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systemd/README
Frantisek Sumsal 4e71714bca README: bump the gcc baseline to 8.4 
We already use __VA_OPT__ in multiple places, which was introduced in
gcc 8 [0], so let's bump the baseline to reflect that. I chose gcc 8.4,
as that was the lowest 8.x version I could easily get my hands on when I
verified this (on Ubuntu Focal with the gcc-8 package).

Closes: #31191

[0] https://gcc.gnu.org/gcc-8/changes.html
2024-02-05 10:45:10 +00:00

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systemd System and Service Manager
WEB SITE:
https://systemd.io
GIT:
git@github.com:systemd/systemd.git
https://github.com/systemd/systemd
MAILING LIST:
https://lists.freedesktop.org/mailman/listinfo/systemd-devel
IRC:
#systemd on irc.libera.chat
BUG REPORTS:
https://github.com/systemd/systemd/issues
OLDER DOCUMENTATION:
https://0pointer.de/blog/projects/systemd.html
https://www.freedesktop.org/wiki/Software/systemd
AUTHOR:
Lennart Poettering
Kay Sievers
...and many others
LICENSE:
LGPL-2.1-or-later for all code, exceptions noted in LICENSES/README.md
REQUIREMENTS:
Linux kernel ≥ 3.15
≥ 4.3 for ambient capabilities
≥ 4.5 for pids controller in cgroup v2
≥ 4.6 for cgroup namespaces
≥ 4.9 for RENAME_NOREPLACE support in vfat
≥ 4.10 for cgroup-bpf egress and ingress hooks
≥ 4.15 for cgroup-bpf device hook and cpu controller in cgroup v2
≥ 4.17 for cgroup-bpf socket address hooks
≥ 4.20 for PSI (used by systemd-oomd)
≥ 5.3 for bounded loops in BPF program
≥ 5.4 for signed Verity images
≥ 5.7 for BPF links and the BPF LSM hook
⛔ Kernel versions below 3.15 ("minimum baseline") are not supported at
all, and are missing required functionality (e.g. CLOCK_BOOTTIME
support for timerfd_create()).
⚠️ Kernel versions below 4.15 ("recommended baseline") have significant
gaps in functionality and are not recommended for use with this version
of systemd (e.g. lack sufficiently comprehensive and working cgroupv2
support). Taint flag 'old-kernel' will be set. systemd will most likely
still function, but upstream support and testing are limited.
Kernel Config Options:
CONFIG_DEVTMPFS
CONFIG_CGROUPS (it is OK to disable all controllers)
CONFIG_INOTIFY_USER
CONFIG_SIGNALFD
CONFIG_TIMERFD
CONFIG_EPOLL
CONFIG_UNIX (it requires CONFIG_NET, but every other flag in it is not necessary)
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= in service units:
CONFIG_NET_NS
Note that systemd-localed.service and other systemd units use
PrivateNetwork so this is effectively required.
Required for PrivateUsers= in service units:
CONFIG_USER_NS
Optional but strongly recommended:
CONFIG_IPV6
CONFIG_AUTOFS_FS
CONFIG_TMPFS_XATTR
CONFIG_{TMPFS,EXT4_FS,XFS,BTRFS_FS,...}_POSIX_ACL
CONFIG_SECCOMP
CONFIG_SECCOMP_FILTER (required for seccomp support)
CONFIG_KCMP (for the kcmp() syscall, used to be under
CONFIG_CHECKPOINT_RESTORE before ~5.12)
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
Required for IPAddressDeny=, IPAddressAllow=, IPIngressFilterPath=,
IPEgressFilterPath= in resource control unit settings unit settings:
CONFIG_BPF
CONFIG_BPF_SYSCALL
CONFIG_BPF_JIT
CONFIG_HAVE_EBPF_JIT
CONFIG_CGROUP_BPF
Required for SocketBind{Allow|Deny}=, RestrictNetworkInterfaces= in
resource control unit settings:
CONFIG_BPF
CONFIG_BPF_SYSCALL
CONFIG_BPF_JIT
CONFIG_HAVE_EBPF_JIT
CONFIG_CGROUP_BPF
For UEFI systems:
CONFIG_EFIVAR_FS
CONFIG_EFI_PARTITION
Required for signed Verity images support:
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG
Required to verify signed Verity images using keys enrolled in the MoK
(Machine-Owner Key) keyring:
CONFIG_DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING
CONFIG_IMA_ARCH_POLICY
CONFIG_INTEGRITY_MACHINE_KEYRING
Required for reading credentials from SMBIOS:
CONFIG_DMI
CONFIG_DMI_SYSFS
Required for RestrictFileSystems= in service units:
CONFIG_BPF
CONFIG_BPF_SYSCALL
CONFIG_BPF_LSM
CONFIG_DEBUG_INFO_BTF
CONFIG_LSM="...,bpf" or kernel booted with lsm="...,bpf".
We recommend to turn off Real-Time group scheduling in the kernel when
using systemd. RT group scheduling effectively makes RT scheduling
unavailable for most userspace, since it requires explicit assignment of
RT budgets to each unit whose processes making use of RT. As there's no
sensible way to assign these budgets automatically this cannot really be
fixed, and it's best to disable group scheduling hence:
CONFIG_RT_GROUP_SCHED=n
It's a good idea to disable the implicit creation of networking bonding
devices by the kernel networking bonding module, so that the
automatically created "bond0" interface doesn't conflict with any such
device created by systemd-networkd (or other tools). Ideally there would
be a kernel compile-time option for this, but there currently isn't. The
next best thing is to make this change through a modprobe.d drop-in.
This is shipped by default, see modprobe.d/systemd.conf.
Required for systemd-nspawn:
CONFIG_DEVPTS_MULTIPLE_INSTANCES or Linux kernel >= 4.7
Required for systemd-oomd:
CONFIG_PSI
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 >= 2.30 (from util-linux)
(util-linux *must* be built without --enable-libmount-support-mtab)
libseccomp >= 2.3.1 (optional)
libblkid >= 2.24 (from util-linux) (optional)
libkmod >= 15 (optional)
PAM >= 1.1.2 (optional)
libcryptsetup (optional), >= 2.3.0 required for signed Verity images support
libaudit (optional)
libacl (optional)
libbpf >= 0.1.0 (optional)
libfdisk >= 2.32 (from util-linux) (optional)
libselinux (optional)
liblzma (optional)
liblz4 >= 1.3.0 / 130 (optional)
libzstd >= 1.4.0 (optional)
libgcrypt (optional)
libqrencode (optional)
libmicrohttpd (optional)
libidn2 or libidn (optional)
gnutls >= 3.1.4 (optional, >= 3.6.0 is required to support DNS-over-TLS with gnutls)
openssl >= 1.1.0 (optional, required to support DNS-over-TLS with openssl)
elfutils >= 158 (optional)
polkit (optional)
tzdata >= 2014f (optional)
pkg-config
gperf
docbook-xsl (optional, required for documentation)
xsltproc (optional, required for documentation)
python >= 3.7 (required by meson too, >= 3.9 is required for ukify)
python-jinja2
python-pefile (optional, required for ukify)
python-lxml (optional, required to build the indices)
pyelftools (optional, required for systemd-boot)
meson >= 0.60.0
ninja
gcc >= 8.4
awk, sed, grep, and similar tools
clang >= 10.0, llvm >= 10.0 (optional, required to build BPF programs
from source code in C)
During runtime, you need the following additional
dependencies:
util-linux >= v2.27.1 required (including but not limited to: mount,
umount, swapon, swapoff, sulogin,
agetty, fsck)
dbus >= 1.4.0 (strictly speaking optional, but recommended)
NOTE: If using dbus < 1.9.18, you should override the default
policy directory (--with-dbuspolicydir=/etc/dbus-1/system.d).
polkit (optional)
To build in directory build/:
meson setup build/ && ninja -C build/
Any configuration options can be specified as -Darg=value... arguments
to meson. After the build directory is initially configured, meson will
refuse to run again, and options must be changed with:
meson configure -Darg=value build/
meson configure without any arguments will print out available options and
their current values.
Useful commands:
ninja -C build -v some/target
meson test -C build/
sudo meson install -C build/ --no-rebuild
DESTDIR=... meson install -C build/
A tarball can be created with:
v=250 && git archive --prefix=systemd-$v/ v$v | zstd >systemd-$v.tar.zstd
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.
nss-systemd must be enabled on systemd systems, as that's required for
DynamicUser= to work. Note that we ship services out-of-the-box that
make use of DynamicUser= now, hence enabling nss-systemd is not
optional.
Note that the build prefix for systemd must be /usr/. (Moreover, packages
systemd relies on — such as D-Bus — really should use the same prefix,
otherwise you are on your own.) Split-usr and unmerged-usr systems are no
longer supported, and moving everything under /usr/ is required. Systems
with a separate /usr/ partition must mount it before transitioning into it
(i.e.: from the initrd). For more information see:
https://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge
Additional packages are necessary to run some tests:
- nc (used by test/TEST-12-ISSUE-3171)
- python (test-udev which is installed is in python)
- python-pyparsing
- python-evdev (used by hwdb parsing tests)
- strace (used by test/test-functions)
- capsh (optional, used by test-execute)
POLICY FOR SUPPORT OF DISTRIBUTIONS AND ARCHITECTURES:
systemd main branch and latest major or stable releases are generally
expected to compile on current versions of popular distributions (at
least all non-EOL versions of Fedora, Debian unstable/testing/stable,
latest Ubuntu LTS and non-LTS releases, openSUSE Tumbleweed/Leap,
CentOS Stream 8 and 9, up-to-date Arch, etc.) We will generally
attempt to support also other non-EOL versions of various distros.
Features which would break compilation on slightly older distributions
will only be introduced if there are significant reasons for this
(i.e. supporting them interferes with development or requires too many
resources to support). In some cases backports of specific libraries or
tools might be required.
The policy is similar for architecture support. systemd is regularly
tested on popular architectures (currently amd64, i386, arm64, ppc64el,
and s390x), but should compile and work also on other architectures, for
which support has been added. systemd will emit warnings when
architecture-specific constants are not defined.
STATIC COMPILATION AND "STANDALONE" BINARIES:
systemd provides a public shared libraries libsystemd.so and
libudev.so. The latter is deprecated, and the sd-device APIs in
libsystemd should be used instead for new code. In addition, systemd is
built with a private shared library, libsystemd-shared-<suffix>.so,
that also includes the libsystemd code, and by default most systemd
binaries are linked to it. Using shared libraries saves disk space and
memory at runtime, because only one copy of the code is needed.
It is possible to build static versions of systemd public shared
libraries (via the configuration options '-Dstatic-libsystemd' and
'-Dstatic-libudev'). This allows the libsystemd and libudev code to be
linked statically into programs. Note that mixing & matching different
versions of libsystemd and systemd is generally not recommended, since
various of its APIs wrap internal state and protocols of systemd
(e.g. logind and udev databases), which are not considered
stable. Hence, using static libraries is not recommended since it
generally means that version of the static libsystemd linked into
applications and the host systemd are not in sync, and will thus create
compatibility problems.
In addition, it is possible to disable the use of
libsystemd-shared-<suffix>.so for various components (via the
configuration options '-Dlink-*-shared'). In this mode, the libsystemd
and libsystemd-shared code is linked statically into selected
binaries. This option is intended for systems where some of the
components are intended to be delivered independently of the main
systemd package. Finally, some binaries can be compiled in a second
version (via the configuration option '-Dstandalone-binaries'). The
version suffixed with ".standalone" has the libsystemd and
libsystemd-shared code linked statically. Those binaries are intended
as replacements to be used in limited installations where the full
systemd suite is not installed. Yet another option is to rebuild
systemd with a different '-Dshared-lib-tag' setting, allowing different
systemd binaries to be linked to instances of the private shared
library that can be installed in parallel.
Again: Using the default shared linking is recommended, building static
or "standalone" versions is not. Mixing versions of systemd components
that would normally be built and used together (in particular various
daemons and the manager) is not recommended: we do not test such
combinations upstream and cannot provide support. Distributors making
use of those options are responsible if things do not work as expected.
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, kvm, lp, render, 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-setup.service.
The journal remote daemon requires the "systemd-journal-remote" 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 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 coredump support requires the "systemd-coredump" system
user and group to exist.
GLIBC NSS:
systemd ships with four glibc 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.
nss-systemd enables resolution of users/group registered via the
User/Group Record Lookup API (https://systemd.io/USER_GROUP_API),
including all dynamically allocated service users. (See the
DynamicUser= setting in unit files.)
To make use of these NSS modules, please add them to the "hosts:",
"passwd:", "group:", "shadow:" and "gshadow:" lines in
/etc/nsswitch.conf.
The four modules should be used in the following order:
passwd: files systemd
group: files [SUCCESS=merge] systemd
shadow: files systemd
gshadow: files systemd
hosts: mymachines resolve [!UNAVAIL=return] files myhostname dns
SYSV INIT.D SCRIPTS:
When calling "systemctl enable/disable/is-enabled" on a unit which is a
SysV init.d script, it calls /usr/lib/systemd/systemd-sysv-install;
this needs to translate the action into the distribution specific
mechanism such as chkconfig or update-rc.d. Packagers need to provide
this script if you need this functionality (you don't if you disabled
SysV init support).
Please see src/systemctl/systemd-sysv-install.SKELETON for how this
needs to look like, and provide an implementation at the marked places.
WARNINGS and TAINT FLAGS:
systemd requires that the /run mount point exists. systemd also
requires that /var/run is a symlink to /run. Taint flag 'var-run-bad'
will be set when this condition is detected.
Systemd will also warn when the cgroup support is unavailable in the
kernel (taint flag 'cgroups-missing'), the system is using the old
cgroup hierarchy (taint flag 'cgroupsv1'), the hardware clock is
running in non-UTC mode (taint flag 'local-hwclock'), the kernel
overflow UID or GID are not 65534 (taint flags 'overflowuid-not-65534'
and 'overflowgid-not-65534'), the UID or GID range assigned to the
running systemd instance covers less than 0…65534 (taint flags
'short-uid-range' and 'short-gid-range').
Taint conditions are logged during boot, but may also be checked at any
time with:
busctl get-property org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager Tainted
See org.freedesktop.systemd1(5) for more information.
VALGRIND:
To run systemd under valgrind, compile systemd with the valgrind
development headers available (i.e. valgrind-devel or equivalent).
Otherwise, false positives will be triggered by code which violates
some rules but is actually safe. Note that valgrind generates nice
output only on exit(), hence on shutdown we don't execve()
systemd-shutdown.
STABLE BRANCHES AND BACKPORTS:
Stable branches with backported patches are available in the
systemd-stable repo at https://github.com/systemd/systemd-stable.
Stable branches are started for certain releases of systemd and named
after them, e.g. v238-stable. Stable branches are managed by
distribution maintainers on an as needed basis. See
https://www.freedesktop.org/wiki/Software/systemd/Backports for some
more information and examples.
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