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authorDaniel Mack <daniel@zonque.org>2015-02-26 20:06:38 (GMT)
committerMaciej Wereski <m.wereski@partner.samsung.com>2015-04-02 15:24:19 (GMT)
commit9d52feac44dccc20209d24c3d1f411c27b7cb92c (patch)
treef4531ca6c9415243ea4e60daea5e79a883cd8362
parent4c81a23b1f43cd1a662da67a8e437b0ea156241a (diff)
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kdbus: add walk-through user space example
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@zonque.org> Signed-off-by: David Herrmann <dh.herrmann@gmail.com> Signed-off-by: Djalal Harouni <tixxdz@opendz.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--samples/Makefile2
-rw-r--r--samples/kdbus/.gitignore1
-rw-r--r--samples/kdbus/Makefile10
-rw-r--r--samples/kdbus/kdbus-api.h114
-rw-r--r--samples/kdbus/kdbus-workers.c1326
5 files changed, 1452 insertions, 1 deletions
diff --git a/samples/Makefile b/samples/Makefile
index 1a60c62..cfbaad7 100644
--- a/samples/Makefile
+++ b/samples/Makefile
@@ -1,4 +1,4 @@
# Makefile for Linux samples code
obj-$(CONFIG_SAMPLES) += kobject/ kprobes/ trace_events/ \
- 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 0000000..ee07d98
--- /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 0000000..d009025
--- /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 0000000..5ed5907
--- /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 0000000..d1d8f7a
--- /dev/null
+++ b/samples/kdbus/kdbus-workers.c
@@ -0,0 +1,1326 @@
+/*
+ * Copyright (C) 2013-2015 David Herrmann <dh.herrmann@gmail.com>
+ *
+ * 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;
+}