12 +
ipc/Makefile | 2 +-
ipc/kdbus/Makefile | 22 +
ipc/kdbus/bus.c | 553 ++++++
ipc/kdbus/bus.h | 103 +
ipc/kdbus/connection.c | 2004 ++++++++++++++++++++
ipc/kdbus/connection.h | 262 +++
ipc/kdbus/domain.c | 350 ++++
ipc/kdbus/domain.h | 84 +
ipc/kdbus/endpoint.c | 232 +++
ipc/kdbus/endpoint.h | 68 +
ipc/kdbus/fs.c | 519 +++++
ipc/kdbus/fs.h | 25 +
ipc/kdbus/handle.c | 1134 +++++++++++
ipc/kdbus/handle.h | 20 +
ipc/kdbus/item.c | 309 +++
ipc/kdbus/item.h | 57 +
ipc/kdbus/limits.h | 95 +
ipc/kdbus/main.c | 72 +
ipc/kdbus/match.c | 535 ++++++
ipc/kdbus/match.h | 32 +
ipc/kdbus/message.c | 598 ++++++
ipc/kdbus/message.h | 133 ++
ipc/kdbus/metadata.c | 1066 +++++++++++
ipc/kdbus/metadata.h | 52 +
ipc/kdbus/names.c | 891 +++++++++
ipc/kdbus/names.h | 82 +
ipc/kdbus/node.c | 910 +++++++++
ipc/kdbus/node.h | 87 +
ipc/kdbus/notify.c | 244 +++
ipc/kdbus/notify.h | 30 +
ipc/kdbus/policy.c | 481 +++++
ipc/kdbus/policy.h | 51 +
ipc/kdbus/pool.c | 784 ++++++++
ipc/kdbus/pool.h | 47 +
ipc/kdbus/queue.c | 505 +++++
ipc/kdbus/queue.h | 108 ++
ipc/kdbus/reply.c | 262 +++
ipc/kdbus/reply.h | 68 +
ipc/kdbus/util.c | 317 ++++
ipc/kdbus/util.h | 133 ++
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/kdbus/.gitignore | 11 +
tools/testing/selftests/kdbus/Makefile | 46 +
tools/testing/selftests/kdbus/kdbus-enum.c | 95 +
tools/testing/selftests/kdbus/kdbus-enum.h | 14 +
tools/testing/selftests/kdbus/kdbus-test.c | 920 +++++++++
tools/testing/selftests/kdbus/kdbus-test.h | 85 +
tools/testing/selftests/kdbus/kdbus-util.c | 1646 ++++++++++++++++
tools/testing/selftests/kdbus/kdbus-util.h | 216 +++
tools/testing/selftests/kdbus/test-activator.c | 319 ++++
tools/testing/selftests/kdbus/test-attach-flags.c | 751 ++++++++
tools/testing/selftests/kdbus/test-benchmark.c | 427 +++++
tools/testing/selftests/kdbus/test-bus.c | 174 ++
tools/testing/selftests/kdbus/test-chat.c | 123 ++
tools/testing/selftests/kdbus/test-connection.c | 611 ++++++
tools/testing/selftests/kdbus/test-daemon.c | 66 +
tools/testing/selftests/kdbus/test-endpoint.c | 344 ++++
tools/testing/selftests/kdbus/test-fd.c | 710 +++++++
tools/testing/selftests/kdbus/test-free.c | 36 +
tools/testing/selftests/kdbus/test-match.c | 442 +++++
tools/testing/selftests/kdbus/test-message.c | 658 +++++++
tools/testing/selftests/kdbus/test-metadata-ns.c | 507 +++++
tools/testing/selftests/kdbus/test-monitor.c | 158 ++
tools/testing/selftests/kdbus/test-names.c | 184 ++
tools/testing/selftests/kdbus/test-policy-ns.c | 633 +++++++
tools/testing/selftests/kdbus/test-policy-priv.c | 1270 +++++++++++++
tools/testing/selftests/kdbus/test-policy.c | 81 +
tools/testing/selftests/kdbus/test-race.c | 313 +++
tools/testing/selftests/kdbus/test-sync.c | 368 ++++
tools/testing/selftests/kdbus/test-timeout.c | 99 +
77 files changed, 27818 insertions(+), 1 deletion(-)
create mode 100644 Documentation/kdbus.txt
create mode 100644 include/uapi/linux/kdbus.h
create mode 100644 ipc/kdbus/Makefile
create mode 100644 ipc/kdbus/bus.c
create mode 100644 ipc/kdbus/bus.h
create mode 100644 ipc/kdbus/connection.c
create mode 100644 ipc/kdbus/connection.h
create mode 100644 ipc/kdbus/domain.c
create mode 100644 ipc/kdbus/domain.h
create mode 100644 ipc/kdbus/endpoint.c
create mode 100644 ipc/kdbus/endpoint.h
create mode 100644 ipc/kdbus/fs.c
create mode 100644 ipc/kdbus/fs.h
create mode 100644 ipc/kdbus/handle.c
create mode 100644 ipc/kdbus/handle.h
create mode 100644 ipc/kdbus/item.c
create mode 100644 ipc/kdbus/item.h
create mode 100644 ipc/kdbus/limits.h
create mode 100644 ipc/kdbus/main.c
create mode 100644 ipc/kdbus/match.c
create mode 100644 ipc/kdbus/match.h
create mode 100644 ipc/kdbus/message.c
create mode 100644 ipc/kdbus/message.h
create mode 100644 ipc/kdbus/metadata.c
create mode 100644 ipc/kdbus/metadata.h
create mode 100644 ipc/kdbus/names.c
create mode 100644 ipc/kdbus/names.h
create mode 100644 ipc/kdbus/node.c
create mode 100644 ipc/kdbus/node.h
create mode 100644 ipc/kdbus/notify.c
create mode 100644 ipc/kdbus/notify.h
create mode 100644 ipc/kdbus/policy.c
create mode 100644 ipc/kdbus/policy.h
create mode 100644 ipc/kdbus/pool.c
create mode 100644 ipc/kdbus/pool.h
create mode 100644 ipc/kdbus/queue.c
create mode 100644 ipc/kdbus/queue.h
create mode 100644 ipc/kdbus/reply.c
create mode 100644 ipc/kdbus/reply.h
create mode 100644 ipc/kdbus/util.c
create mode 100644 ipc/kdbus/util.h
create mode 100644 tools/testing/selftests/kdbus/.gitignore
create mode 100644 tools/testing/selftests/kdbus/Makefile
create mode 100644 tools/testing/selftests/kdbus/kdbus-enum.c
create mode 100644 tools/testing/selftests/kdbus/kdbus-enum.h
create mode 100644 tools/testing/selftests/kdbus/kdbus-test.c
create mode 100644 tools/testing/selftests/kdbus/kdbus-test.h
create mode 100644 tools/testing/selftests/kdbus/kdbus-util.c
create mode 100644 tools/testing/selftests/kdbus/kdbus-util.h
create mode 100644 tools/testing/selftests/kdbus/test-activator.c
create mode 100644 tools/testing/selftests/kdbus/test-attach-flags.c
create mode 100644 tools/testing/selftests/kdbus/test-benchmark.c
create mode 100644 tools/testing/selftests/kdbus/test-bus.c
create mode 100644 tools/testing/selftests/kdbus/test-chat.c
create mode 100644 tools/testing/selftests/kdbus/test-connection.c
create mode 100644 tools/testing/selftests/kdbus/test-daemon.c
create mode 100644 tools/testing/selftests/kdbus/test-endpoint.c
create mode 100644 tools/testing/selftests/kdbus/test-fd.c
create mode 100644 tools/testing/selftests/kdbus/test-free.c
create mode 100644 tools/testing/selftests/kdbus/test-match.c
create mode 100644 tools/testing/selftests/kdbus/test-message.c
create mode 100644 tools/testing/selftests/kdbus/test-metadata-ns.c
create mode 100644 tools/testing/selftests/kdbus/test-monitor.c
create mode 100644 tools/testing/selftests/kdbus/test-names.c
create mode 100644 tools/testing/selftests/kdbus/test-policy-ns.c
create mode 100644 tools/testing/selftests/kdbus/test-policy-priv.c
create mode 100644 tools/testing/selftests/kdbus/test-policy.c
create mode 100644 tools/testing/selftests/kdbus/test-race.c
create mode 100644 tools/testing/selftests/kdbus/test-sync.c
create mode 100644 tools/testing/selftests/kdbus/test-timeout.c
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