What is Linux?¶

Linux is a clone of the operating system Unix, written from scratch byLinus Torvalds with assistance from a loosely-knit team of hackers acrossthe Net. It aims towards POSIX and Single UNIX Specification compliance.

It has all the features you would expect in a modern fully-fledged Unix,including true multitasking, virtual memory, shared libraries, demandloading, shared copy-on-write executables, proper memory management,and multistack networking including IPv4 and IPv6.

It is distributed under the GNU General Public License v2 - see theaccompanying COPYING file for more details.

On what hardware does it run?¶

Although originally developed first for 32-bit x86-based PCs (386 or higher),today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC andUltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell,IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64 Xtensa, andARC architectures.

Linux is easily portable to most general-purpose 32- or 64-bit architecturesas long as they have a paged memory management unit (PMMU) and a port of theGNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux hasalso been ported to a number of architectures without a PMMU, althoughfunctionality is then obviously somewhat limited.Linux has also been ported to itself. You can now run the kernel as auserspace application - this is called UserMode Linux (UML).

Documentation¶

• There is a lot of documentation available both in electronic form onthe Internet and in books, both Linux-specific and pertaining togeneral UNIX questions. I’d recommend looking into the documentationsubdirectories on any Linux FTP site for the LDP (Linux DocumentationProject) books. This README is not meant to be documentation on thesystem: there are much better sources available.

• There are various README files in the Documentation/ subdirectory:these typically contain kernel-specific installation notes for somedrivers for example. Please read theDocumentation/process/changes.rst file, as itcontains information about the problems, which may result by upgradingyour kernel.

Installing the kernel source¶

• If you install the full sources, put the kernel tarball in adirectory where you have permissions (e.g. your home directory) andunpack it:

  xz -cd linux-6.x.tar.xz | tar xvf -

  Replace “X” with the version number of the latest kernel.

  Do NOT use the /usr/src/linux area! This area has a (usuallyincomplete) set of kernel headers that are used by the library headerfiles. They should match the library, and not get messed up bywhatever the kernel-du-jour happens to be.

• You can also upgrade between 6.x releases by patching. Patches aredistributed in the xz format. To install by patching, get all thenewer patch files, enter the top level directory of the kernel source(linux-6.x) and execute:

  xz -cd ../patch-6.x.xz | patch -p1

  Replace “x” for all versions bigger than the version “x” of your currentsource tree,in_order, and you should be ok. You may want to removethe backup files (some-file-name~ or some-file-name.orig), and make surethat there are no failed patches (some-file-name# or some-file-name.rej).If there are, either you or I have made a mistake.

  Unlike patches for the 6.x kernels, patches for the 6.x.y kernels(also known as the -stable kernels) are not incremental but instead applydirectly to the base 6.x kernel. For example, if your base kernel is 6.0and you want to apply the 6.0.3 patch, you must not first apply the 6.0.1and 6.0.2 patches. Similarly, if you are running kernel version 6.0.2 andwant to jump to 6.0.3, you must first reverse the 6.0.2 patch (that is,patch -R)before applying the 6.0.3 patch. You can read more on this inDocumentation/process/applying-patches.rst.

  Alternatively, the script patch-kernel can be used to automate thisprocess. It determines the current kernel version and applies anypatches found:

  linux/scripts/patch-kernel linux

  The first argument in the command above is the location of thekernel source. Patches are applied from the current directory, butan alternative directory can be specified as the second argument.

• Make sure you have no stale .o files and dependencies lying around:

  cd linuxmake mrproper

  You should now have the sources correctly installed.

Software requirements¶

Compiling and running the 6.x kernels requires up-to-dateversions of various software packages. ConsultDocumentation/process/changes.rst for the minimum version numbersrequired and how to get updates for these packages. Beware that usingexcessively old versions of these packages can cause indirecterrors that are very difficult to track down, so don’t assume thatyou can just update packages when obvious problems arise duringbuild or operation.

Build directory for the kernel¶

When compiling the kernel, all output files will per default bestored together with the kernel source code.Using the optionmakeO=output/dir allows you to specify an alternateplace for the output files (including .config).Example:

kernel source code: /usr/src/linux-6.xbuild directory:    /home/name/build/kernel

To configure and build the kernel, use:

cd /usr/src/linux-6.xmake O=/home/name/build/kernel menuconfigmake O=/home/name/build/kernelsudo make O=/home/name/build/kernel modules_install install

Please note: If theO=output/dir option is used, then it must beused for all invocations of make.

Configuring the kernel¶

Do not skip this step even if you are only upgrading one minorversion. New configuration options are added in each release, andodd problems will turn up if the configuration files are not set upas expected. If you want to carry your existing configuration to anew version with minimal work, usemakeoldconfig, which willonly ask you for the answers to new questions.

• Alternative configuration commands are:

  "make config"      Plain text interface."make menuconfig"  Text based color menus, radiolists & dialogs."make nconfig"     Enhanced text based color menus."make xconfig"     Qt based configuration tool."make gconfig"     GTK based configuration tool."make oldconfig"   Default all questions based on the contents of                   your existing ./.config file and asking about                   new config symbols."make olddefconfig"                   Like above, but sets new symbols to their default                   values without prompting."make defconfig"   Create a ./.config file by using the default                   symbol values from either arch/$ARCH/configs/defconfig                   or arch/$ARCH/configs/${PLATFORM}_defconfig,                   depending on the architecture."make ${PLATFORM}_defconfig"                   Create a ./.config file by using the default                   symbol values from                   arch/$ARCH/configs/${PLATFORM}_defconfig.                   Use "make help" to get a list of all available                   platforms of your architecture."make allyesconfig"                   Create a ./.config file by setting symbol                   values to 'y' as much as possible."make allmodconfig"                   Create a ./.config file by setting symbol                   values to 'm' as much as possible."make allnoconfig" Create a ./.config file by setting symbol                   values to 'n' as much as possible."make randconfig"  Create a ./.config file by setting symbol                   values to random values."make localmodconfig" Create a config based on current config and                      loaded modules (lsmod). Disables any module                      option that is not needed for the loaded modules.                      To create a localmodconfig for another machine,                      store the lsmod of that machine into a file                      and pass it in as a LSMOD parameter.                      Also, you can preserve modules in certain folders                      or kconfig files by specifying their paths in                      parameter LMC_KEEP.              target$ lsmod > /tmp/mylsmod              target$ scp /tmp/mylsmod host:/tmp              host$ make LSMOD=/tmp/mylsmod \                      LMC_KEEP="drivers/usb:drivers/gpu:fs" \                      localmodconfig                      The above also works when cross compiling."make localyesconfig" Similar to localmodconfig, except it will convert                      all module options to built in (=y) options. You can                      also preserve modules by LMC_KEEP."make kvm_guest.config"   Enable additional options for kvm guest kernel                          support."make xen.config"   Enable additional options for xen dom0 guest kernel                    support."make tinyconfig"  Configure the tiniest possible kernel.

  You can find more information on using the Linux kernel config toolsinConfiguration targets and editors.

• NOTES onmakeconfig:

  • Having unnecessary drivers will make the kernel bigger, and canunder some circumstances lead to problems: probing for anonexistent controller card may confuse your other controllers.

  • A kernel with math-emulation compiled in will still use thecoprocessor if one is present: the math emulation will justnever get used in that case. The kernel will be slightly larger,but will work on different machines regardless of whether theyhave a math coprocessor or not.

  • The “kernel hacking” configuration details usually result in abigger or slower kernel (or both), and can even make the kernelless stable by configuring some routines to actively try tobreak bad code to find kernel problems (kmalloc()). Thus youshould probably answer ‘n’ to the questions for “development”,“experimental”, or “debugging” features.

Compiling the kernel¶

• Make sure you have at least gcc 8.1 available.For more information, refer toDocumentation/process/changes.rst.

• Do amake to create a compressed kernel image. It is also possible to domakeinstall if you have lilo installed or if your distribution has aninstall script recognised by the kernel’s installer. Most populardistributions will have a recognized install script. You may want tocheck your distribution’s setup first.

  To do the actual install, you have to be root, but none of the normalbuild should require that. Don’t take the name of root in vain.

• If you configured any of the parts of the kernel asmodules, youwill also have to domakemodules_install.

• Verbose kernel compile/build output:

  Normally, the kernel build system runs in a fairly quiet mode (but nottotally silent). However, sometimes you or other kernel developers needto see compile, link, or other commands exactly as they are executed.For this, use “verbose” build mode. This is done by passingV=1 to themake command, e.g.:

  make V=1 all

  To have the build system also tell the reason for the rebuild of eachtarget, useV=2. The default isV=0.

• Keep a backup kernel handy in case something goes wrong. This isespecially true for the development releases, since each new releasecontains new code which has not been debugged. Make sure you keep abackup of the modules corresponding to that kernel, as well. If youare installing a new kernel with the same version number as yourworking kernel, make a backup of your modules directory before youdo amakemodules_install.

  Alternatively, before compiling, use the kernel config option“LOCALVERSION” to append a unique suffix to the regular kernel version.LOCALVERSION can be set in the “General Setup” menu.

• In order to boot your new kernel, you’ll need to copy the kernelimage (e.g. .../linux/arch/x86/boot/bzImage after compilation)to the place where your regular bootable kernel is found.

• Booting a kernel directly from a storage device without the assistanceof a bootloader such as LILO or GRUB, is no longer supported in BIOS(non-EFI systems). On UEFI/EFI systems, however, you can use EFISTUBwhich allows the motherboard to boot directly to the kernel.On modern workstations and desktops, it’s generally recommended to use abootloader as difficulties can arise with multiple kernels and secure boot.For more details on EFISTUB,see “The EFI Boot Stub”.

• It’s important to note that as of 2016 LILO (LInux LOader) is no longer inactive development, though as it was extremely popular, it often comes upin documentation. Popular alternatives include GRUB2, rEFInd, Syslinux,systemd-boot, or EFISTUB. For various reasons, it’s not recommended to usesoftware that’s no longer in active development.

• Chances are your distribution includes an install script and runningmakeinstall will be all that’s needed. Should that not be the caseyou’ll have to identify your bootloader and reference its documentation orconfigure your EFI.

Legacy LILO Instructions¶

• If you use LILO the kernel images are specified in the file /etc/lilo.conf.The kernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or/boot/bzImage. To use the new kernel, save a copy of the old image and copythe new image over the old one. Then, you MUST RERUN LILO to update theloading map! If you don’t, you won’t be able to boot the new kernel image.

• Reinstalling LILO is usually a matter of running /sbin/lilo. You may wishto edit /etc/lilo.conf to specify an entry for your old kernel image(say, /vmlinux.old) in case the new one does not work. See the LILO docsfor more information.

• After reinstalling LILO, you should be all set. Shutdown the system,reboot, and enjoy!

• If you ever need to change the default root device, video mode, etc. in thekernel image, use your bootloader’s boot options where appropriate. No needto recompile the kernel to change these parameters.

• Reboot with the new kernel and enjoy.

If something goes wrong¶

If you have problems that seem to be due to kernel bugs, please follow theinstructions at ‘Reporting issues’.

Hints on understanding kernel bug reports are in‘Bug hunting’. More on debugging the kernelwith gdb is in ‘Debugging kernel and modules via gdb’ and‘Using kgdb, kdb and the kernel debugger internals’.