David Herrmann
98a4c30ba1
Whenever we provide a bus API that allows clients to create and manage
server-side objects, we need to provide a unique name for these objects.
There are two ways to provide them:
1) Let the server choose a name and return it as method reply.
2) Let the client pass its name of choice in the method arguments.
The first method is the easiest one to implement. However, it suffers from
a race condition: If a client creates an object asynchronously, it cannot
destroy that object until it received the method reply. It cannot know the
name of the new object, thus, it cannot destroy it. Furthermore, this
method enforces a round-trip. If the client _depends_ on the method call
to succeed (eg., it would close() the connection if it failed), the client
usually has no reason to wait for the method reply. Instead, the client
can immediately schedule further method calls on the newly created object
(in case the API guarantees in-order method-call handling).
The second method fixes both problems: The client passes an object name
with the method-call. The server uses it to create the object. Therefore,
the client can schedule object destruction even if the object-creation
hasn't finished, yet (again, requiring in-order method-call handling).
Furthermore, the client can schedule further method calls on the newly
created object, before the constructor returned.
There're two problems to solve, though:
1) Object names are usually defined via dbus object paths, which are
usually globally namespaced. Therefore, multiple clients must be able
to choose unique object names without interference.
2) If multiple libraries share the same bus connection, they must be
able to choose unique object names without interference.
The first problem is solved easily by prefixing a name with the
unique-bus-name of a connection. The server side must enforce this and
reject any other name.
The second problem is solved by providing unique suffixes from within
sd-bus. As long as sd-bus always returns a fresh new ID, if requested,
multiple libraries will never interfere. This implementation re-uses
bus->cookie as ID generator, which already provides unique IDs for each
bus connection.
This patch introduces two new helpers:
bus_path_encode_unique(sd_bus *bus,
const char *prefix,
const char *sender_id,
const char *external_id,
char **ret_path);
This creates a new object-path via the template
'/prefix/sender_id/external_id'. That is, it appends two new labels to
the given prefix. If 'sender_id' is NULL, it will use
bus->unique_name, if 'external_id' is NULL, it will allocate a fresh,
unique cookie from bus->cookie.
bus_path_decode_unique(const char *path,
const char *prefix,
char **ret_sender,
char **ret_external);
This reverses what bus_path_encode_unique() did. It parses 'path' from
the template '/prefix/sender/external' and returns both suffix-labels
in 'ret_sender' and 'ret_external'. In case the template does not
match, 0 is returned and both output arguments are set to NULL.
Otherwise, 1 is returned and the output arguments contain the decoded
labels.
Note: Client-side allocated IDs are inspired by the Wayland protocol
(which itself was inspired by X11). Wayland uses those IDs heavily
to avoid round-trips. Clients can create server-side objects and
send method calls without any round-trip and waiting for any object
IDs to be returned. But unlike Wayland, DBus uses gobally namespaced
object names. Therefore, we have to add the extra step by adding the
unique-name of the bus connection.
systemd System and Service Manager
DETAILS:
http://0pointer.de/blog/projects/systemd.html
WEB SITE:
http://www.freedesktop.org/wiki/Software/systemd
GIT:
git://anongit.freedesktop.org/systemd/systemd
ssh://git.freedesktop.org/git/systemd/systemd
GITWEB:
http://cgit.freedesktop.org/systemd/systemd
MAILING LIST:
http://lists.freedesktop.org/mailman/listinfo/systemd-devel
http://lists.freedesktop.org/mailman/listinfo/systemd-commits
IRC:
#systemd on irc.freenode.org
BUG REPORTS:
https://bugs.freedesktop.org/enter_bug.cgi?product=systemd
AUTHOR:
Lennart Poettering
Kay Sievers
...and many others
LICENSE:
LGPLv2.1+ for all code
- except src/shared/MurmurHash2.c which is Public Domain
- except src/shared/siphash24.c which is CC0 Public Domain
- except src/journal/lookup3.c which is Public Domain
- except src/udev/* which is (currently still) GPLv2, GPLv2+
REQUIREMENTS:
Linux kernel >= 3.7
Linux kernel >= 3.8 for Smack support
Kernel Config Options:
CONFIG_DEVTMPFS
CONFIG_CGROUPS (it is OK to disable all controllers)
CONFIG_INOTIFY_USER
CONFIG_SIGNALFD
CONFIG_TIMERFD
CONFIG_EPOLL
CONFIG_NET
CONFIG_SYSFS
CONFIG_PROC_FS
CONFIG_FHANDLE (libudev, mount and bind mount handling)
udev will fail to work with the legacy sysfs layout:
CONFIG_SYSFS_DEPRECATED=n
Legacy hotplug slows down the system and confuses udev:
CONFIG_UEVENT_HELPER_PATH=""
Userspace firmware loading is not supported and should
be disabled in the kernel:
CONFIG_FW_LOADER_USER_HELPER=n
Some udev rules and virtualization detection relies on it:
CONFIG_DMIID
Support for some SCSI devices serial number retrieval, to
create additional symlinks in /dev/disk/ and /dev/tape:
CONFIG_BLK_DEV_BSG
Required for PrivateNetwork and PrivateDevices in service units:
CONFIG_NET_NS
CONFIG_DEVPTS_MULTIPLE_INSTANCES
Note that systemd-localed.service and other systemd units use
PrivateNetwork and PrivateDevices so this is effectively required.
Optional but strongly recommended:
CONFIG_IPV6
CONFIG_AUTOFS4_FS
CONFIG_TMPFS_XATTR
CONFIG_{TMPFS,EXT4,XFS,BTRFS_FS,...}_POSIX_ACL
CONFIG_SECCOMP
Required for CPUShares in resource control unit settings
CONFIG_CGROUP_SCHED
CONFIG_FAIR_GROUP_SCHED
Required for CPUQuota in resource control unit settings
CONFIG_CFS_BANDWIDTH
For systemd-bootchart, several proc debug interfaces are required:
CONFIG_SCHEDSTATS
CONFIG_SCHED_DEBUG
For UEFI systems:
CONFIG_EFIVAR_FS
CONFIG_EFI_PARTITION
Note that kernel auditing is broken when used with systemd's
container code. When using systemd in conjunction with
containers, please make sure to either turn off auditing at
runtime using the kernel command line option "audit=0", or
turn it off at kernel compile time using:
CONFIG_AUDIT=n
If systemd is compiled with libseccomp support on
architectures which do not use socketcall() and where seccomp
is supported (this effectively means x86-64 and ARM, but
excludes 32-bit x86!), then nspawn will now install a
work-around seccomp filter that makes containers boot even
with audit being enabled. This works correctly only on kernels
3.14 and newer though. TL;DR: turn audit off, still.
glibc >= 2.16
libcap
libmount >= 2.20 (from util-linux)
libseccomp >= 1.0.0 (optional)
libblkid >= 2.24 (from util-linux) (optional)
libkmod >= 15 (optional)
PAM >= 1.1.2 (optional)
libcryptsetup (optional)
libaudit (optional)
libacl (optional)
libselinux (optional)
liblzma (optional)
liblz4 >= 119 (optional)
libgcrypt (optional)
libqrencode (optional)
libmicrohttpd (optional)
libpython (optional)
libidn (optional)
gobject-introspection > 1.40.0 (optional)
elfutils >= 158 (optional)
make, gcc, and similar tools
During runtime, you need the following additional
dependencies:
util-linux >= v2.25 required
dbus >= 1.4.0 (strictly speaking optional, but recommended)
dracut (optional)
PolicyKit (optional)
When building from git, you need the following additional
dependencies:
pkg-config
docbook-xsl
xsltproc
automake
autoconf
libtool
intltool
gperf
gtkdocize (optional)
python (optional)
python-lxml (optional, but required to build the indices)
sphinx (optional)
When systemd-hostnamed is used, it is strongly recommended to
install nss-myhostname to ensure that, in a world of
dynamically changing hostnames, the hostname stays resolvable
under all circumstances. In fact, systemd-hostnamed will warn
if nss-myhostname is not installed.
To build HTML documentation for python-systemd using sphinx,
please first install systemd (using 'make install'), and then
invoke sphinx-build with 'make sphinx-<target>', with <target>
being 'html' or 'latexpdf'. If using DESTDIR for installation,
pass the same DESTDIR to 'make sphinx-html' invocation.
USERS AND GROUPS:
Default udev rules use the following standard system group
names, which need to be resolvable by getgrnam() at any time,
even in the very early boot stages, where no other databases
and network are available:
audio, cdrom, dialout, disk, input, kmem, lp, tape, tty, video
During runtime, the journal daemon requires the
"systemd-journal" system group to exist. New journal files will
be readable by this group (but not writable), which may be used
to grant specific users read access. In addition, system
groups "wheel" and "adm" will be given read-only access to
journal files using systemd-tmpfiles.service.
The journal gateway daemon requires the
"systemd-journal-gateway" system user and group to
exist. During execution this network facing service will drop
privileges and assume this uid/gid for security reasons.
Similarly, the NTP daemon requires the "systemd-timesync" system
user and group to exist.
Similarly, the network management daemon requires the
"systemd-network" system user and group to exist.
Similarly, the name resolution daemon requires the
"systemd-resolve" system user and group to exist.
Similarly, the kdbus dbus1 proxy daemon requires the
"systemd-bus-proxy" system user and group to exist.
NSS:
systemd ships with three NSS modules:
nss-myhostname resolves the local hostname to locally
configured IP addresses, as well as "localhost" to
127.0.0.1/::1.
nss-resolve enables DNS resolution via the systemd-resolved
DNS/LLMNR caching stub resolver "systemd-resolved".
nss-mymachines enables resolution of all local containers
registered with machined to their respective IP addresses.
To make use of these NSS modules, please add them to the
"hosts: " line in /etc/nsswitch.conf. The "resolve" module
should replace the glibc "dns" module in this file.
The three modules should be used in the following order:
hosts: files mymachines resolve myhostname
WARNINGS:
systemd will warn you during boot if /etc/mtab is not a
symlink to /proc/mounts. Please ensure that /etc/mtab is a
proper symlink.
systemd will warn you during boot if /usr is on a different
file system than /. While in systemd itself very little will
break if /usr is on a separate partition, many of its
dependencies very likely will break sooner or later in one
form or another. For example, udev rules tend to refer to
binaries in /usr, binaries that link to libraries in /usr or
binaries that refer to data files in /usr. Since these
breakages are not always directly visible, systemd will warn
about this, since this kind of file system setup is not really
supported anymore by the basic set of Linux OS components.
systemd requires that the /run mount point exists. systemd also
requires that /var/run is a a symlink to /run.
For more information on this issue consult
http://freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
To run systemd under valgrind, compile with VALGRIND defined
(e.g. ./configure CPPFLAGS='... -DVALGRIND=1'). Otherwise,
false positives will be triggered by code which violates
some rules but is actually safe.
ENGINEERING AND CONSULTING SERVICES:
ENDOCODE <https://endocode.com/> offers professional
engineering and consulting services for systemd. Please
contact Chris Kühl <chris@endocode.com> for more information.