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.

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).

• 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.

• 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-5.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 5.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-5.x) and execute:

  xz -cd ../patch-5.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 5.x kernels, patches for the 5.x.y kernels(also known as the -stable kernels) are not incremental but instead applydirectly to the base 5.x kernel. For example, if your base kernel is 5.0and you want to apply the 5.0.3 patch, you must not first apply the 5.0.1and 5.0.2 patches. Similarly, if you are running kernel version 5.0.2 andwant to jump to 5.0.3, you must first reverse the 5.0.2 patch (that is,patch -R)before applying the 5.0.3 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.

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-5.xbuild directory:    /home/name/build/kernel

To configure and build the kernel, use:

cd /usr/src/linux-5.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.

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/$ARCH/defconfig                   or arch/$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 kvmconfig"   Enable additional options for kvm guest kernel support."make xenconfig"   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 toolsin Documentation/kbuild/kconfig.rst.

• 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.

• Make sure you have at least gcc 4.9 available.For more information, refer toDocumentation/process/changes.rst.

  Please note that you can still run a.out user programs with this kernel.

• Do amake to create a compressed kernel image. It is alsopossible to domakeinstall if you have lilo installed to suit thekernel makefiles, but you may want to check your particular lilo 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 floppy without the assistance of abootloader such as LILO, is no longer supported.

  If you boot Linux from the hard drive, chances are you use LILO, whichuses the kernel image as specified in the file /etc/lilo.conf. Thekernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or/boot/bzImage. To use the new kernel, save a copy of the old imageand copy the new image over the old one. Then, you MUST RERUN LILOto update the loading map! If you don’t, you won’t be able to bootthe new kernel image.

  Reinstalling LILO is usually a matter of running /sbin/lilo.You may wish to edit /etc/lilo.conf to specify an entry for yourold kernel image (say, /vmlinux.old) in case the new one does notwork. See the LILO docs for 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,ramdisk size, etc. in the kernel image, use therdev program (oralternatively the LILO boot options when appropriate). No need torecompile the kernel to change these parameters.

• Reboot with the new kernel and enjoy.

• If you have problems that seem to be due to kernel bugs, please checkthe file MAINTAINERS to see if there is a particular person associatedwith the part of the kernel that you are having trouble with. If thereisn’t anyone listed there, then the second best thing is to mailthem to me (torvalds@linux-foundation.org), and possibly to any otherrelevant mailing-list or to the newsgroup.

• In all bug-reports,please tell what kernel you are talking about,how to duplicate the problem, and what your setup is (use your commonsense). If the problem is new, tell me so, and if the problem isold, please try to tell me when you first noticed it.

• If the bug results in a message like:

  unable to handle kernel paging request at address C0000010Oops: 0002EIP:   0010:XXXXXXXXeax: xxxxxxxx   ebx: xxxxxxxx   ecx: xxxxxxxx   edx: xxxxxxxxesi: xxxxxxxx   edi: xxxxxxxx   ebp: xxxxxxxxds: xxxx  es: xxxx  fs: xxxx  gs: xxxxPid: xx, process nr: xxxx xx xx xx xx xx xx xx xx xx

  or similar kernel debugging information on your screen or in yoursystem log, please duplicate itexactly. The dump may lookincomprehensible to you, but it does contain information that mayhelp debugging the problem. The text above the dump is alsoimportant: it tells something about why the kernel dumped code (inthe above example, it’s due to a bad kernel pointer). More informationon making sense of the dump is in Documentation/admin-guide/bug-hunting.rst

• If you compiled the kernel with CONFIG_KALLSYMS you can send the dumpas is, otherwise you will have to use theksymoops program to makesense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred).This utility can be downloaded fromhttps://www.kernel.org/pub/linux/utils/kernel/ksymoops/ .Alternatively, you can do the dump lookup by hand:

• In debugging dumps like the above, it helps enormously if you canlook up what the EIP value means. The hex value as such doesn’t helpme or anybody else very much: it will depend on your particularkernel setup. What you should do is take the hex value from the EIPline (ignore the0010:), and look it up in the kernel namelist tosee which kernel function contains the offending address.

  To find out the kernel function name, you’ll need to find the systembinary associated with the kernel that exhibited the symptom. This isthe file ‘linux/vmlinux’. To extract the namelist and match it againstthe EIP from the kernel crash, do:

  nm vmlinux | sort | less

  This will give you a list of kernel addresses sorted in ascendingorder, from which it is simple to find the function that contains theoffending address. Note that the address given by the kerneldebugging messages will not necessarily match exactly with thefunction addresses (in fact, that is very unlikely), so you can’tjust ‘grep’ the list: the list will, however, give you the startingpoint of each kernel function, so by looking for the function thathas a starting address lower than the one you are searching for butis followed by a function with a higher address you will find the oneyou want. In fact, it may be a good idea to include a bit of“context” in your problem report, giving a few lines around theinteresting one.

  If you for some reason cannot do the above (you have a pre-compiledkernel image or similar), telling me as much about your setup aspossible will help. Please read theadmin-guide/reporting-bugs.rstdocument for details.

• Alternatively, you can use gdb on a running kernel. (read-only; i.e. youcannot change values or set break points.) To do this, first compile thekernel with -g; edit arch/x86/Makefile appropriately, then do amakeclean. You’ll also need to enable CONFIG_PROC_FS (viamakeconfig).

  After you’ve rebooted with the new kernel, dogdbvmlinux/proc/kcore.You can now use all the usual gdb commands. The command to look up thepoint where your system crashed isl*0xXXXXXXXX. (Replace the XXXeswith the EIP value.)

  gdb’ing a non-running kernel currently fails becausegdb (wrongly)disregards the starting offset for which the kernel is compiled.