8.3.1. Installation of the kernel
Building the kernel involves a few steps—configuration,
compilation, and installation. Read the README file in the kernel source tree for
alternative methods to the way this book configures the kernel.
Prepare for compilation by running the following command:
make mrproper
This ensures that the kernel tree is absolutely clean. The kernel
team recommends that this command be issued prior to each kernel
compilation. Do not rely on the source tree being clean after
un-tarring.
Configure the kernel via a menu-driven interface. For general
information on kernel configuration see
http://www.linuxfromscratch.org/hints/downloads/files/kernel-configuration.txt.
BLFS has some information regarding particular kernel configuration
requirements of packages outside of LFS at
http://www.linuxfromscratch.org/blfs/view/7.9-systemd-rc1/longindex.html#kernel-config-index.
Additional information about configuring and building the kernel
can be found at http://www.kroah.com/lkn/
![[Note]](../images/note.png)
Note
A good starting place for setting up the kernel configuration is
to run make
defconfig. This will set the base configuration
to a good state that takes your current system architecture into
account.
Be sure to enable or disable following features or the system
might not work correctly or boot at all:
General setup --->
[*] open by fhandle syscalls [CONFIG_FHANDLE]
[ ] Auditing support [CONFIG_AUDIT]
[*] Control Group support [CONFIG_CGROUPS]
Processor type and features --->
[*] Enable seccomp to safely compute untrusted bytecode [CONFIG_SECCOMP]
Networking support --->
Networking options --->
<*> The IPv6 protocol [CONFIG_IPV6]
Device Drivers --->
Generic Driver Options --->
[ ] Support for uevent helper [CONFIG_UEVENT_HELPER]
[*] Maintain a devtmpfs filesystem to mount at /dev [CONFIG_DEVTMPFS]
[ ] Fallback user-helper invocation for firmware loading [CONFIG_FW_LOADER_USER_HELPER]
Firmware Drivers --->
[*] Export DMI identification via sysfs to userspace [CONFIG_DMIID]
File systems --->
[*] Inotify support for userspace [CONFIG_INOTIFY_USER]
<*> Kernel automounter version 4 support (also supports v3) [CONFIG_AUTOFS4_FS]
Pseudo filesystems --->
[*] Tmpfs POSIX Access Control Lists [CONFIG_TMPFS_POSIX_ACL]
[*] Tmpfs extended attributes [CONFIG_TMPFS_XATTR]
Note
While "The IPv6 Protocol" is not strictly required, it is highly
recommended by the systemd developers.
The rationale for the above configuration items:
-
Support for
uevent helper
-
Having this option set may interfere with device management
when using Udev/Eudev.
-
Maintain a
devtmpfs
-
This will create automated device nodes which are populated
by the kernel, even without Udev running. Udev then runs on
top of this, managing permissions and adding symlinks. This
configuration item is required for all users of Udev/Eudev.
make LANG=<host_LANG_value> LC_ALL= menuconfig
The meaning of the make parameters:
-
LANG=<host_LANG_value>
LC_ALL=
-
This establishes the locale setting to the one used on the
host. This is needed for a proper menuconfig ncurses
interface line drawing on UTF-8 linux text console.
Be sure to replace <host_LANG_value> by
the value of the $LANG variable
from your host. If not set, you could use instead the host's
value of $LC_ALL or $LC_CTYPE.
Alternatively, make
oldconfig may be more appropriate in some
situations. See the README file for
more information.
If desired, skip kernel configuration by copying the kernel config
file, .config, from the host system
(assuming it is available) to the unpacked linux-4.4.1 directory. However, we do not
recommend this option. It is often better to explore all the
configuration menus and create the kernel configuration from
scratch.
Compile the kernel image and modules:
make
If using kernel modules, module configuration in /etc/modprobe.d may be required. Information
pertaining to modules and kernel configuration is located in
Section 7.3,
“Device and Module Handling on an LFS System” and
in the kernel documentation in the linux-4.4.1/Documentation directory. Also,
modprobe.d(5) may be of interest.
Install the modules, if the kernel configuration uses them:
make modules_install
After kernel compilation is complete, additional steps are required
to complete the installation. Some files need to be copied to the
/boot directory.
The path to the kernel image may vary depending on the platform
being used. The filename below can be changed to suit your taste,
but the stem of the filename should be vmlinuz to be compatible with the
automatic setup of the boot process described in the next section.
The following command assumes an x86 architecture:
cp -v arch/x86/boot/bzImage /boot/vmlinuz-4.4.1-lfs-7.9-systemd-rc1
System.map is a symbol file for the
kernel. It maps the function entry points of every function in the
kernel API, as well as the addresses of the kernel data structures
for the running kernel. It is used as a resource when investigating
kernel problems. Issue the following command to install the map
file:
cp -v System.map /boot/System.map-4.4.1
The kernel configuration file .config
produced by the make
menuconfig step above contains all the
configuration selections for the kernel that was just compiled. It
is a good idea to keep this file for future reference:
cp -v .config /boot/config-4.4.1
Install the documentation for the Linux kernel:
install -d /usr/share/doc/linux-4.4.1
cp -r Documentation/* /usr/share/doc/linux-4.4.1
It is important to note that the files in the kernel source
directory are not owned by root. Whenever a package is unpacked as
user root (like we did
inside chroot), the files have the user and group IDs of whatever
they were on the packager's computer. This is usually not a problem
for any other package to be installed because the source tree is
removed after the installation. However, the Linux source tree is
often retained for a long time. Because of this, there is a chance
that whatever user ID the packager used will be assigned to
somebody on the machine. That person would then have write access
to the kernel source.
Note
In many cases, the configuration of the kernel will need to be
updated for packages that will be installed later in BLFS. Unlike
other packages, it is not necessary to remove the kernel source
tree after the newly built kernel is installed.
If the kernel source tree is going to be retained, run
chown -R 0:0 on the
linux-4.4.1 directory to ensure all
files are owned by user root.
![[Warning]](../images/warning.png)
Warning
Some kernel documentation recommends creating a symlink from
/usr/src/linux pointing to the
kernel source directory. This is specific to kernels prior to the
2.6 series and must not be
created on an LFS system as it can cause problems for packages
you may wish to build once your base LFS system is complete.
Warning
The headers in the system's include
directory (/usr/include) should
always be the ones against
which Glibc was compiled, that is, the sanitised headers
installed in Section 6.7,
“Linux-4.4.1 API Headers”. Therefore, they should
never be replaced by
either the raw kernel headers or any other kernel sanitized
headers.
8.3.2. Configuring
Linux Module Load Order
Most of the time Linux modules are loaded automatically, but
sometimes it needs some specific direction. The program that loads
modules, modprobe or
insmod, uses
/etc/modprobe.d/usb.conf for this
purpose. This file needs to be created so that if the USB drivers
(ehci_hcd, ohci_hcd and uhci_hcd) have been built as modules, they
will be loaded in the correct order; ehci_hcd needs to be loaded
prior to ohci_hcd and uhci_hcd in order to avoid a warning being
output at boot time.
Create a new file /etc/modprobe.d/usb.conf by running the
following:
install -v -m755 -d /etc/modprobe.d
cat > /etc/modprobe.d/usb.conf << "EOF"
# Begin /etc/modprobe.d/usb.conf
install ohci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i ohci_hcd ; true
install uhci_hcd /sbin/modprobe ehci_hcd ; /sbin/modprobe -i uhci_hcd ; true
# End /etc/modprobe.d/usb.conf
EOF