10.3.1. 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.
![[Important]](../images/important.png)
Important
Building the linux kernel for the first time is one of the most
challenging tasks in LFS. Getting it right depends on the
specific hardware for the target system and your specific needs.
There are almost 12,000 configuration items that are available
for the kernel although only about a third of them are needed for
most computers. The LFS editors recommend that users not familiar
with this process follow the procedures below fairly closely. The
objective is to get an initial system to a point where you can
log in at the command line when you reboot later in Section 11.3,
“Rebooting the System”. At this point
optimization and customization is not a goal.
For general information on kernel configuration see
https://www.linuxfromscratch.org/hints/downloads/files/kernel-configuration.txt.
Additional information about configuring and building the kernel
can be found at https://anduin.linuxfromscratch.org/LFS/kernel-nutshell/.
These references are a bit dated, but still give a reasonable
overview of the process.
If all else fails, you can ask for help on the lfs-support
mailing list. Note that subscribing is required in order for the
list to avoid spam.
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.
There are several ways to configure the kernel options. Usually,
This is done through a menu-driven interface, for example:
make menuconfig
The meaning of optional make environment
variables:
-
LANG=<host_LANG_value>
LC_ALL=
-
This establishes the locale setting to the one used on the
host. This may be needed for a proper menuconfig ncurses
interface line drawing on a UTF-8 linux text console.
If used, be sure to replace <host_LANG_value> by
the value of the $LANG variable
from your host. You can alternatively use instead the host's
value of $LC_ALL or $LC_CTYPE.
-
make
menuconfig
-
This launches an ncurses menu-driven interface. For other
(graphical) interfaces, type make help.
![[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/disable/set the following features or the
system might not work correctly or boot at all:
General setup --->
[ ] Compile the kernel with warnings as errors [WERROR]
[ ] Auditing support [AUDIT]
CPU/Task time and stats accounting --->
[*] Pressure stall information tracking [PSI]
[ ] Require boot parameter to enable pressure stall information tracking
... [PSI_DEFAULT_DISABLED]
< > Enable kernel headers through /sys/kernel/kheaders.tar.xz [IKHEADERS]
[*] Control Group support ---> [CGROUPS]
[*] Memory controller [MEMCG]
[ ] Configure standard kernel features (expert users) ---> [EXPERT]
Kernel Features --->
[*] Build a relocatable kernel image [RELOCATABLE]
[*] Randomize the address of the kernel image [RANDOMIZE_BASE]
General architecture-dependent options --->
[*] Stack Protector buffer overflow detection [STACKPROTECTOR]
[*] Strong Stack Protector [STACKPROTECTOR_STRONG]
[*] Networking support ---> [NET]
Networking options --->
[*] TCP/IP networking [INET]
<*> The IPv6 protocol ---> [IPV6]
Device Drivers --->
Generic Driver Options --->
[ ] Support for uevent helper [UEVENT_HELPER]
[*] Maintain a devtmpfs filesystem to mount at /dev [DEVTMPFS]
[*] Automount devtmpfs at /dev, after the kernel mounted the rootfs
... [DEVTMPFS_MOUNT]
Firmware loader --->
< /*> Firmware loading facility [FW_LOADER]
[ ] Enable the firmware sysfs fallback mechanism
... [FW_LOADER_USER_HELPER]
Firmware Drivers --->
[*] Export DMI identification via sysfs to userspace [DMIID]
Graphics support --->
Frame buffer Devices --->
<*> Support for frame buffer devices ---> [FB]
Console display driver support --->
[*] Framebuffer Console support [FRAMEBUFFER_CONSOLE]
File systems --->
[*] Inotify support for userspace [INOTIFY_USER]
Pseudo filesystems --->
[*] Tmpfs virtual memory file system support (former shm fs) [TMPFS]
[*] Tmpfs POSIX Access Control Lists [TMPFS_POSIX_ACL]
If the partition for the LFS system is in a NVME SSD (i. e. the
device node for the partition is /dev/nvme* instead of /dev/sd*), enable NVME support or the LFS
system won't boot:
Device Drivers --->
NVME Support --->
<*> NVM Express block device [BLK_DEV_NVME]
Note
While "The IPv6 Protocol" is not strictly required, it is highly
recommended by the systemd developers.
There are several other options that may be desired depending on
the requirements for the system. For a list of options needed for
BLFS packages, see the
BLFS Index of Kernel Settings.
Note
If your host hardware is using UEFI and you wish to boot the LFS
system with it, you should adjust some kernel configuration
following
the BLFS page even if you'll use
the UEFI bootloader from the host distro.
The rationale for the above configuration items:
-
Randomize the
address of the kernel image (KASLR)
-
Enable ASLR for kernel image, to mitigate some attacks based
on fixed addresses of sensitive data or code in the kernel.
-
Compile the
kernel with warnings as errors
-
This may cause building failure if the compiler and/or
configuration are different from those of the kernel
developers.
-
Enable kernel
headers through /sys/kernel/kheaders.tar.xz
-
This will require cpio building the kernel.
cpio is not
installed by LFS.
-
Configure
standard kernel features (expert users)
-
This will make some options show up in the configuration
interface but changing those options may be dangerous. Do not
use this unless you know what you are doing.
-
Strong Stack
Protector
-
Enable SSP for the kernel. We've enabled it for the entire
userspace with --enable-default-ssp
configuring GCC, but the kernel does not use GCC default
setting for SSP. We enable it explicitly here.
-
Support for
uevent helper
-
Having this option set may interfere with device management
when using Udev.
-
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.
-
Automount
devtmpfs at /dev
-
This will mount the kernel view of the devices on /dev upon
switching to root filesystem just before starting init.
-
Enable the
generic EFI decompressor
-
Create the bootable image as an EFI application that carries
the actual kernel image in compressed form. It can make the
bootable image 50% smaller.
-
Framebuffer
Console support
-
This is needed to display the Linux console on a frame buffer
device. To allow the kernel to print debug messages at an
early boot stage, it shouldn't be built as a kernel module
unless an initramfs will be used. And, if CONFIG_DRM (Direct Rendering Manager) is
enabled, it's likely CONFIG_DRM_FBDEV_EMULATION (Enable legacy
fbdev support for your modesetting driver) should be enabled
as well.
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-6.4.12 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 9.3,
“Overview of Device and Module Handling” and in the
kernel documentation in the linux-6.4.12/Documentation directory. Also,
modprobe.d(5) may be of interest.
Unless module support has been disabled in the kernel
configuration, install the modules with:
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.
![[Caution]](../images/caution.png)
Caution
If you've decided to use a separate /boot partition for the LFS system (maybe
sharing a /boot partition with the
host distro) , the files copied below should go there. The
easiest way to do that is to create the entry for /boot in /etc/fstab first (read the previous section for
details), then issue the following command as the root user in the chroot environment:
mount /boot
The path to the device node is omitted in the command because
mount can read it
from /etc/fstab.
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 ARM64 architecture with an EFI
boot loader (for example, GRUB built in Chapter 8):
cp -iv arch/arm64/boot/vmlinuz.efi /boot/vmlinuz-6.4.12-lfs-12.0-systemd
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 -iv System.map /boot/System.map-6.4.12
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 -iv .config /boot/config-6.4.12
Install the documentation for the Linux kernel:
cp -r Documentation -T /usr/share/doc/linux-6.4.12
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-6.4.12 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 5.4,
“Linux-6.4.12 API Headers”. Therefore, they
should never be replaced
by either the raw kernel headers or any other kernel sanitized
headers.