[PATCH 13/14] kdbus: add walk-through user space example

[Greg Kroah-Hartman]

From: Daniel Mack <daniel@xxxxxxxxxx>

Provide a walk-through example that explains how to use the low-level
ioctl API that kdbus offers. This example is meant to be useful for
developers who want to gain a in-depth understanding of how the kdbus
API works by reading a well-documented real-world example.

This program computes prime-numbers based on the sieve of Eratosthenes.
The master sets up a shared memory region and spawns workers which clear
out the non-primes. The master reacts to keyboard input and to
client-requests to control what each worker does. Note that this is in
no way meant as efficient way to compute primes. It should only serve as
example how a master/worker concept can be implemented with kdbus used
as control messages.

The main process is called the 'master'. It creates a new, private bus
which will be used between the master and its workers to communicate.
The master then spawns a fixed number of workers. Whenever a worker dies
(detected via SIGCHLD), the master spawns a new worker. When done, the
master waits for all workers to exit, prints a status report and exits
itself.

The master process does *not* keep track of its workers. Instead, this
example implements a PULL model. That is, the master acquires a
well-known name on the bus which each worker uses to request tasks from
the master. If there are no more tasks, the master will return an empty
task-list, which casues a worker to exit immediately.

As tasks can be computationally expensive, we support cancellation.
Whenever the master process is interrupted, it will drop its well-known
name on the bus. This causes kdbus to broadcast a name-change
notification. The workers check for broadcast messages regularly and
will exit if they receive one.

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>
---
 samples/Makefile              |    3 +-
 samples/kdbus/.gitignore      |    1 +
 samples/kdbus/Makefile        |   10 +
 samples/kdbus/kdbus-api.h     |  114 ++++
 samples/kdbus/kdbus-workers.c | 1326 +++++++++++++++++++++++++++++++++++++++++
 5 files changed, 1453 insertions(+), 1 deletion(-)
 create mode 100644 samples/kdbus/.gitignore
 create mode 100644 samples/kdbus/Makefile
 create mode 100644 samples/kdbus/kdbus-api.h
 create mode 100644 samples/kdbus/kdbus-workers.c

diff --git a/samples/Makefile b/samples/Makefile
index f00257bcc5a7..f0ad51e5b342 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -1,4 +1,5 @@
 # Makefile for Linux samples code
 
 obj-$(CONFIG_SAMPLES)	+= kobject/ kprobes/ trace_events/ livepatch/ \
-			   hw_breakpoint/ kfifo/ kdb/ hidraw/ rpmsg/ seccomp/
+			   hw_breakpoint/ kfifo/ kdb/ kdbus/ hidraw/ rpmsg/ \
+			   seccomp/
diff --git a/samples/kdbus/.gitignore b/samples/kdbus/.gitignore
new file mode 100644
index 000000000000..ee07d9857086
--- /dev/null
+++ b/samples/kdbus/.gitignore
@@ -0,0 +1 @@
+kdbus-workers
diff --git a/samples/kdbus/Makefile b/samples/kdbus/Makefile
new file mode 100644
index 000000000000..d009025369f4
--- /dev/null
+++ b/samples/kdbus/Makefile
@@ -0,0 +1,10 @@
+# kbuild trick to avoid linker error. Can be omitted if a module is built.
+obj- := dummy.o
+
+hostprogs-y += kdbus-workers
+
+always := $(hostprogs-y)
+
+HOSTCFLAGS_kdbus-workers.o +=		\
+	-I$(objtree)/usr/include/	\
+	-I$(objtree)/include/uapi/
diff --git a/samples/kdbus/kdbus-api.h b/samples/kdbus/kdbus-api.h
new file mode 100644
index 000000000000..5ed5907c5cb4
--- /dev/null
+++ b/samples/kdbus/kdbus-api.h
@@ -0,0 +1,114 @@
+#ifndef KDBUS_API_H
+#define KDBUS_API_H
+
+#include <sys/ioctl.h>
+#include <linux/kdbus.h>
+
+#define KDBUS_ALIGN8(l) (((l) + 7) & ~7)
+#define KDBUS_ITEM_HEADER_SIZE offsetof(struct kdbus_item, data)
+#define KDBUS_ITEM_SIZE(s) KDBUS_ALIGN8((s) + KDBUS_ITEM_HEADER_SIZE)
+#define KDBUS_ITEM_NEXT(item) \
+	(typeof(item))(((uint8_t *)item) + KDBUS_ALIGN8((item)->size))
+#define KDBUS_FOREACH(iter, first, _size)				\
+	for (iter = (first);						\
+	     ((uint8_t *)(iter) < (uint8_t *)(first) + (_size)) &&	\
+	       ((uint8_t *)(iter) >= (uint8_t *)(first));		\
+	     iter = (void*)(((uint8_t *)iter) + KDBUS_ALIGN8((iter)->size)))
+
+static inline int kdbus_cmd_bus_make(int control_fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(control_fd, KDBUS_CMD_BUS_MAKE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_endpoint_make(int bus_fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(bus_fd, KDBUS_CMD_ENDPOINT_MAKE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_endpoint_update(int ep_fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(ep_fd, KDBUS_CMD_ENDPOINT_UPDATE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_hello(int bus_fd, struct kdbus_cmd_hello *cmd)
+{
+	int ret = ioctl(bus_fd, KDBUS_CMD_HELLO, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_update(int fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(fd, KDBUS_CMD_UPDATE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_byebye(int conn_fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_BYEBYE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_free(int conn_fd, struct kdbus_cmd_free *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_FREE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_conn_info(int conn_fd, struct kdbus_cmd_info *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_CONN_INFO, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_bus_creator_info(int conn_fd, struct kdbus_cmd_info *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_BUS_CREATOR_INFO, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_list(int fd, struct kdbus_cmd_list *cmd)
+{
+	int ret = ioctl(fd, KDBUS_CMD_LIST, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_send(int conn_fd, struct kdbus_cmd_send *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_SEND, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_recv(int conn_fd, struct kdbus_cmd_recv *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_RECV, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_name_acquire(int conn_fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_NAME_ACQUIRE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_name_release(int conn_fd, struct kdbus_cmd *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_NAME_RELEASE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_match_add(int conn_fd, struct kdbus_cmd_match *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_MATCH_ADD, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+static inline int kdbus_cmd_match_remove(int conn_fd, struct kdbus_cmd_match *cmd)
+{
+	int ret = ioctl(conn_fd, KDBUS_CMD_MATCH_REMOVE, cmd);
+	return (ret < 0) ? (errno > 0 ? -errno : -EINVAL) : 0;
+}
+
+#endif /* KDBUS_API_H */
diff --git a/samples/kdbus/kdbus-workers.c b/samples/kdbus/kdbus-workers.c
new file mode 100644
index 000000000000..d1d8f7a7697b
--- /dev/null
+++ b/samples/kdbus/kdbus-workers.c
@@ -0,0 +1,1326 @@
+/*
+ * Copyright (C) 2013-2015 David Herrmann <dh.herrmann@xxxxxxxxx>
+ *
+ * kdbus is free software; you can redistribute it and/or modify it under
+ * the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ */
+
+/*
+ * Example: Workers
+ * This program computes prime-numbers based on the sieve of Eratosthenes. The
+ * master sets up a shared memory region and spawns workers which clear out the
+ * non-primes. The master reacts to keyboard input and to client-requests to
+ * control what each worker does. Note that this is in no way meant as efficient
+ * way to compute primes. It should only serve as example how a master/worker
+ * concept can be implemented with kdbus used as control messages.
+ *
+ * The main process is called the 'master'. It creates a new, private bus which
+ * will be used between the master and its workers to communicate. The master
+ * then spawns a fixed number of workers. Whenever a worker dies (detected via
+ * SIGCHLD), the master spawns a new worker. When done, the master waits for all
+ * workers to exit, prints a status report and exits itself.
+ *
+ * The master process does *not* keep track of its workers. Instead, this
+ * example implements a PULL model. That is, the master acquires a well-known
+ * name on the bus which each worker uses to request tasks from the master. If
+ * there are no more tasks, the master will return an empty task-list, which
+ * casues a worker to exit immediately.
+ *
+ * As tasks can be computationally expensive, we support cancellation. Whenever
+ * the master process is interrupted, it will drop its well-known name on the
+ * bus. This causes kdbus to broadcast a name-change notification. The workers
+ * check for broadcast messages regularly and will exit if they receive one.
+ *
+ * This example exists of 4 objects:
+ *  * master: The master object contains the context of the master process. This
+ *            process manages the prime-context, spawns workers and assigns
+ *            prime-ranges to each worker to compute.
+ *            The master itself does not do any prime-computations itself.
+ *  * child:  The child object contains the context of a worker. It inherits the
+ *            prime context from its parent (the master) and then creates a new
+ *            bus context to request prime-ranges to compute.
+ *  * prime:  The "prime" object is used to abstract how we compute primes. When
+ *            allocated, it prepares a memory region to hold 1 bit for each
+ *            natural number up to a fixed maximum ('MAX_PRIMES').
+ *            The memory region is backed by a memfd which we share between
+ *            processes. Each worker now gets assigned a range of natural
+ *            numbers which it clears multiples of off the memory region. The
+ *            master process is responsible of distributing all natural numbers
+ *            up to the fixed maximum to its workers.
+ *  * bus:    The bus object is an abstraction of the kdbus API. It is pretty
+ *            straightfoward and only manages the connection-fd plus the
+ *            memory-mapped pool in a single object.
+ *
+ * This example is in reversed order, which should make it easier to read
+ * top-down, but requires some forward-declarations. Just ignore those.
+ */
+
+#include <ctype.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/memfd.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/mman.h>
+#include <sys/poll.h>
+#include <sys/signalfd.h>
+#include <sys/syscall.h>
+#include <sys/time.h>
+#include <sys/wait.h>
+#include <time.h>
+#include <unistd.h>
+#include "kdbus-api.h"
+
+/* FORWARD DECLARATIONS */
+
+#define POOL_SIZE (16 * 1024 * 1024)
+#define MAX_PRIMES (2UL << 24)
+#define WORKER_COUNT (16)
+#define PRIME_STEPS (65536 * 4)
+
+static const char *arg_busname = "example-workers";
+static const char *arg_modname = "kdbus";
+static const char *arg_master = "org.freedesktop.master";
+
+static int err_assert(int r_errno, const char *msg, const char *func, int line,
+		      const char *file)
+{
+	r_errno = (r_errno != 0) ? -abs(r_errno) : -EFAULT;
+	if (r_errno < 0) {
+		errno = -r_errno;
+		fprintf(stderr, "ERR: %s: %m (%s:%d in %s)\n",
+			msg, func, line, file);
+	}
+	return r_errno;
+}
+
+#define err_r(_r, _msg) err_assert((_r), (_msg), __func__, __LINE__, __FILE__)
+#define err(_msg) err_r(errno, (_msg))
+
+struct prime;
+struct bus;
+struct master;
+struct child;
+
+struct prime {
+	int fd;
+	uint8_t *area;
+	size_t max;
+	size_t done;
+	size_t status;
+};
+
+static int prime_new(struct prime **out);
+static void prime_free(struct prime *p);
+static bool prime_done(struct prime *p);
+static void prime_consume(struct prime *p, size_t amount);
+static int prime_run(struct prime *p, struct bus *cancel, size_t number);
+static void prime_print(struct prime *p);
+
+struct bus {
+	int fd;
+	uint8_t *pool;
+};
+
+static int bus_open_connection(struct bus **out, uid_t uid, const char *name,
+			       uint64_t recv_flags);
+static void bus_close_connection(struct bus *b);
+static void bus_poool_free_slice(struct bus *b, uint64_t offset);
+static int bus_acquire_name(struct bus *b, const char *name);
+static int bus_install_name_loss_match(struct bus *b, const char *name);
+static int bus_poll(struct bus *b);
+static int bus_make(uid_t uid, const char *name);
+
+struct master {
+	size_t n_workers;
+	size_t max_workers;
+
+	int signal_fd;
+	int control_fd;
+
+	struct prime *prime;
+	struct bus *bus;
+};
+
+static int master_new(struct master **out);
+static void master_free(struct master *m);
+static int master_run(struct master *m);
+static int master_poll(struct master *m);
+static int master_handle_stdin(struct master *m);
+static int master_handle_signal(struct master *m);
+static int master_handle_bus(struct master *m);
+static int master_reply(struct master *m, const struct kdbus_msg *msg);
+static int master_waitpid(struct master *m);
+static int master_spawn(struct master *m);
+
+struct child {
+	struct bus *bus;
+	struct prime *prime;
+};
+
+static int child_new(struct child **out, struct prime *p);
+static void child_free(struct child *c);
+static int child_run(struct child *c);
+
+/* END OF FORWARD DECLARATIONS */
+
+/*
+ * This is the main entrypoint of this example. It is pretty straightforward. We
+ * create a master object, run the computation, print a status report and then
+ * exit. Nothing particularly interesting here, so lets look into the master
+ * object...
+ */
+int main(int argc, char **argv)
+{
+	struct master *m = NULL;
+	int r;
+
+	r = master_new(&m);
+	if (r < 0)
+		goto out;
+
+	r = master_run(m);
+	if (r < 0)
+		goto out;
+
+	if (0)
+		prime_print(m->prime);
+
+out:
+	master_free(m);
+	if (r < 0 && r != -EINTR)
+		fprintf(stderr, "failed\n");
+	else
+		fprintf(stderr, "done\n");
+	return r < 0 ? EXIT_FAILURE : EXIT_SUCCESS;
+}
+
+/*
+ * ...this will allocate a new master context. It keeps track of the current
+ * number of children/workers that are running, manages a signalfd to track
+ * SIGCHLD, and creates a private kdbus bus. Afterwards, it opens its connection
+ * to the bus and acquires a well known-name (arg_master).
+ */
+static int master_new(struct master **out)
+{
+	struct master *m;
+	sigset_t smask;
+	int r;
+
+	m = calloc(1, sizeof(*m));
+	if (!m)
+		return err("cannot allocate master");
+
+	m->max_workers = WORKER_COUNT;
+	m->signal_fd = -1;
+	m->control_fd = -1;
+
+	/* Block SIGINT and SIGCHLD signals */
+	sigemptyset(&smask);
+	sigaddset(&smask, SIGINT);
+	sigaddset(&smask, SIGCHLD);
+	sigprocmask(SIG_BLOCK, &smask, NULL);
+
+	m->signal_fd = signalfd(-1, &smask, SFD_CLOEXEC);
+	if (m->signal_fd < 0) {
+		r = err("cannot create signalfd");
+		goto error;
+	}
+
+	r = prime_new(&m->prime);
+	if (r < 0)
+		goto error;
+
+	m->control_fd = bus_make(getuid(), arg_busname);
+	if (m->control_fd < 0) {
+		r = m->control_fd;
+		goto error;
+	}
+
+	/*
+	 * Open a bus connection for the master, and require each received
+	 * message to have a metadata item of type KDBUS_ITEM_PIDS attached.
+	 * The current UID is needed to compute the name of the bus node to
+	 * connect to.
+	 */
+	r = bus_open_connection(&m->bus, getuid(),
+				arg_busname, KDBUS_ATTACH_PIDS);
+	if (r < 0)
+		goto error;
+
+	/*
+	 * Acquire a well-known name on the bus, so children can address
+	 * messages to the master using KDBUS_DST_ID_NAME as destination-ID
+	 * of messages.
+	 */
+	r = bus_acquire_name(m->bus, arg_master);
+	if (r < 0)
+		goto error;
+
+	*out = m;
+	return 0;
+
+error:
+	master_free(m);
+	return r;
+}
+
+/* pretty straightforward destructor of a master object */
+static void master_free(struct master *m)
+{
+	if (!m)
+		return;
+
+	bus_close_connection(m->bus);
+	if (m->control_fd >= 0)
+		close(m->control_fd);
+	prime_free(m->prime);
+	if (m->signal_fd >= 0)
+		close(m->signal_fd);
+	free(m);
+}
+
+static int master_run(struct master *m)
+{
+	int res, r = 0;
+
+	while (!prime_done(m->prime)) {
+		while (m->n_workers < m->max_workers) {
+			r = master_spawn(m);
+			if (r < 0)
+				break;
+		}
+
+		r = master_poll(m);
+		if (r < 0)
+			break;
+	}
+
+	if (r < 0) {
+		bus_close_connection(m->bus);
+		m->bus = NULL;
+	}
+
+	while (m->n_workers > 0) {
+		res = master_poll(m);
+		if (res < 0) {
+			if (m->bus) {
+				bus_close_connection(m->bus);
+				m->bus = NULL;
+			}
+			r = res;
+		}
+	}
+
+	return r == -EINTR ? 0 : r;
+}
+
+static int master_poll(struct master *m)
+{
+	struct pollfd fds[3] = {};
+	int r = 0, n = 0;
+
+	/*
+	 * Add stdin, the eventfd and the connection owner file descriptor to
+	 * the pollfd table, and handle incoming traffic on the latter in
+	 * master_handle_bus().
+	 */
+	fds[n].fd = STDIN_FILENO;
+	fds[n++].events = POLLIN;
+	fds[n].fd = m->signal_fd;
+	fds[n++].events = POLLIN;
+	if (m->bus) {
+		fds[n].fd = m->bus->fd;
+		fds[n++].events = POLLIN;
+	}
+
+	r = poll(fds, n, -1);
+	if (r < 0)
+		return err("poll() failed");
+
+	if (fds[0].revents & POLLIN)
+		r = master_handle_stdin(m);
+	else if (fds[0].revents)
+		r = err("ERR/HUP on stdin");
+	if (r < 0)
+		return r;
+
+	if (fds[1].revents & POLLIN)
+		r = master_handle_signal(m);
+	else if (fds[1].revents)
+		r = err("ERR/HUP on signalfd");
+	if (r < 0)
+		return r;
+
+	if (fds[2].revents & POLLIN)
+		r = master_handle_bus(m);
+	else if (fds[2].revents)
+		r = err("ERR/HUP on bus");
+
+	return r;
+}
+
+static int master_handle_stdin(struct master *m)
+{
+	char buf[128];
+	ssize_t l;
+	int r = 0;
+
+	l = read(STDIN_FILENO, buf, sizeof(buf));
+	if (l < 0)
+		return err("cannot read stdin");
+	if (l == 0)
+		return err_r(-EINVAL, "EOF on stdin");
+
+	while (l-- > 0) {
+		switch (buf[l]) {
+		case 'q':
+			/* quit */
+			r = -EINTR;
+			break;
+		case '\n':
+		case ' ':
+			/* ignore */
+			break;
+		default:
+			if (isgraph(buf[l]))
+				fprintf(stderr, "invalid input '%c'\n", buf[l]);
+			else
+				fprintf(stderr, "invalid input 0x%x\n", buf[l]);
+			break;
+		}
+	}
+
+	return r;
+}
+
+static int master_handle_signal(struct master *m)
+{
+	struct signalfd_siginfo val;
+	ssize_t l;
+
+	l = read(m->signal_fd, &val, sizeof(val));
+	if (l < 0)
+		return err("cannot read signalfd");
+	if (l != sizeof(val))
+		return err_r(-EINVAL, "invalid data from signalfd");
+
+	switch (val.ssi_signo) {
+	case SIGCHLD:
+		return master_waitpid(m);
+	case SIGINT:
+		return err_r(-EINTR, "interrupted");
+	default:
+		return err_r(-EINVAL, "caught invalid signal");
+	}
+}
+
+static int master_handle_bus(struct master *m)
+{
+	struct kdbus_cmd_recv recv = { .size = sizeof(recv) };
+	const struct kdbus_msg *msg = NULL;
+	const struct kdbus_item *item;
+	const struct kdbus_vec *vec = NULL;
+	int r = 0;
+
+	/*
+	 * To receive a message, the KDBUS_CMD_RECV ioctl is used.
+	 * It takes an argument of type 'struct kdbus_cmd_recv', which
+	 * will contain information on the received message when the call
+	 * returns. See kdbus.message(7).
+	 */
+	r = kdbus_cmd_recv(m->bus->fd, &recv);
+	/*
+	 * EAGAIN is returned when there is no message waiting on this
+	 * connection. This is not an error - simply bail out.
+	 */
+	if (r == -EAGAIN)
+		return 0;
+	if (r < 0)
+		return err_r(r, "cannot receive message");
+
+	/*
+	 * Messages received by a connection are stored inside the connection's
+	 * pool, at an offset that has been returned in the 'recv' command
+	 * struct above. The value describes the relative offset from the
+	 * start address of the pool. A message is described with
+	 * 'struct kdbus_msg'. See kdbus.message(7).
+	 */
+	msg = (void *)(m->bus->pool + recv.msg.offset);
+
+	/*
+	 * A messages describes its actual payload in an array of items.
+	 * KDBUS_FOREACH() is a simple iterator that walks such an array.
+	 * struct kdbus_msg has a field to denote its total size, which is
+	 * needed to determine the number of items in the array.
+	 */
+	KDBUS_FOREACH(item, msg->items,
+		      msg->size - offsetof(struct kdbus_msg, items)) {
+		/*
+		 * An item of type PAYLOAD_OFF describes in-line memory
+		 * stored in the pool at a described offset. That offset is
+		 * relative to the start address of the message header.
+		 * This example program only expects one single item of that
+		 * type, remembers the struct kdbus_vec member of the item
+		 * when it sees it, and bails out if there is more than one
+		 * of them.
+		 */
+		if (item->type == KDBUS_ITEM_PAYLOAD_OFF) {
+			if (vec) {
+				r = err_r(-EEXIST,
+					  "message with multiple vecs");
+				break;
+			}
+			vec = &item->vec;
+			if (vec->size != 1) {
+				r = err_r(-EINVAL, "invalid message size");
+				break;
+			}
+
+		/*
+		 * MEMFDs are transported as items of type PAYLOAD_MEMFD.
+		 * If such an item is attached, a new file descriptor was
+		 * installed into the task when KDBUS_CMD_RECV was called, and
+		 * its number is stored in item->memfd.fd.
+		 * Implementers *must* handle this item type and close the
+		 * file descriptor when no longer needed in order to prevent
+		 * file descriptor exhaustion. This example program just bails
+		 * out with an error in this case, as memfds are not expected
+		 * in this context.
+		 */
+		} else if (item->type == KDBUS_ITEM_PAYLOAD_MEMFD) {
+			r = err_r(-EINVAL, "message with memfd");
+			break;
+		}
+	}
+	if (r < 0)
+		goto exit;
+	if (!vec) {
+		r = err_r(-EINVAL, "empty message");
+		goto exit;
+	}
+
+	switch (*((const uint8_t *)msg + vec->offset)) {
+	case 'r': {
+		r = master_reply(m, msg);
+		break;
+	}
+	default:
+		r = err_r(-EINVAL, "invalid message type");
+		break;
+	}
+
+exit:
+	/*
+	 * We are done with the memory slice that was given to us through
+	 * recv.msg.offset. Tell the kernel it can use it for other content
+	 * in the future. See kdbus.pool(7).
+	 */
+	bus_poool_free_slice(m->bus, recv.msg.offset);
+	return r;
+}
+
+static int master_reply(struct master *m, const struct kdbus_msg *msg)
+{
+	struct kdbus_cmd_send cmd;
+	struct kdbus_item *item;
+	struct kdbus_msg *reply;
+	size_t size, status, p[2];
+	int r;
+
+	/*
+	 * This functions sends a message over kdbus. To do this, it uses the
+	 * KDBUS_CMD_SEND ioctl, which takes a command struct argument of type
+	 * 'struct kdbus_cmd_send'. This struct stores a pointer to the actual
+	 * message to send. See kdbus.message(7).
+	 */
+	p[0] = m->prime->done;
+	p[1] = prime_done(m->prime) ? 0 : PRIME_STEPS;
+
+	size = sizeof(*reply);
+	size += KDBUS_ITEM_SIZE(sizeof(struct kdbus_vec));
+
+	/* Prepare the message to send */
+	reply = alloca(size);
+	memset(reply, 0, size);
+	reply->size = size;
+
+	/* Each message has a cookie that can be used to send replies */
+	reply->cookie = 1;
+
+	/* The payload_type is arbitrary, but it must be non-zero */
+	reply->payload_type = 0xdeadbeef;
+
+	/*
+	 * We are sending a reply. Let the kernel know the cookie of the
+	 * message we are replying to.
+	 */
+	reply->cookie_reply = msg->cookie;
+
+	/*
+	 * Messages can either be directed to a well-known name (stored as
+	 * string) or to a unique name (stored as number). This example does
+	 * the latter. If the message would be directed to a well-known name
+	 * instead, the message's dst_id field would be set to
+	 * KDBUS_DST_ID_NAME, and the name would be attaches in an item of type
+	 * KDBUS_ITEM_DST_NAME. See below for an example, and also refer to
+	 * kdbus.message(7).
+	 */
+	reply->dst_id = msg->src_id;
+
+	/* Our message has exactly one item to store its payload */
+	item = reply->items;
+	item->type = KDBUS_ITEM_PAYLOAD_VEC;
+	item->size = KDBUS_ITEM_HEADER_SIZE + sizeof(struct kdbus_vec);
+	item->vec.address = (uintptr_t)p;
+	item->vec.size = sizeof(p);
+
+	/*
+	 * Now prepare the command struct, and reference the message we want
+	 * to send.
+	 */
+	memset(&cmd, 0, sizeof(cmd));
+	cmd.size = sizeof(cmd);
+	cmd.msg_address = (uintptr_t)reply;
+
+	/*
+	 * Finally, employ the command on the connection owner
+	 * file descriptor.
+	 */
+	r = kdbus_cmd_send(m->bus->fd, &cmd);
+	if (r < 0)
+		return err_r(r, "cannot send reply");
+
+	if (p[1]) {
+		prime_consume(m->prime, p[1]);
+		status = m->prime->done * 10000 / m->prime->max;
+		if (status != m->prime->status) {
+			m->prime->status = status;
+			fprintf(stderr, "status: %7.3lf%%\n",
+				(double)status / 100);
+		}
+	}
+
+	return 0;
+}
+
+static int master_waitpid(struct master *m)
+{
+	pid_t pid;
+	int r;
+
+	while ((pid = waitpid(-1, &r, WNOHANG)) > 0) {
+		if (m->n_workers > 0)
+			--m->n_workers;
+		if (!WIFEXITED(r))
+			r = err_r(-EINVAL, "child died unexpectedly");
+		else if (WEXITSTATUS(r) != 0)
+			r = err_r(-WEXITSTATUS(r), "child failed");
+	}
+
+	return r;
+}
+
+static int master_spawn(struct master *m)
+{
+	struct child *c = NULL;
+	struct prime *p = NULL;
+	pid_t pid;
+	int r;
+
+	/* Spawn off one child and call child_run() inside it */
+
+	pid = fork();
+	if (pid < 0)
+		return err("cannot fork");
+	if (pid > 0) {
+		/* parent */
+		++m->n_workers;
+		return 0;
+	}
+
+	/* child */
+
+	p = m->prime;
+	m->prime = NULL;
+	master_free(m);
+
+	r = child_new(&c, p);
+	if (r < 0)
+		goto exit;
+
+	r = child_run(c);
+
+exit:
+	child_free(c);
+	exit(abs(r));
+}
+
+static int child_new(struct child **out, struct prime *p)
+{
+	struct child *c;
+	int r;
+
+	c = calloc(1, sizeof(*c));
+	if (!c)
+		return err("cannot allocate child");
+
+	c->prime = p;
+
+	/*
+	 * Open a connection to the bus and require each received message to
+	 * carry a list of the well-known names the sendind connection currently
+	 * owns. The current UID is needed in order to determine the name of the
+	 * bus node to connect to.
+	 */
+	r = bus_open_connection(&c->bus, getuid(),
+				arg_busname, KDBUS_ATTACH_NAMES);
+	if (r < 0)
+		goto error;
+
+	/*
+	 * Install a kdbus match so the child's connection gets notified when
+	 * the master loses its well-known name.
+	 */
+	r = bus_install_name_loss_match(c->bus, arg_master);
+	if (r < 0)
+		goto error;
+
+	*out = c;
+	return 0;
+
+error:
+	child_free(c);
+	return r;
+}
+
+static void child_free(struct child *c)
+{
+	if (!c)
+		return;
+
+	bus_close_connection(c->bus);
+	prime_free(c->prime);
+	free(c);
+}
+
+static int child_run(struct child *c)
+{
+	struct kdbus_cmd_send cmd;
+	struct kdbus_item *item;
+	struct kdbus_vec *vec = NULL;
+	struct kdbus_msg *msg;
+	struct timespec spec;
+	size_t n, steps, size;
+	int r = 0;
+
+	/*
+	 * Let's send a message to the master and ask for work. To do this,
+	 * we use the KDBUS_CMD_SEND ioctl, which takes an argument of type
+	 * 'struct kdbus_cmd_send'. This struct stores a pointer to the actual
+	 * message to send. See kdbus.message(7).
+	 */
+	size = sizeof(*msg);
+	size += KDBUS_ITEM_SIZE(strlen(arg_master) + 1);
+	size += KDBUS_ITEM_SIZE(sizeof(struct kdbus_vec));
+
+	msg = alloca(size);
+	memset(msg, 0, size);
+	msg->size = size;
+
+	/*
+	 * Tell the kernel that we expect a reply to this message. This means
+	 * that
+	 *
+	 * a) The remote peer will gain temporary permission to talk to us
+	 *    even if it would not be allowed to normally.
+	 *
+	 * b) A timeout value is required.
+	 *
+	 *    For asynchronous send commands, if no reply is received, we will
+	 *    get a kernel notification with an item of type
+	 *    KDBUS_ITEM_REPLY_TIMEOUT attached.
+	 *
+	 *    For synchronous send commands (which this example does), the
+	 *    ioctl will block until a reply is received or the timeout is
+	 *    exceeded.
+	 */
+	msg->flags = KDBUS_MSG_EXPECT_REPLY;
+
+	/* Set our cookie. Replies must use this cookie to send their reply. */
+	msg->cookie = 1;
+
+	/* The payload_type is arbitrary, but it must be non-zero */
+	msg->payload_type = 0xdeadbeef;
+
+	/*
+	 * We are sending our message to the current owner of a well-known
+	 * name. This makes an item of type KDBUS_ITEM_DST_NAME mandatory.
+	 */
+	msg->dst_id = KDBUS_DST_ID_NAME;
+
+	/*
+	 * Set the reply timeout to 5 seconds. Timeouts are always set in
+	 * absolute timestamps, based con CLOCK_MONOTONIC. See kdbus.message(7).
+	 */
+	clock_gettime(CLOCK_MONOTONIC_COARSE, &spec);
+	msg->timeout_ns += (5 + spec.tv_sec) * 1000ULL * 1000ULL * 1000ULL;
+	msg->timeout_ns += spec.tv_nsec;
+
+	/*
+	 * Fill the appended items. First, set the well-known name of the
+	 * destination we want to talk to.
+	 */
+	item = msg->items;
+	item->type = KDBUS_ITEM_DST_NAME;
+	item->size = KDBUS_ITEM_HEADER_SIZE + strlen(arg_master) + 1;
+	strcpy(item->str, arg_master);
+
+	/*
+	 * The 2nd item contains a vector to memory we want to send. It
+	 * can be content of any type. In our case, we're sending a one-byte
+	 * string only. The memory referenced by this item will be copied into
+	 * the pool of the receveiver connection, and does not need to be
+	 * valid after the command is employed.
+	 */
+	item = KDBUS_ITEM_NEXT(item);
+	item->type = KDBUS_ITEM_PAYLOAD_VEC;
+	item->size = KDBUS_ITEM_HEADER_SIZE + sizeof(struct kdbus_vec);
+	item->vec.address = (uintptr_t)"r";
+	item->vec.size = 1;
+
+	/* Set up the command struct and reference the message we prepared */
+	memset(&cmd, 0, sizeof(cmd));
+	cmd.size = sizeof(cmd);
+	cmd.msg_address = (uintptr_t)msg;
+
+	/*
+	 * The send commands knows a mode in which it will block until a
+	 * reply to a message is received. This example uses that mode.
+	 * The pool offset to the received reply will be stored in the command
+	 * struct after the send command returned. See below.
+	 */
+	cmd.flags = KDBUS_SEND_SYNC_REPLY;
+
+	/*
+	 * Finally, employ the command on the connection owner
+	 * file descriptor.
+	 */
+	r = kdbus_cmd_send(c->bus->fd, &cmd);
+	if (r == -ESRCH || r == -EPIPE || r == -ECONNRESET)
+		return 0;
+	if (r < 0)
+		return err_r(r, "cannot send request to master");
+
+	/*
+	 * The command was sent with the KDBUS_SEND_SYNC_REPLY flag set,
+	 * and returned successfully, which means that cmd.reply.offset now
+	 * points to a message inside our connection's pool where the reply
+	 * is found. This is equivalent to receiving the reply with
+	 * KDBUS_CMD_RECV, but it doesn't require waiting for the reply with
+	 * poll() and also saves the ioctl to receive the message.
+	 */
+	msg = (void *)(c->bus->pool + cmd.reply.offset);
+
+	/*
+	 * A messages describes its actual payload in an array of items.
+	 * KDBUS_FOREACH() is a simple iterator that walks such an array.
+	 * struct kdbus_msg has a field to denote its total size, which is
+	 * needed to determine the number of items in the array.
+	 */
+	KDBUS_FOREACH(item, msg->items,
+		      msg->size - offsetof(struct kdbus_msg, items)) {
+		/*
+		 * An item of type PAYLOAD_OFF describes in-line memory
+		 * stored in the pool at a described offset. That offset is
+		 * relative to the start address of the message header.
+		 * This example program only expects one single item of that
+		 * type, remembers the struct kdbus_vec member of the item
+		 * when it sees it, and bails out if there is more than one
+		 * of them.
+		 */
+		if (item->type == KDBUS_ITEM_PAYLOAD_OFF) {
+			if (vec) {
+				r = err_r(-EEXIST,
+					  "message with multiple vecs");
+				break;
+			}
+			vec = &item->vec;
+			if (vec->size != 2 * sizeof(size_t)) {
+				r = err_r(-EINVAL, "invalid message size");
+				break;
+			}
+		/*
+		 * MEMFDs are transported as items of type PAYLOAD_MEMFD.
+		 * If such an item is attached, a new file descriptor was
+		 * installed into the task when KDBUS_CMD_RECV was called, and
+		 * its number is stored in item->memfd.fd.
+		 * Implementers *must* handle this item type close the
+		 * file descriptor when no longer needed in order to prevent
+		 * file descriptor exhaustion. This example program just bails
+		 * out with an error in this case, as memfds are not expected
+		 * in this context.
+		 */
+		} else if (item->type == KDBUS_ITEM_PAYLOAD_MEMFD) {
+			r = err_r(-EINVAL, "message with memfd");
+			break;
+		}
+	}
+	if (r < 0)
+		goto exit;
+	if (!vec) {
+		r = err_r(-EINVAL, "empty message");
+		goto exit;
+	}
+
+	n = ((size_t *)((const uint8_t *)msg + vec->offset))[0];
+	steps = ((size_t *)((const uint8_t *)msg + vec->offset))[1];
+
+	while (steps-- > 0) {
+		++n;
+		r = prime_run(c->prime, c->bus, n);
+		if (r < 0)
+			break;
+		r = bus_poll(c->bus);
+		if (r != 0) {
+			r = r < 0 ? r : -EINTR;
+			break;
+		}
+	}
+
+exit:
+	/*
+	 * We are done with the memory slice that was given to us through
+	 * cmd.reply.offset. Tell the kernel it can use it for other content
+	 * in the future. See kdbus.pool(7).
+	 */
+	bus_poool_free_slice(c->bus, cmd.reply.offset);
+	return r;
+}
+
+/*
+ * Prime Computation
+ *
+ */
+
+static int prime_new(struct prime **out)
+{
+	struct prime *p;
+	int r;
+
+	p = calloc(1, sizeof(*p));
+	if (!p)
+		return err("cannot allocate prime memory");
+
+	p->fd = -1;
+	p->area = MAP_FAILED;
+	p->max = MAX_PRIMES;
+
+	/*
+	 * Prepare and map a memfd to store the bit-fields for the number
+	 * ranges we want to perform the prime detection on.
+	 */
+	p->fd = syscall(__NR_memfd_create, "prime-area", MFD_CLOEXEC);
+	if (p->fd < 0) {
+		r = err("cannot create memfd");
+		goto error;
+	}
+
+	r = ftruncate(p->fd, p->max / 8 + 1);
+	if (r < 0) {
+		r = err("cannot ftruncate area");
+		goto error;
+	}
+
+	p->area = mmap(NULL, p->max / 8 + 1, PROT_READ | PROT_WRITE,
+		       MAP_SHARED, p->fd, 0);
+	if (p->area == MAP_FAILED) {
+		r = err("cannot mmap memfd");
+		goto error;
+	}
+
+	*out = p;
+	return 0;
+
+error:
+	prime_free(p);
+	return r;
+}
+
+static void prime_free(struct prime *p)
+{
+	if (!p)
+		return;
+
+	if (p->area != MAP_FAILED)
+		munmap(p->area, p->max / 8 + 1);
+	if (p->fd >= 0)
+		close(p->fd);
+	free(p);
+}
+
+static bool prime_done(struct prime *p)
+{
+	return p->done >= p->max;
+}
+
+static void prime_consume(struct prime *p, size_t amount)
+{
+	p->done += amount;
+}
+
+static int prime_run(struct prime *p, struct bus *cancel, size_t number)
+{
+	size_t i, n = 0;
+	int r;
+
+	if (number < 2 || number > 65535)
+		return 0;
+
+	for (i = number * number;
+	     i < p->max && i > number;
+	     i += number) {
+		p->area[i / 8] |= 1 << (i % 8);
+
+		if (!(++n % (1 << 20))) {
+			r = bus_poll(cancel);
+			if (r != 0)
+				return r < 0 ? r : -EINTR;
+		}
+	}
+
+	return 0;
+}
+
+static void prime_print(struct prime *p)
+{
+	size_t i, l = 0;
+
+	fprintf(stderr, "PRIMES:");
+	for (i = 0; i < p->max; ++i) {
+		if (!(p->area[i / 8] & (1 << (i % 8))))
+			fprintf(stderr, "%c%7zu", !(l++ % 16) ? '\n' : ' ', i);
+	}
+	fprintf(stderr, "\nEND\n");
+}
+
+static int bus_open_connection(struct bus **out, uid_t uid, const char *name,
+			       uint64_t recv_flags)
+{
+	struct kdbus_cmd_hello hello;
+	char path[128];
+	struct bus *b;
+	int r;
+
+	/*
+	 * The 'bus' object is our representation of a kdbus connection which
+	 * stores two details: the connection owner file descriptor, and the
+	 * mmap()ed memory of its associated pool. See kdbus.connection(7) and
+	 * kdbus.pool(7).
+	 */
+	b = calloc(1, sizeof(*b));
+	if (!b)
+		return err("cannot allocate bus memory");
+
+	b->fd = -1;
+	b->pool = MAP_FAILED;
+
+	/* Compute the name of the bus node to connect to. */
+	snprintf(path, sizeof(path), "/sys/fs/%s/%lu-%s/bus",
+		 arg_modname, (unsigned long)uid, name);
+	b->fd = open(path, O_RDWR | O_CLOEXEC);
+	if (b->fd < 0) {
+		r = err("cannot open bus");
+		goto error;
+	}
+
+	/*
+	 * To make a connection to the bus, the KDBUS_CMD_HELLO ioctl is used.
+	 * It takes an argument of type 'struct kdbus_cmd_hello'.
+	 */
+	memset(&hello, 0, sizeof(hello));
+	hello.size = sizeof(hello);
+
+	/*
+	 * Specify a mask of metadata attach flags, describing metadata items
+	 * that this new connection allows to be sent.
+	 */
+	hello.attach_flags_send = _KDBUS_ATTACH_ALL;
+
+	/*
+	 * Specify a mask of metadata attach flags, describing metadata items
+	 * that this new connection wants to be receive along with each message.
+	 */
+	hello.attach_flags_recv = recv_flags;
+
+	/*
+	 * A connection may choose the size of its pool, but the number has to
+	 * comply with two rules: a) it must be greater than 0, and b) it must
+	 * be a mulitple of PAGE_SIZE. See kdbus.pool(7).
+	 */
+	hello.pool_size = POOL_SIZE;
+
+	/*
+	 * Now employ the command on the file descriptor opened above.
+	 * This command will turn the file descriptor into a connection-owner
+	 * file descriptor that controls the life-time of the connection; once
+	 * it's closed, the connection is shut down.
+	 */
+	r = kdbus_cmd_hello(b->fd, &hello);
+	if (r < 0) {
+		err_r(r, "HELLO failed");
+		goto error;
+	}
+
+	bus_poool_free_slice(b, hello.offset);
+
+	/*
+	 * Map the pool of the connection. Its size has been set in the
+	 * command struct above. See kdbus.pool(7).
+	 */
+	b->pool = mmap(NULL, POOL_SIZE, PROT_READ, MAP_SHARED, b->fd, 0);
+	if (b->pool == MAP_FAILED) {
+		r = err("cannot mmap pool");
+		goto error;
+	}
+
+	*out = b;
+	return 0;
+
+error:
+	bus_close_connection(b);
+	return r;
+}
+
+static void bus_close_connection(struct bus *b)
+{
+	if (!b)
+		return;
+
+	/*
+	 * A bus connection is closed by simply calling close() on the
+	 * connection owner file descriptor. The unique name and all owned
+	 * well-known names of the conneciton will disappear.
+	 * See kdbus.connection(7).
+	 */
+	if (b->pool != MAP_FAILED)
+		munmap(b->pool, POOL_SIZE);
+	if (b->fd >= 0)
+		close(b->fd);
+	free(b);
+}
+
+static void bus_poool_free_slice(struct bus *b, uint64_t offset)
+{
+	struct kdbus_cmd_free cmd = {
+		.size = sizeof(cmd),
+		.offset = offset,
+	};
+	int r;
+
+	/*
+	 * Once we're done with a piece of pool memory that was returned
+	 * by a command, we have to call the KDBUS_CMD_FREE ioctl on it so it
+	 * can be reused. The command takes an argument of type
+	 * 'struct kdbus_cmd_free', in which the pool offset of the slice to
+	 * free is stored. The ioctl is employed on the connection owner
+	 * file descriptor. See kdbus.pool(7),
+	 */
+	r = kdbus_cmd_free(b->fd, &cmd);
+	if (r < 0)
+		err_r(r, "cannot free pool slice");
+}
+
+static int bus_acquire_name(struct bus *b, const char *name)
+{
+	struct kdbus_item *item;
+	struct kdbus_cmd *cmd;
+	size_t size;
+	int r;
+
+	/*
+	 * This function acquires a well-known name on the bus through the
+	 * KDBUS_CMD_NAME_ACQUIRE ioctl. This ioctl takes an argument of type
+	 * 'struct kdbus_cmd', which is assembled below. See kdbus.name(7).
+	 */
+	size = sizeof(*cmd);
+	size += KDBUS_ITEM_SIZE(strlen(name) + 1);
+
+	cmd = alloca(size);
+	memset(cmd, 0, size);
+	cmd->size = size;
+
+	/*
+	 * The command requires an item of type KDBUS_ITEM_NAME, and its
+	 * content must be a valid bus name.
+	 */
+	item = cmd->items;
+	item->type = KDBUS_ITEM_NAME;
+	item->size = KDBUS_ITEM_HEADER_SIZE + strlen(name) + 1;
+	strcpy(item->str, name);
+
+	/*
+	 * Employ the command on the connection owner file descriptor.
+	 */
+	r = kdbus_cmd_name_acquire(b->fd, cmd);
+	if (r < 0)
+		return err_r(r, "cannot acquire name");
+
+	return 0;
+}
+
+static int bus_install_name_loss_match(struct bus *b, const char *name)
+{
+	struct kdbus_cmd_match *match;
+	struct kdbus_item *item;
+	size_t size;
+	int r;
+
+	/*
+	 * In order to install a match for signal messages, we have to
+	 * assemble a 'struct kdbus_cmd_match' and use it along with the
+	 * KDBUS_CMD_MATCH_ADD ioctl. See kdbus.match(7).
+	 */
+	size = sizeof(*match);
+	size += KDBUS_ITEM_SIZE(sizeof(item->name_change) + strlen(name) + 1);
+
+	match = alloca(size);
+	memset(match, 0, size);
+	match->size = size;
+
+	/*
+	 * A match is comprised of many 'rules', each of which describes a
+	 * mandatory detail of the message. All rules of a match must be
+	 * satified in order to make a message pass.
+	 */
+	item = match->items;
+
+	/*
+	 * In this case, we're interested in notifications that inform us
+	 * about a well-known name being removed from the bus.
+	 */
+	item->type = KDBUS_ITEM_NAME_REMOVE;
+	item->size = KDBUS_ITEM_HEADER_SIZE +
+			sizeof(item->name_change) + strlen(name) + 1;
+
+	/*
+	 * We could limit the match further and require a specific unique-ID
+	 * to be the new or the old owner of the name. In this case, however,
+	 * we don't, and allow 'any' id.
+	 */
+	item->name_change.old_id.id = KDBUS_MATCH_ID_ANY;
+	item->name_change.new_id.id = KDBUS_MATCH_ID_ANY;
+
+	/* Copy in the well-known name we're interested in */
+	strcpy(item->name_change.name, name);
+
+	/*
+	 * Add the match through the KDBUS_CMD_MATCH_ADD ioctl, employed on
+	 * the connection owner fd.
+	 */
+	r = kdbus_cmd_match_add(b->fd, match);
+	if (r < 0)
+		return err_r(r, "cannot add match");
+
+	return 0;
+}
+
+static int bus_poll(struct bus *b)
+{
+	struct pollfd fds[1] = {};
+	int r;
+
+	/*
+	 * A connection endpoint supports poll() and will wake-up the
+	 * task with POLLIN set once a message has arrived.
+	 */
+	fds[0].fd = b->fd;
+	fds[0].events = POLLIN;
+	r = poll(fds, sizeof(fds) / sizeof(*fds), 0);
+	if (r < 0)
+		return err("cannot poll bus");
+	return !!(fds[0].revents & POLLIN);
+}
+
+static int bus_make(uid_t uid, const char *name)
+{
+	struct kdbus_item *item;
+	struct kdbus_cmd *make;
+	char path[128], busname[128];
+	size_t size;
+	int r, fd;
+
+	/*
+	 * Compute the full path to the 'control' node. 'arg_modname' may be
+	 * set to a different value than 'kdbus' for development purposes.
+	 * The 'control' node is the primary entry point to kdbus that must be
+	 * used in order to create a bus. See kdbus(7) and kdbus.bus(7).
+	 */
+	snprintf(path, sizeof(path), "/sys/fs/%s/control", arg_modname);
+
+	/*
+	 * Compute the bus name. A valid bus name must always be prefixed with
+	 * the EUID of the currently running process in order to avoid name
+	 * conflicts. See kdbus.bus(7).
+	 */
+	snprintf(busname, sizeof(busname), "%lu-%s", (unsigned long)uid, name);
+
+	fd = open(path, O_RDWR | O_CLOEXEC);
+	if (fd < 0)
+		return err("cannot open control file");
+
+	/*
+	 * The KDBUS_CMD_BUS_MAKE ioctl takes an argument of type
+	 * 'struct kdbus_cmd', and expects at least two items attached to
+	 * it: one to decribe the bloom parameters to be propagated to
+	 * connections of the bus, and the name of the bus that was computed
+	 * above. Assemble this struct now, and fill it with values.
+	 */
+	size = sizeof(*make);
+	size += KDBUS_ITEM_SIZE(sizeof(struct kdbus_bloom_parameter));
+	size += KDBUS_ITEM_SIZE(strlen(busname) + 1);
+
+	make = alloca(size);
+	memset(make, 0, size);
+	make->size = size;
+
+	/*
+	 * Each item has a 'type' and 'size' field, and must be stored at an
+	 * 8-byte aligned address. The KDBUS_ITEM_NEXT macro is used to advance
+	 * the pointer. See kdbus.item(7) for more details.
+	 */
+	item = make->items;
+	item->type = KDBUS_ITEM_BLOOM_PARAMETER;
+	item->size = KDBUS_ITEM_HEADER_SIZE + sizeof(item->bloom_parameter);
+	item->bloom_parameter.size = 8;
+	item->bloom_parameter.n_hash = 1;
+
+	/* The name of the new bus is stored in the next item. */
+	item = KDBUS_ITEM_NEXT(item);
+	item->type = KDBUS_ITEM_MAKE_NAME;
+	item->size = KDBUS_ITEM_HEADER_SIZE + strlen(busname) + 1;
+	strcpy(item->str, busname);
+
+	/*
+	 * Now create the bus via the KDBUS_CMD_BUS_MAKE ioctl and return the
+	 * fd that was used back to the caller of this function. This fd is now
+	 * called a 'bus owner file descriptor', and it controls the life-time
+	 * of the newly created bus; once the file descriptor is closed, the
+	 * bus goes away, and all connections are shut down. See kdbus.bus(7).
+	 */
+	r = kdbus_cmd_bus_make(fd, make);
+	if (r < 0) {
+		err_r(r, "cannot make bus");
+		close(fd);
+		return r;
+	}
+
+	return fd;
+}
-- 
2.3.1

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