8.74. Udev from Systemd-254

The Udev package contains programs for dynamic creation of device nodes.

Approximate build time: 0.2 SBU
Required disk space: 138 MB

8.74.1. Installation of Udev

Udev is part of the systemd-254 package. Use the systemd-254.tar.xz file as the source tarball.

Remove two unneeded groups, render and sgx, from the default udev rules:

sed -i -e 's/GROUP="render"/GROUP="video"/' \
       -e 's/GROUP="sgx", //' rules.d/50-udev-default.rules.in

Remove one udev rule requiring a full Systemd installation:

sed '/systemd-sysctl/s/^/#/' -i rules.d/99-systemd.rules.in

Prepare Udev for compilation:

mkdir -p build
cd       build

meson setup \
      --prefix=/usr                 \
      --buildtype=release           \
      -Dmode=release                \
      -Ddev-kvm-mode=0660           \
      -Dlink-udev-shared=false      \
      ..

The meaning of the meson options:

--buildtype=release

This switch overrides the default buildtype (debug), which produces unoptimized binaries.

-Dmode=release

Disable some features considered experimental by upstream.

-Ddev-kvm-mode=0660

The default udev rule would allow all users to access /dev/kvm. The editors consider it dangerous. This option overrides it.

-Dlink-udev-shared=false

This option prevents udev from linking to the internal systemd shared library, libsystemd-shared. This library is designed to be shared by many Systemd components and it's too overkill for a udev-only installation.

Only build the components needed for udev:

ninja udevadm systemd-hwdb \
      $(grep -o -E "^build (src/libudev|src/udev|rules.d|hwdb.d)[^:]*" \
        build.ninja | awk '{ print $2 }')                              \
      $(realpath libudev.so --relative-to .)

Remove one udev rule file requiring a full Systemd installation:

rm rules.d/90-vconsole.rules

Install the package:

install -vm755 -d {/usr/lib,/etc}/udev/{hwdb,rules}.d
install -vm755 -d /usr/{lib,share}/pkgconfig
install -vm755 udevadm                     /usr/bin/
install -vm755 systemd-hwdb                /usr/bin/udev-hwdb
ln      -svfn  ../bin/udevadm              /usr/sbin/udevd
cp      -av    libudev.so{,*[0-9]}         /usr/lib/
install -vm644 ../src/libudev/libudev.h    /usr/include/
install -vm644 src/libudev/*.pc            /usr/lib/pkgconfig/
install -vm644 src/udev/*.pc               /usr/share/pkgconfig/
install -vm644 ../src/udev/udev.conf       /etc/udev/
install -vm644 rules.d/* ../rules.d/{*.rules,README} /usr/lib/udev/rules.d/
install -vm644 hwdb.d/*  ../hwdb.d/{*.hwdb,README}   /usr/lib/udev/hwdb.d/
install -vm755 $(find src/udev -type f | grep -F -v ".") /usr/lib/udev

Install some custom rules and support files useful in an LFS environment:

tar -xvf ../../udev-lfs-20230818.tar.xz
make -f udev-lfs-20230818/Makefile.lfs install

Install the man pages:

tar -xf ../../systemd-man-pages-254.tar.xz                            \
    --no-same-owner --strip-components=1                              \
    -C /usr/share/man --wildcards '*/udev*' '*/libudev*'              \
                                  '*/systemd-'{hwdb,udevd.service}.8
sed 's/systemd\(\\\?-\)/udev\1/' /usr/share/man/man8/systemd-hwdb.8   \
                               > /usr/share/man/man8/udev-hwdb.8
sed 's|lib.*udevd|sbin/udevd|'                                        \
    /usr/share/man/man8/systemd-udevd.service.8                       \
  > /usr/share/man/man8/udevd.8
rm  /usr/share/man/man8/systemd-*.8

8.74.2. Configuring Udev

Information about hardware devices is maintained in the /etc/udev/hwdb.d and /usr/lib/udev/hwdb.d directories. Udev needs that information to be compiled into a binary database /etc/udev/hwdb.bin. Create the initial database:

udev-hwdb update

This command needs to be run each time the hardware information is updated.

8.74.3. Contents of Udev

Installed programs: udevadm, udevd (symlink to udevadm), and udev-hwdb
Installed libraries: libudev.so
Installed directories: /etc/udev and /usr/lib/udev

Short Descriptions

udevadm

Generic udev administration tool: controls the udevd daemon, provides info from the Udev database, monitors uevents, waits for uevents to finish, tests Udev configuration, and triggers uevents for a given device

udevd

A daemon that listens for uevents on the netlink socket, creates devices and runs the configured external programs in response to these uevents

udev-hwdb

Updates or queries the hardware database.

libudev

A library interface to udev device information

/etc/udev

Contains Udev configuration files, device permissions, and rules for device naming