8.76. Udev from Systemd-260.2

#### Linux From Scratch - Version r13.0-131

### Chapter 8. Installing Basic System Software

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# 8.76. Udev from Systemd-260.2

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

Approximate build time: 0.1 SBU
Required disk space: 162 MB

## 8.76.1. Installation of Udev

Udev is part of the systemd-260.2 package. Use the systemd-260.2.tar.xz file as the source tarball.

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

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

Remove one udev rule requiring a full Systemd installation:

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

Adjust the hardcoded paths to network configuration files for the standalone udev installation:

    sed -e '/NETWORK_DIRS/s/systemd/udev/' \
        -i src/libsystemd/sd-network/network-util.h

Prepare Udev for compilation:

    mkdir -p build
    cd       build

    meson setup ..                  \
          --prefix=/usr             \
          --buildtype=release       \
          -D mode=release           \
          -D dev-kvm-mode=0660      \
          -D link-udev-shared=false \
          -D logind=false           \
          -D vconsole=false

The meaning of the meson options:

`--buildtype=release`

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

`-D mode=release`

  Disable some features considered experimental by upstream.

`-D dev-kvm-mode=0660`

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

`-D link-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.

`-D logind=false -D vconsole=false`

  These options prevent the generation of several udev rule files belonging to the other Systemd components that we won't install.

Get the list of the shipped udev helpers and save it into an environment variable (exporting it is not strictly necessary, but it makes building as a regular user or using a package manager easier):

    export udev_helpers=$(grep "'name' :" ../src/udev/meson.build | \
                          awk '{print $3}' | tr -d ",'" | grep -v 'udevadm')

Only build the components needed for udev:

    ninja udevadm systemd-hwdb                                           \
          $(ninja -n | grep -Eo '(src/(lib)?udev|rules.d|hwdb.d)/[^ ]*') \
          $(realpath libudev.so --relative-to .)                         \
          $udev_helpers

Install the package:

    install -vm755 -d {/usr/lib,/etc}/udev/{hwdb.d,rules.d,network}
    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/README         /usr/lib/udev/rules.d/
    install -vm644 $(find ../rules.d/*.rules \
                          -not -name '*power-switch*') /usr/lib/udev/rules.d/
    install -vm644 hwdb.d/*  ../hwdb.d/{*.hwdb,README} /usr/lib/udev/hwdb.d/
    install -vm755 $udev_helpers                       /usr/lib/udev
    install -vm644 ../network/99-default.link          /usr/lib/udev/network

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-260.2.tar.xz                          \
        --no-same-owner --strip-components=1                              \
        -C /usr/share/man --wildcards '*/udev*' '*/libudev*'              \
                                      '*/systemd.link.5'                  \
                                      '*/systemd-'{hwdb,udevd.service}.8

    sed 's|systemd/network|udev/network|'                                 \
        /usr/share/man/man5/systemd.link.5                                \
      > /usr/share/man/man5/udev.link.5

    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/man*/systemd*

Finally, unset the `udev_helpers` variable:

    unset udev_helpers

## 8.76.2. 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.

## 8.76.3. 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

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