Linux kernel build requirements

Kernel/Traditional compilation

This article is an introduction to building custom kernels from kernel.org sources. This method of compiling kernels is the traditional method common to all distributions. It can be, depending on your background, more complicated than using the Kernels/Arch Build System. Consider the Arch Build System tools are developed and maintained to make repeatable compilation tasks efficient and safe.

Contents

Preparation

It is not necessary (or recommended) to use the root account or root privileges (i.e. via Sudo) for kernel preparation.

Install the core packages

Install the base-devel package group, which contains necessary packages such as make and gcc . It is also recommended to install the following packages, as listed in the default Arch kernel PKGBUILD: xmlto , kmod , inetutils , bc , libelf , git , cpio , perl , tar , xz .

Create a kernel compilation directory

It is recommended to create a separate build directory for your kernel(s). In this example, the directory kernelbuild will be created in the home directory:

Download the kernel source

Download the kernel source from https://www.kernel.org. This should be the tarball ( tar.xz ) file for your chosen kernel.

It can be downloaded by simply right-clicking the tar.xz link in your browser and selecting Save Link As. , or any other number of ways via alternative graphical or command-line tools that utilise HTTP, TFTP, Rsync, or Git.

In the following command-line example, wget has been installed and is used inside the

/kernelbuild directory to obtain kernel 4.8.6:

You should also verify the correctness of the download before trusting it. First grab the signature, then use that to grab the fingerprint of the signing key, then use the fingerprint to obtain the actual signing key:

Note the signature was generated for the tar archive (i.e. extension .tar ), not the compressed .tar.xz file that you have downloaded. You need to decompress the latter without untarring it. Verify that you have xz installed, then you can proceed like so:

Do not proceed if this does not result in output that includes the string «Good signature».

If wget was not used inside the build directory, it will be necessary to move the tarball into it, e.g.

Unpack the kernel source

Within the build directory, unpack the kernel tarball:

To finalise the preparation, ensure that the kernel tree is absolutely clean; do not rely on the source tree being clean after unpacking. To do so, first change into the new kernel source directory created, and then run the make mrproper command:

Kernel configuration

This is the most crucial step in customizing the default kernel to reflect your computer’s precise specifications. Kernel configuration is set in its .config file, which includes the use of Kernel modules. By setting the options in .config properly, your kernel and computer will function most efficiently.

You can do a mixture of two things:

• Use the default Arch settings from an official kernel (recommended)
• Manually configure the kernel options (optional, advanced and not recommended)

Default Arch configuration

This method will create a .config file for the custom kernel using the default Arch kernel settings. If a stock Arch kernel is running, you can use the following command inside the custom kernel source directory:

Otherwise, the default configuration can be found online in the official Arch Linux kernel package.

Advanced configuration

There are several tools available to fine-tune the kernel configuration, which provide an alternative to otherwise spending hours manually configuring each and every one of the options available during compilation.

Those tools are:

• make menuconfig : Command-line ncurses interface superseded by nconfig
• make nconfig : Newer ncurses interface for the command-line
• make xconfig : User-friendly graphical interface that requires packagekit-qt5 to be installed as a dependency. This is the recommended method — especially for less experienced users — as it is easier to navigate, and information about each option is also displayed.
• make gconfig : Graphical configuration similar to xconfig but using gtk. This requires gtk2 , glib2 and libgladeAUR .

The chosen method should be run inside the kernel source directory, and all will either create a new .config file, or overwrite an existing one where present. All optional configurations will be automatically enabled, although any newer configuration options (i.e. with an older kernel .config ) may not be automatically selected.

Once the changes have been made save the .config file. It is a good idea to make a backup copy outside the source directory. You may need to do this multiple times before you get all the options right.

If unsure, only change a few options between compilations. If you cannot boot your newly built kernel, see the list of necessary config items here.

Running lspci -k # from liveCD lists names of kernel modules in use. Most importantly, you must maintain cgroups support. This is necessary for systemd. For more detailed information, see Gentoo:Kernel/Gentoo Kernel Configuration Guide and Gentoo:Intel#Kernel or Gentoo:Ryzen#Kernel for Intel or AMD Ryzen processors.

Compilation

Compilation time will vary from as little as fifteen minutes to over an hour, depending on your kernel configuration and processor capability. Once the .config file has been set for the custom kernel, within the source directory run the following command to compile:

Installation

Install the modules

Once the kernel has been compiled, the modules for it must follow. First build the modules:

Then install the modules. As root or with root privileges, run the following command to do so:

This will copy the compiled modules into /lib/modules/ — . For example, for kernel version 4.8 installed above, they would be copied to /lib/modules/4.8.6-ARCH . This keeps the modules for individual kernels used separated.

Copy the kernel to /boot directory

The kernel compilation process will generate a compressed bzImage (big zImage) of that kernel, which must be copied to the /boot directory and renamed in the process. Provided the name is prefixed with vmlinuz- , you may name the kernel as you wish. In the examples below, the installed and compiled 4.8 kernel has been copied over and renamed to vmlinuz-linux48 :

Make initial RAM disk

If you do not know what making an initial RAM disk is, see Initramfs on Wikipedia and mkinitcpio.

Automated preset method

An existing mkinitcpio preset can be copied and modified so that the custom kernel initramfs images can be generated in the same way as for an official kernel. This is useful where intending to recompile the kernel (e.g. where updated). In the example below, the preset file for the stock Arch kernel will be copied and modified for kernel 4.8, installed above.

First, copy the existing preset file, renaming it to match the name of the custom kernel specified as a suffix to /boot/vmlinuz- when copying the bzImage (in this case, linux48 ):

Second, edit the file and amend for the custom kernel. Note (again) that the ALL_kver= parameter also matches the name of the custom kernel specified when copying the bzImage :

Finally, generate the initramfs images for the custom kernel in the same way as for an official kernel:

Manual method

Rather than use a preset file, mkinitcpio can also be used to generate an initramfs file manually. The syntax of the command is:

• -k ( —kernel ): Specifies the modules to use when generating the initramfs image. The name will be the same as the name of the custom kernel source directory (and the modules directory for it, located in /usr/lib/modules/ ).
• -g ( —generate ): Specifies the name of the initramfs file to generate in the /boot directory. Again, using the naming convention mentioned above is recommended.

For example, the command for the 4.8 custom kernel installed above would be:

Copy System.map

The System.map file is not required for booting Linux. It is a type of «phone directory» list of functions in a particular build of a kernel. The System.map contains a list of kernel symbols (i.e function names, variable names etc) and their corresponding addresses. This «symbol-name to address mapping» is used by:

• Some processes like klogd, ksymoops, etc.
• By OOPS handler when information has to be dumped to the screen during a kernel crash (i.e info like in which function it has crashed).

If your /boot is on a filesystem which supports symlinks (i.e., not FAT32), copy System.map to /boot , appending your kernel’s name to the destination file. Then create a symlink from /boot/System.map to point to /boot/System.map- :

After completing all steps above, you should have the following 3 files and 1 soft symlink in your /boot directory along with any other previously existing files:

• Kernel: vmlinuz-
• Initramfs: Initramfs- .img
• System Map: System.map-
• System Map kernel symlink

Bootloader configuration

Add an entry for your new kernel in your bootloader’s configuration file. See Arch boot process#Feature comparison for possible boot loaders, their wiki articles and other information.

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Minimal requirements to compile the Kernel¶

Intro¶

This document is designed to provide a list of the minimum levels of software necessary to run the 4.x kernels.

This document is originally based on my “Changes” file for 2.0.x kernels and therefore owes credit to the same people as that file (Jared Mauch, Axel Boldt, Alessandro Sigala, and countless other users all over the ‘net).

Current Minimal Requirements¶

Upgrade to at least these software revisions before thinking you’ve encountered a bug! If you’re unsure what version you’re currently running, the suggested command should tell you.

Again, keep in mind that this list assumes you are already functionally running a Linux kernel. Also, not all tools are necessary on all systems; obviously, if you don’t have any ISDN hardware, for example, you probably needn’t concern yourself with isdn4k-utils.

Program	Minimal version	Command to check the version
GNU C	4.6	gcc –version
GNU make	3.81	make –version
binutils	2.20	ld -v
flex	2.5.35	flex –version
bison	2.0	bison –version
util-linux	2.10o	fdformat –version
kmod	13	depmod -V
e2fsprogs	1.41.4	e2fsck -V
jfsutils	1.1.3	fsck.jfs -V
reiserfsprogs	3.6.3	reiserfsck -V
xfsprogs	2.6.0	xfs_db -V
squashfs-tools	4.0	mksquashfs -version
btrfs-progs	0.18	btrfsck
pcmciautils	004	pccardctl -V
quota-tools	3.09	quota -V
PPP	2.4.0	pppd –version
isdn4k-utils	3.1pre1	isdnctrl 2>&1|grep version
nfs-utils	1.0.5	showmount –version
procps	3.2.0	ps –version
oprofile	0.9	oprofiled –version
udev	081	udevd –version
grub	0.93	grub –version || grub-install –version
mcelog	0.6	mcelog –version
iptables	1.4.2	iptables -V
openssl & libcrypto	1.0.0	openssl version
bc	1.06.95	bc –version
Sphinx[1]	1.3	sphinx-build –version

[1]	Sphinx is needed only to build the Kernel documentation

Kernel compilation¶

The gcc version requirements may vary depending on the type of CPU in your computer.

Make¶

You will need GNU make 3.81 or later to build the kernel.

Binutils¶

The build system has, as of 4.13, switched to using thin archives ( ar T ) rather than incremental linking ( ld -r ) for built-in.a intermediate steps. This requires binutils 2.20 or newer.

pkg-config¶

The build system, as of 4.18, requires pkg-config to check for installed kconfig tools and to determine flags settings for use in ‘make config’. Previously pkg-config was being used but not verified or documented.

Flex¶

Since Linux 4.16, the build system generates lexical analyzers during build. This requires flex 2.5.35 or later.

Bison¶

Since Linux 4.16, the build system generates parsers during build. This requires bison 2.0 or later.

Perl¶

You will need perl 5 and the following modules: Getopt::Long , Getopt::Std , File::Basename , and File::Find to build the kernel.

You will need bc to build kernels 3.10 and higher

OpenSSL¶

Module signing and external certificate handling use the OpenSSL program and crypto library to do key creation and signature generation.

You will need openssl to build kernels 3.7 and higher if module signing is enabled. You will also need openssl development packages to build kernels 4.3 and higher.

System utilities¶

Architectural changes¶

32-bit UID support is now in place. Have fun!

Linux documentation for functions is transitioning to inline documentation via specially-formatted comments near their definitions in the source. These comments can be combined with ReST files the Documentation/ directory to make enriched documentation, which can then be converted to PostScript, HTML, LaTex, ePUB and PDF files. In order to convert from ReST format to a format of your choice, you’ll need Sphinx.

Util-linux¶

New versions of util-linux provide fdisk support for larger disks, support new options to mount, recognize more supported partition types, have a fdformat which works with 2.4 kernels, and similar goodies. You’ll probably want to upgrade.

Ksymoops¶

If the unthinkable happens and your kernel oopses, you may need the ksymoops tool to decode it, but in most cases you don’t. It is generally preferred to build the kernel with CONFIG_KALLSYMS so that it produces readable dumps that can be used as-is (this also produces better output than ksymoops). If for some reason your kernel is not build with CONFIG_KALLSYMS and you have no way to rebuild and reproduce the Oops with that option, then you can still decode that Oops with ksymoops.

Mkinitrd¶

These changes to the /lib/modules file tree layout also require that mkinitrd be upgraded.

E2fsprogs¶

The latest version of e2fsprogs fixes several bugs in fsck and debugfs. Obviously, it’s a good idea to upgrade.

JFSutils¶

The jfsutils package contains the utilities for the file system. The following utilities are available:

• fsck.jfs — initiate replay of the transaction log, and check and repair a JFS formatted partition.
• mkfs.jfs — create a JFS formatted partition.
• other file system utilities are also available in this package.

Reiserfsprogs¶

The reiserfsprogs package should be used for reiserfs-3.6.x (Linux kernels 2.4.x). It is a combined package and contains working versions of mkreiserfs , resize_reiserfs , debugreiserfs and reiserfsck . These utils work on both i386 and alpha platforms.

Xfsprogs¶

The latest version of xfsprogs contains mkfs.xfs , xfs_db , and the xfs_repair utilities, among others, for the XFS filesystem. It is architecture independent and any version from 2.0.0 onward should work correctly with this version of the XFS kernel code (2.6.0 or later is recommended, due to some significant improvements).

PCMCIAutils¶

PCMCIAutils replaces pcmcia-cs . It properly sets up PCMCIA sockets at system startup and loads the appropriate modules for 16-bit PCMCIA devices if the kernel is modularized and the hotplug subsystem is used.

Quota-tools¶

Support for 32 bit uid’s and gid’s is required if you want to use the newer version 2 quota format. Quota-tools version 3.07 and newer has this support. Use the recommended version or newer from the table above.

Intel IA32 microcode¶

A driver has been added to allow updating of Intel IA32 microcode, accessible as a normal (misc) character device. If you are not using udev you may need to:

as root before you can use this. You’ll probably also want to get the user-space microcode_ctl utility to use with this.

udev¶

udev is a userspace application for populating /dev dynamically with only entries for devices actually present. udev replaces the basic functionality of devfs, while allowing persistent device naming for devices.

FUSE¶

Needs libfuse 2.4.0 or later. Absolute minimum is 2.3.0 but mount options direct_io and kernel_cache won’t work.

Networking¶

General changes¶

If you have advanced network configuration needs, you should probably consider using the network tools from ip-route2.

Packet Filter / NAT¶

The packet filtering and NAT code uses the same tools like the previous 2.4.x kernel series (iptables). It still includes backwards-compatibility modules for 2.2.x-style ipchains and 2.0.x-style ipfwadm.

The PPP driver has been restructured to support multilink and to enable it to operate over diverse media layers. If you use PPP, upgrade pppd to at least 2.4.0.

If you are not using udev, you must have the device file /dev/ppp which can be made by:

Isdn4k-utils¶

Due to changes in the length of the phone number field, isdn4k-utils needs to be recompiled or (preferably) upgraded.

NFS-utils¶

In ancient (2.4 and earlier) kernels, the nfs server needed to know about any client that expected to be able to access files via NFS. This information would be given to the kernel by mountd when the client mounted the filesystem, or by exportfs at system startup. exportfs would take information about active clients from /var/lib/nfs/rmtab .

This approach is quite fragile as it depends on rmtab being correct which is not always easy, particularly when trying to implement fail-over. Even when the system is working well, rmtab suffers from getting lots of old entries that never get removed.

With modern kernels we have the option of having the kernel tell mountd when it gets a request from an unknown host, and mountd can give appropriate export information to the kernel. This removes the dependency on rmtab and means that the kernel only needs to know about currently active clients.

To enable this new functionality, you need to:

before running exportfs or mountd. It is recommended that all NFS services be protected from the internet-at-large by a firewall where that is possible.

mcelog¶

On x86 kernels the mcelog utility is needed to process and log machine check events when CONFIG_X86_MCE is enabled. Machine check events are errors reported by the CPU. Processing them is strongly encouraged.

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