Linux kernel

The Linux kernel is a free and open-source, monolithic, Unix-like operating system kernel. It is deployed on a wide variety of computing systems, from personal computers, mobile devices, mainframes, and supercomputer[6] to embedded devices, such as routers, wireless access points, private branch exchanges, set-top boxes, FTA receivers, smart TVs, personal video recorders, and NAS appliances. Its availability and continuous development and support have spawned a plethora of operating system distributions, commonly also called Linux.[7]

Linux kernel
Tux the penguin, mascot of Linux[1]
Linux kernel 3.0.0 booting
DeveloperLinus Torvalds and thousands of collaborators
Written inC and assembly[2]
OS familyUnix-like
Initial release0.02 (5 October 1991 (1991-10-05))
Latest release5.6.7[3] (24 April 2020 (2020-04-24)) [±]
Latest preview5.7-rc2[4] (19 April 2020 (2020-04-19)) [±]
Repository
Available inEnglish
Kernel typeMonolithic
LicenseGPLv2 with some code under compatible GPL variants or under permissive licenses like BSD, MIT etc.[5]
Official websitewww.kernel.org

While the adoption of the Linux kernel in desktop computer operating systems is low, Linux-based operating systems dominate nearly every other segment of computing, including its use in the Android operating system for tablet computers, smartphones, and smartwatches.

The Linux kernel was conceived and created in 1991 by Linus Torvalds[8] for his personal computer and with no cross-platform intentions, but has since expanded to support a huge array of computer architectures, many more than other kernels do. Linux rapidly attracted developers and users who adopted it as the kernel for other free software projects, notably the GNU Operating System,[9] which was created as a free, non-proprietary operating system, and based on UNIX as a by-product of the fallout of the Unix wars.[10]

The Linux kernel, developed by contributors worldwide, is a prominent example of free and open source software.[11] Day-to-day development discussions take place on the Linux kernel mailing list (LKML). The Linux kernel as a whole, as it is clearly stated in the COPYING file,[12] is released under the GNU General Public License version 2 (GPLv2), but it also contains several files under other compatible licenses[5] and an ad hoc exemption for the User-space API header files (UAPI).

The Linux ABI (i.e., the Application Binary Interface which also includes Application Program Interface or API at the code source level)[13] between the kernel and the user space has four degrees of stability (stable, testing, obsolete, removed),[14] however the system calls are expected to never change in order to not break the userspace programs that rely on them.[15] As far as in-kernel APIs are regarded, there's no guarantee of stability. Device drivers included in the mainline Linux must be kept updated by their maintainers in order to stay at pace with the kernel evolution. Furthermore, the interface between the kernel and loadable kernel modules (LKMs), unlike in many other kernels, is not meant to be stable by design.[16]

History
See also: History of Linux and Linux kernel version history
Linus Torvalds

In April 1991, Linus Torvalds, at the time a 21-year-old computer science student at the University of Helsinki, Finland, started working on some simple ideas for an operating system. He started with a task switcher in Intel 80386 assembly language and a terminal driver. On 25 August 1991, Torvalds posted the following to comp.os.minix, a newsgroup on Usenet:[17]

I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since April, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things). I've currently ported bash(1.08) and gcc(1.40), and things seem to work. This implies that I'll get something practical within a few months [...] Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT portable (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.

On 17 September 1991, Torvalds prepared version 0.01 of the Linux kernel and put on the "ftp.funet.fi" – FTP server of the Finnish University and Research Network (FUNET). It was not even executable since its code still needed Minix for compilation and play.[18]

On 5 October 1991, Linus announced the first "official" version of Linux, version 0.02.[19] At this point, Linux was able to run Bash, GCC, and some other GNU utilities:[19][18]

[As] I mentioned a month ago, I'm working on a free version of a Minix-lookalike for AT-386 computers. It has finally reached the stage where it's even usable (though may not be depending on what you want), and I am willing to put out the sources for wider distribution. It is just version 0.02...but I've successfully run bash, gcc, gnu-make, gnu-sed, compress, etc. under it.

After that, many people contributed code to the project. Early on, the MINIX community contributed code and ideas to the Linux kernel. At the time, the GNU Project had created many of the components required for a free operating system, but its own kernel, GNU Hurd, was incomplete and unavailable. The Berkeley Software Distribution had not yet freed itself from legal encumbrances. Despite the limited functionality of the early versions, Linux rapidly gained developers and users.

Torvalds assigned version 0 to the kernel to indicate that it was mainly for testing and not intended for productive use.[20] In December 1991, Linux kernel 0.11 was released. This version was the first to be self-hosted as Linux kernel 0.11 could be compiled by a computer running the same kernel version.

When Torvalds released version 0.12 in February 1992, he adopted the GNU General Public License version 2 (GPLv2) over his previous self-drafted license, which had not permitted commercial redistribution.[21] In contrast to Unix, all source files of the Linux kernel are freely available, including device drivers.[22] The initial success of the Linux kernel was driven by the support of programmers and testers across the world. By structuring the Linux kernel along POSIX standards, it was compatible with a range of free software and applications that had been developed for universities.[23]

The Linux kernel now supports various hardware architectures, providing a common platform for software, including proprietary.

On 19 January 1992, the first post to the new newsgroup alt.os.linux was submitted.[24] On 31 March 1992, the newsgroup was renamed comp.os.linux.[25] The fact that Linux is a monolithic kernel rather than a microkernel was the topic of a debate between Andrew S. Tanenbaum, the creator of MINIX, and Torvalds.[26] This discussion is known as the Tanenbaum–Torvalds debate and started in 1992 on the Usenet discussion group comp.os.minix as a general debate about Linux and kernel architecture.[27][28]

Linux version 0.95 was the first to be capable of running the X Window System.[29] In March 1994, Linux 1.0.0 was released with 176,250 lines of code.[30] It was the first version suitable for use in production environments.[20]

It started a versioning system for the kernel with three or four numbers separated by dots where the first represented the major release, the second was the minor release, and the third was the revision.[31] At that time odd-numbered minor releases were for development and tests, whilst even numbered minor releases were for production. The optional fourth digit indicated a set of patches to a revision.[20]

The current version numbering is a little different from the above. First of all, the even vs. odd numbering has been dropped. Second, the first two numbers, taken as a whole, indicate a specific major version. While the time-frame is open for the development of the next major, the -rcN suffix is used to identify the n'th release candidate for the next version.[32] For example, the release of the version 4.16 was preceded by seven 4.16-rcN (from -rc1 to -rc7). Once a stable release is made, its maintenance is passed off to the “stable team". Occasional updates to stable releases are identified by a three numbering scheme (e.g., 4.13.1, 4.13.2, ..., 4.13.16).[32]

After version 1.3 of the kernel, Torvalds decided that Linux had evolved enough to warrant a new major number, so he released version 2.0.0 in June 1996.[33]

The developers contributing to the Linux kernel have thought it important that the kernel which Torvalds had written for Intel PCs supported different hardware architectures. The kernel can now run on CPUs from Intel (80386, 80486, 80686), Digital Equipment Corporation (Alpha), Motorola (MC680x0 and PowerPC), Silicon Graphics (MIPS) and Sun Microsystems (SPARC). Through an integrated FPU emulator, the Linux kernel can even run on hardware architectures that lack a floating point math coprocessor.[34] Developer teams were established for different architectures and with the release of Linux kernel version 2.0 the sources of the kernel include all components to configure the kernel for different architectures before compiling it.[23] Making the Linux kernel compatible with different file systems has also been a priority. The Linux kernel can operate file systems that have been formatted for Minix, Xenix or System V, while the umsdos format even allows for Linux to be installed within an MS-DOS partition.[34] In 1999, IBM published its patches to the Linux 2.2.13 code for the support of the S/390 architecture.[35]

In December 2003, with the release of kernel version 2.6.0, Torvalds regarded the Linux kernels to be so stable that he started the 2.6.x.y release cycle. Every 2.6 release was a kernel for production, development releases were indicated with -rc ("release candidate") appended to the version number. Stable 2.6 kernel releases started coming out on a regular schedule every 2–3 months, up through 2.6.39 in May 2011.[33] The shorter release cycle was the result of discussions among kernel developers regarding the release and version scheme in 2004.[36][37]

As a response to the lack of a stable kernel tree where people could coordinate the collection of bug fixes as such, in December 2005 Adrian Bunk announced that he would keep releasing 2.6.16.y kernels when the stable team moved on to 2.6.17.[38][39][40] Since then, the "stable team" had been formed, and it would keep updating stable kernel versions with bug fixes. In February 2008 Stephen Rothwell created the linux-next tree to serve as a place where patches aimed to be merged during the next development cycle are gathered.[41][42] Several subsystem maintainers also adopted the suffix -next for trees containing code which is meant to be submitted for inclusion in the next release cycle. As of January 2014, the in-development version of the Linux kernel is held in an unstable branch named linux-next.[43]

The Linux kernel source code used to be maintained without the help of an automated source code management system. In 2002, Linux kernel development switched to BitKeeper. It was made available to Torvalds and several others free of charge but was not free software. In April 2005, efforts to reverse-engineer the BitKeeper system by Andrew Tridgell led BitMover, the company which maintained BitKeeper, to stop supporting the Linux development community. In response, Torvalds and others wrote a new source code control system for the purpose, called Git. The new system was written within weeks, and in two months the first official kernel release was made using Git.[44]

The 20th anniversary of the Linux kernel was celebrated by Torvalds in July 2011 with the release of the 3.0.0 kernel version.[33] The Linux Foundation celebrated the 20th anniversary of the kernel in the 2011 edition of its kernel development study. Kernel 3.0 had 15 million lines of code and over 1,300 individual developers had contributed to this version of the Linux kernel. Volunteer developers contributed 16 percent of the total changes to the kernel in 2011. The other changes were contributions by professional software developers who were paid by a company to submit code to the kernel. In 2011, Red Hat developers contributed 10 percent of the changes made to the kernel, the second largest corporate contributor being Intel, followed by IBM and Novell. While Nokia had by that time abandoned its Linux kernel based smartphone operating system MeeGo, in 2011 Nokia developers still contributed more code to the kernel than the developers employed by Google, which had released Android in 2007 and Chrome OS in 2009. By 2011, Microsoft ranked as the 17th most prolific corporate contributor to the kernel. Its developers had first started contributing to the kernel in 2009 to improve the performance of Linux virtual machines running on top of the Windows Hyper-V hypervisor.[45]

Stable 3.x.y kernels were released until 3.19 in February 2015, with development releases carrying the -rc designation. To account for the occasional special patch release, the version 3 series of the kernel added a fourth digit to the version numbering. In April 2015, Torvalds released kernel version 4.0.[33] By February 2015 the Linux kernel had received contributions from nearly 12,000 programmers from more than 1,200 companies, including some of the world's largest software and hardware vendors.[46] Version 4.1 of the Linux kernel, released in June 2015, contains over 19.5 million lines of code contributed by almost 14,000 programmers.[47]

Popularity
Main article: Usage share of operating systems

According to the Stack Overflow’s annual Developer Survey of 2019, more than the 53% of all respondents have developed software for Linux OS and about 27% for Android,[48] although only about 25% develop with Linux-based operating systems.[49]

Linux distributions that bundle the Linux kernel with system software (eg., the GNU C Library, Systemd, and most of the others Unix utilities and daemons) and application software, and the Android operating system, which counts the majority of the installed base of all other operating systems for mobile devices,[50][51][52] are responsible for the rising popularity of Linux operating systems among users.[23]

The vast majority of website servers use Linux[53][54] and all of the world's 500 most powerful supercomputers use some kind of OS based on the Linux kernel.[6] However, the usage share of Linux distributions in desktops is low in comparison to other operating systems.

The Linux kernel is used in a wide variety of embedded devices, such as routers, wireless access points, PBXes, set-top boxes, FTA receivers, smart TVs, PVRs, NAS appliances, and webcams.

Architecture
Map of the Linux kernel

Linux is a monolithic kernel, supporting most features once only available in closed source kernels of non-free operating systems:

Device drivers and kernel extensions run in kernel space (ring 0 in many CPU architectures), with full access to the hardware, although some exceptions run in user space, for example, filesystems based on FUSE/CUSE, and parts of UIO.[67][68] The graphics system most people use with Linux does not run within the kernel. Unlike standard monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while the system is running. Also, unlike standard monolithic kernels, device drivers can be pre-empted under certain conditions; this feature was added to handle hardware interrupts correctly and to better support symmetric multiprocessing.[56] By choice, the Linux kernel has no stable device driver application binary interface.[16]

Linux typically makes use of memory protection and virtual memory and can also handle non-uniform memory access,[69] however the project has absorbed μClinux which also makes it possible to run Linux on microcontrollers without virtual memory.[70]

The hardware is also incorporated into the file hierarchy. Device drivers interface to user applications via an entry in the /dev or /sys directories.[71] Process information as well is mapped to the file system through the /proc directory.[71]

Various layers within Linux, also showing separation between the userland and kernel space
User mode User applications For example, bash, LibreOffice, GIMP, Blender, 0 A.D., Mozilla Firefox, etc.
Low-level system components: System daemons:
systemd, runit, logind, networkd, PulseAudio, ...		 Windowing system:
X11, Wayland, SurfaceFlinger (Android)		 Other libraries:
GTK+, Qt, EFL, SDL, SFML, FLTK, GNUstep, etc.		 Graphics:
Mesa, AMD Catalyst, ...
C standard library open(), exec(), sbrk(), socket(), fopen(), calloc(), ... (up to 2000 subroutines)
glibc aims to be POSIX/SUS-compatible, musl and uClibc target embedded systems, bionic written for Android, etc.
Kernel mode Linux kernel stat, splice, dup, read, open, ioctl, write, mmap, close, exit, etc. (about 380 system calls)
The Linux kernel System Call Interface (SCI, aims to be POSIX/SUS-compatible)
Process scheduling
subsystem		 IPC
subsystem		 Memory management
subsystem		 Virtual files
subsystem		 Network
subsystem
Other components: ALSA, DRI, evdev, LVM, device mapper, Linux Network Scheduler, Netfilter
Linux Security Modules: SELinux, TOMOYO, AppArmor, Smack
Hardware (CPU, main memory, data storage devices, etc.)

Programming language and coding style

The Linux kernel is written in the version of the C programming language supported by GCC (which has introduced a number of extensions and changes to standard C), together with a number of short sections of code written in the assembly language (in GCC's "AT&T-style" syntax) of the target architecture. Because of the extensions to C it supports, GCC was for a long time the only compiler capable of correctly building the Linux kernel. Since 2002 all the code must adhere to the 21 rules comprising the Linux Kernel Coding Style.[72][73]

GNU toolchain

The GNU Compiler Collection (GCC or GNU cc) is the default compiler for the mainline Linux kernel source and it is invoked by a utility called make. The GNU Assembler (more often called GAS or GNU as) is then used to produce object files from the GCC generated assembly. The GNU Linker (GNU ld) is then used to produce a statically linked executable Linux kernel file called vmlinux. as and ld are part of a package called GNU binutils. The above-mentioned tools are collectively known as the GNU toolchain.

Compiler compatibility

In 2004, Intel claimed to have modified the kernel so that its C compiler was also capable of compiling it.[74] There was another such reported success in 2009, with a modified 2.6.22 version of the kernel.[75][76]

Since 2010, effort has been underway to build the Linux kernel with Clang, an alternative compiler for the C language;[77] as of 12 April 2014, the official kernel could almost be compiled by Clang.[78][79] The project dedicated to this effort is named LLVMLinux after the LLVM compiler infrastructure upon which Clang is built.[80] LLVMLinux does not aim to fork either the Linux kernel or the LLVM, therefore it is a meta-project composed of patches that are eventually submitted to the upstream projects. By enabling the Linux kernel to be compiled by Clang that, among other advantages, is known for faster compilation compared with GCC, kernel developers may benefit from a faster workflow due to shorter compilation times.[81]

Interfaces
Four interfaces are distinguished: two internal to the kernel, and two between the kernel and userspace.

Linux is a clone of UNIX, therefore it aims towards POSIX and Single UNIX Specification (SUSv3) compliance.[82] Furthermore, since Linux is not UNIX, the kernel provides additional system calls and other interfaces that are Linux specific.[83] In order to be included in the official Linux kernel, the code must comply with a set of well defined licensing rules.[12][5]

Kernel-to-userspace API
Main articles: POSIX and Single UNIX Specification

The relevant standards aiming to achieve source code portability of user programs, that the development of the Linux kernel, the GNU C Library, and associated utilities try to adhere to, are POSIX and Single UNIX Specification.

The subset of the Linux kernel API that regard the interfaces exposed to user applications is fundamentally composed by UNIX and Linux-specific system calls. For example, among the Linux-specific there is the family of the clone system calls.[84] Most extensions must be enabled by defining the _GNU_SOURCE macro in a header file or when the user-land code is being compiled.[85]

System calls can only be invoked by using assembly instructions which enable the transition from unprivileged user space to privileged kernel space in ring 0, for example using the "int 0x80" instruction after copying a system call identifier and its parameters to some CPU registers. For this reason, the C Standard Library (libC) acts as a wrapper to most Linux system calls, by exposing C functions that, only whether it is needed[86], can transparently enter into the kernel which will execute on behalf of the calling process.[83] For those system calls not exposed by libC, e.g. the fast userspace mutex (futex),[87] the library provides a function called syscall which can be used to explicitly invoke them.[88]

Pseudo filesystems (e.g., the sysfs and procfs filesystems) and special files (e.g., /dev/random, /dev/sda, /dev/tty, and many others) constitute another layer of interface to kernel data structures representing hardware or logical (software) devices.[89][90]

Kernel-to-userspace ABI

Because of the differences existing between the hundreds of various implementations of the Linux OS, executable objects, even though they are compiled, assembled, and linked for running on a specific hardware architecture (that is, they use the ISA of the target hardware), often cannot run on different Linux Distributions. This issue is mainly due to distribution-specific configurations and set of patches applied to the code of the Linux kernel, differences in system libraries, services (daemons), filesystem hierarchies, and environment variables.

The main standard concerning application and binary compatibility of Linux distributions is the Linux Standard Base (LSB).[91][92] However, the LSB goes beyond what concerns the Linux kernel, because it also defines the desktop specifications, the X libraries and Qt that have little to do with it.[93] The LSB version 5 is built upon several standards and drafts (POSIX, SUS, X/Open, File System Hierarchy (FHS), and others).[94]

The parts of the LSB largely relevant to the Linux kernel are the General ABI (gABI)[95], especially the System V ABI[96][97] and the Executable and Linkable Format (ELF),[98] and the Processor Specific ABI (psABI), for example the Core Specification for X86-64.[99][100]

In-kernel API
At XDC2014, Alex Deucher from AMD announced the unified kernel-mode driver.[101] The proprietary Linux graphic driver, libGL-fglrx-glx, will share the same DRM infrastructure with Mesa 3D. As there is no stable in-kernel ABI, AMD had to constantly adapt the former binary blob used by Catalyst.

There are several kernel internal APIs utilized between the different subsystems. Some are available only within the kernel subsystems, while a somewhat limited set of in-kernel symbols (i.e., variables, data structures and functions) is exposed also to dynamically loadable modules (e.g., device drivers loaded on demand) whether they're exported with the EXPORT_SYMBOL() and EXPORT_SYMBOL_GPL() macros[102][103] (the latter reserved to modules released under a GPL-compatible license).[104]

Some in-kernel APIs have been kept stable over several releases, others have not. There are no guarantees regarding the in-kernel APIs. Maintainers and contributors are free to augment or change them at any time.[16] However, there are in-kernel APIs which manipulates data structures (e.g., lists, trees, queues) or perform common routines (e.g., copy data from and to user space, allocate memory, print lines to the system log, and so on) that have remained stable at least since Linux version 2.6.[105][106][107]

Examples of in-kernel APIs include software frameworks/APIs for the following classes of device drivers:

  • Video4Linux – for video capture hardware
  • Advanced Linux Sound Architecture (ALSA) – for sound cards
  • New API – for network interface controllers
  • Direct Rendering Manager – for graphics accelerators
  • KMS driver – for display controllers
  • mac80211 – for wireless network interface controllers[108]
  • WEXT – for wireless network interface controllers (obsolete).

In-kernel ABI

The Linux kernel developers choose not to maintain a stable in-kernel ABI.[109]

Technical features
The Linux Storage Stack Diagram[110]


Preemption

The Linux kernel provides preemptive scheduling under certain conditions. Until kernel version 2.4, only user processes were preemptive, i.e., in addition to time quantum expiration, an execution of current process in user mode would be interrupted if higher dynamic priority processes entered TASK_RUNNING state.[111] Toward 2.6 series of the Linux kernel, an ability to interrupt a task executing kernel code was added, although with that not all sections of the kernel code can be preempted.[112]

The Linux kernel contains different scheduler classes.[113] By default the kernel uses a scheduler mechanism called the Completely Fair Scheduler introduced in the 2.6.23 version of the kernel.[57] Internally this default-scheduler class is also known as SCHED_OTHER, but the kernel also contains two POSIX-compliant[114] real-time scheduling classes named SCHED_FIFO (realtime first-in-first-out) and SCHED_RR (realtime round-robin), both of which take precedence over the default class.[113] An additional scheduling policy known as SCHED_DEADLINE, implementing the earliest deadline first algorithm (EDF), was added in kernel version 3.14, released on 30 March 2014.[115][116] SCHED_DEADLINE takes precedence over all the other scheduling classes.

Through the use of the real-time Linux kernel patch PREEMPT_RT, support for full preemption of critical sections, interrupt handlers, and "interrupt disable" code sequences can be supported.[117] Partial mainline integration of the real-time Linux kernel patch already brought some functionality to the kernel mainline.[118] Preemption improves latency, increases responsiveness, and makes Linux more suitable for desktop and real-time applications. Older versions of the kernel had a so-called big kernel lock for synchronization across the entire kernel, which was finally removed by Arnd Bergmann in 2011.[119]

Portability
TiVo DVR, a consumer device running Linux

While not originally designed to be portable,[17][120] Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the ARM architecture to IBM z/Architecture mainframe computers. The first port beyond Linux's original 386 architecture was performed on the Motorola 68000 platform by Amiga users, who accomplished this by replacing major parts of the kernel. The modifications to the kernel were so fundamental that Torvalds viewed the Motorola version as a fork and a "Linux-like operating system"[120] rather than as an actual port. It was, however, the impetus that Torvalds needed to lead a major restructure of the kernel code to facilitate porting to competing computing architectures. The first Linux endorsed port was to the DEC Alpha AXP 64-bit platform which was demonstrated at DECUS in May 1995,[121] supporting both 386 and Alpha in a single source tree.[120] DEC was responsible for supplying the hardware necessary to Torvalds to enable a port of Linux to 64 bits[122] that same year.

Linux runs as the main operating system on IBM's Summit and all other fastest supercomputers, including the Chinese-designed and built Sunway TaihuLight (formerly fastest); as of October 2019, all of the world's 500 fastest supercomputers run some variant of Linux,[6] a big change from 1998 when the first Linux supercomputer got added to the list then ranked 113.[123] Unix had dominated the list previously; the list hit a roughly even split between Unix and Linux in about 2003.

Linux was also been ported to various handheld devices such as Apple's iPhone 3G and iPod.[124]

Kernel variants
An iPod booting iPodLinux

There are certain variants of the Linux kernel that provide additional functionality but do not belong to the Linux kernel mainline. Such variants of the Linux kernel include Linux-libre, Compute Node Linux, Cooperative Linux, Longene, grsecurity, INK, L4Linux, MkLinux, RTLinux, and User-Mode Linux (UML). Some of these variants have been partially merged into the mainline.[125] Some operating systems developed for mobile phones initially used heavily modified versions of the Linux kernel, including Google Android, Firefox OS, HP webOS, Nokia Maemo and Jolla Sailfish OS. In 2010, the Linux kernel community criticised Google for effectively starting its own kernel tree:[126][127]

This means that any drivers written for Android hardware platforms, can not get merged into the main kernel tree because they have dependencies on code that only lives in Google's kernel tree, causing it to fail to build in the kernel.org tree. Because of this, Google has now prevented a large chunk of hardware drivers and platform code from ever getting merged into the main kernel tree. Effectively creating a kernel branch that a number of different vendors are now relying on.[128]

— Greg Kroah-Hartman, 2010

Today Android uses a slightly customized Linux kernel[129] where changes are implemented in device drivers so that little or no change to the core kernel code is required. Android developers also submit patches to the mainline Linux kernel and the mainline kernel can boot the Android operating system. A Nexus 7 can boot and run a mainline Linux kernel.[129]

Kernel panic and oopses
An example of Linux kernel panic

In Linux, a "panic" is an unrecoverable system error detected by the kernel, as opposed to similar errors detected by user space code. It is possible for kernel code to indicate such a condition by calling the panic function located in the header file sys/system.h. However, most panics are the result of unhandled processor exceptions in kernel code, such as references to invalid memory addresses. These are typically indicative of a bug somewhere in the call chain leading to the panic. They can also indicate a failure of hardware, such as a failed RAM cell or errors in arithmetic functions in the processor caused by a processor bug, overheating/damaged processor, or a soft error.

A report of a non-fatal bug in the kernel is called an "oops"; such deviations from correct behavior of the Linux kernel may allow continued operation with compromised reliability.[130] These crash reports are automatically collected and can be sent upstream by various software, such as kerneloops,[131] ABRT (Fedora)[132] and apport (Ubuntu). KernelOops.org collects these reports and publishes statistics on their website.[133]

The kernel panic message might not be printed visibly in some conditions, such as when using a graphical desktop. To debug such conditions, other methods such as attaching a serial port console can be used.

Live patching

Rebootless updates can even be applied to the kernel by using live patching technologies such as Ksplice, kpatch and kGraft. Minimalistic foundations for live kernel patching were merged into the Linux kernel mainline in kernel version 4.0, which was released on 12 April 2015. Those foundations, known as livepatch and based primarily on the kernel's ftrace functionality, form a common core capable of supporting hot patching by both kGraft and kpatch, by providing an application programming interface (API) for kernel modules that contain hot patches and an application binary interface (ABI) for the userspace management utilities. However, the common core included into Linux kernel 4.0 supports only the x86 architecture and does not provide any mechanisms for ensuring function-level consistency while the hot patches are applied. As of April 2015, there is ongoing work on porting kpatch and kGraft to the common live patching core provided by the Linux kernel mainline.[134][135][136]

Security

Computer security is a much-publicized topic in relation to the Linux kernel because a large portion of the kernel bugs present potential security flaws. For example, they may allow for privilege escalation or create denial-of-service attack vectors. Over the years, numerous such flaws were found and fixed in the Linux kernel.[137] New security features are frequently implemented to improve the Linux kernel's security.[138][139]

Linux offers a wealth of mechanisms to reduce kernel attack surface and improve security which are collectively known as the Linux Security Modules (LSM).[140] They comprise the Security-Enhanced Linux (SELinux) module, whose code has been originally developed and then released to the public by the NSA[141], and AppArmor[66] among others. SELinux is now actively developed and maintained on GitHub.[65] SELinux and AppArmor provide support to access control security policies, including mandatory access control (MAC), though they profoundly differ in complexity and scope.

Another security feature is the Seccomp BPF (SECure COMPuting with Berkeley Packet Filters) which works by filtering parameters and reducing the set of system calls available to user-land applications.[142]

Critics have accused kernel developers of covering up security flaws or at least not announcing them; in 2008, Linus Torvalds responded to this with the following:[143][144]

I personally consider security bugs to be just "normal bugs". I don't cover them up, but I also don't have any reason what-so-ever to think it's a good idea to track them and announce them as something special...one reason I refuse to bother with the whole security circus is that I think it glorifies—and thus encourages—the wrong behavior. It makes "heroes" out of security people, as if the people who don't just fix normal bugs aren't as important. In fact, all the boring normal bugs are way more important, just because there's a lot more of them. I don't think some spectacular security hole should be glorified or cared about as being any more "special" than a random spectacular crash due to bad locking.

Linux distributions typically release security updates to fix vulnerabilities in the Linux kernel. Many offer long-term support releases that receive security updates for a certain Linux kernel version for an extended period of time.

Feature history

Version 1.0 of the Linux kernel was released on 14 March 1994.[145] This release of the Linux kernel only supported single-processor i386-based computer systems. Portability became a concern, and so version 1.2 (released 7 March 1995)[146] gained support for computer systems using processors based on the Alpha, SPARC, and MIPS architectures.

Version 2.0 was released on 9 June 1996.[147] The series included 41 releases. The major feature of 2.0 was support for symmetric multiprocessing (SMP) and support for more types of processors.

Version 2.2, released on 20 January 1999,[148] removed the global spinlock and provided improved SMP support, added support for the m68k and PowerPC architectures, and added new file systems (including read-only support for Microsoft's NTFS).[149]

Version 2.4.0, released on 4 January 2001,[150] contained support for ISA Plug and Play, USB, and PC Cards.[151] It also included support for the PA-RISC processor from Hewlett-Packard. Development for 2.4.x changed a bit in that more features were made available throughout the duration of the series, including support for Bluetooth, Logical Volume Manager (LVM) version 1, RAID support, InterMezzo and ext3 file systems.

Version 2.6.0 was released on 17 December 2003.[152] The development for 2.6.x changed further towards including new features throughout the duration of the series. Among the changes that have been made in the 2.6 series are: integration of µClinux into the mainline kernel sources, PAE support, support for several new lines of CPUs, integration of Advanced Linux Sound Architecture (ALSA) into the mainline kernel sources, support for up to 232 users (up from 216), support for up to 229 process IDs (64-bit only, 32-bit arches still limited to 215),[153] substantially increased the number of device types and the number of devices of each type, improved 64-bit support, support for file systems which support file sizes of up to 16 terabytes, in-kernel preemption, support for the Native POSIX Thread Library (NPTL), User-mode Linux integration into the mainline kernel sources, SELinux integration into the mainline kernel sources, InfiniBand support, and considerably more. Also notable are the addition of several file systems throughout the 2.6.x releases: FUSE, JFS, XFS, ext4 and more. Details on the history of the 2.6 kernel series can be found in the ChangeLog files on the 2.6 kernel series source code release area of kernel.org.[154]

Version 3.0 was released on 22 July 2011.[155] On 30 May 2011, Torvalds announced that the big change was "NOTHING. Absolutely nothing." and asked, "...let's make sure we really make the next release not just an all new shiny number, but a good kernel too."[156] After the expected 6–7 weeks of the development process, it would be released near the 20th anniversary of Linux.

On 11 December 2012, Torvalds decided to reduce kernel complexity by removing support for i386 processors, making the 3.7 kernel series the last one still supporting the original processor.[157][158] The same series unified support for the ARM processor.[159]

Version 3.11, released on 2 September 2013,[160] adds many new features such as new O_TMPFILE flag for open(2) to reduce temporary file vulnerabilities, experimental AMD Radeon dynamic power management, low-latency network polling, and zswap (compressed swap cache).[161]

Version 5.0, released in 2019,[162] includes patches for the Spectre and Meltdown security vulnerabilities and other feature enhancements.[163]

The numbering change from 2.6.39 to 3.0, and from 3.19 to 4.0, involved no meaningful technical differentiation. The major version number was increased to avoid large minor numbers.[155][164]


Development

Developer community

As of 2007, the development of the kernel had shifted from the top 20 most active developers writing 80% of the code to the top 30 writing 30% of the code, with top developers spending more time reviewing changes.[165] Developers can also be categorized by affiliation; in 2007, the top category was unknown while the top corporation was Red Hat with 12% of contributions, and known amateurs at 3.9%.[165] The kernel changes made in the year 2007 have been submitted by over 1900 developers, which may be a significant underestimate because developers working in teams usually count as one.

It is generally assumed that the community of Linux kernel developers comprises 5000 or 6000 members. Update from the 2016 Linux Kernel Development Report, issued by the Linux Foundation, covering the period from 3.18 (December 2014) to 4.7 (July 2016): About 1500 developers are contributing to each release from about 200-250 companies on average per release. The top 30 developers contributed a little more than 16% of the code. As of companies, the top contributors are Intel (12.9%) and Red Hat (8.0%), the third and fourth places are held by the 'none' (7.7%) and 'unknown' (6.8%) categories.

Instead of a roadmap, there are technical guidelines. Instead of a central resource allocation, there are persons and companies who all have a stake in the further development of the Linux kernel, quite independently from one another: People like Linus Torvalds and I don’t plan the kernel evolution. We don’t sit there and think up the roadmap for the next two years, then assign resources to the various new features. That's because we don’t have any resources. The resources are all owned by the various corporations who use and contribute to Linux, as well as by the various independent contributors out there. It's those people who own the resources who decide...

— Andrew Morton, 2005

Submitting code to the kernel

A developer who wants to change the Linux kernel starts with developing and testing that change. Depending on how significant the change is and how many subsystems it modifies, the change will either be submitted as a single patch or in multiple patches of source code. In case of a single subsystem that is maintained by a single maintainer, these patches are sent as e-mails to the maintainer of the subsystem with the appropriate mailing list in Cc. The maintainer and the readers of the mailing list will review the patches and provide feedback. Once the review process has finished the subsystem maintainer accepts the patches in the relevant Git kernel tree. If the changes to the Linux kernel are bug fixes that are considered important enough, a pull request for the patches will be sent to Torvalds within a few days. Otherwise, a pull request will be sent to Torvalds during the next merge window. The merge window usually lasts two weeks and starts immediately after the release of the previous kernel version.[166] The Git kernel source tree names all developers who have contributed to the Linux kernel in the Credits directory and all subsystem maintainers are listed in Maintainers.[167]

Development model

The Linux kernel project integrates new code on a rolling basis. Software checked into the project must work and compile without error. For each kernel subsystem there is a maintainer who is responsible for reviewing patches against the kernel code standards and keeps a queue of patches that can be submitted to Linus Torvalds within a merge window of several weeks. Patches are merged by Torvalds into the source code of the prior stable Linux kernel release, creating the -rc release candidate for the next stable kernel. Once the merge window is closed only fixes to the new code in the development release are accepted. The -rc development release of the kernel goes through regression tests and once it is judged to be stable by Torvalds and the kernel subsystem maintainers a new Linux kernel is released and the development process starts all over again.[168]

Developers who feel treated unfairly can report this to the Linux Foundation's Technical Advisory Board.[169] In July 2013 the maintainer of the USB 3.0 driver Sarah Sharp asked Torvalds to address the abusive commentary in the kernel development community. In 2014, Sharp backed out of Linux kernel development, saying that "The focus on technical excellence, in combination with overloaded maintainers, and people with different cultural and social norms, means that Linux kernel maintainers are often blunt, rude, or brutal to get their job done".[170] At the linux.conf.au (LCA) conference in 2018, developers expressed the view that the culture of the community has gotten much better in the past few years. Daniel Vetter, the maintainer of the Intel drm/i915 graphics kernel driver, commented that the "rather violent language and discussion" in the kernel community has decreased or disappeared.[171]

Laurent Pinchart asked developers for feedback on their experience with the kernel community at the 2017 Embedded Linux Conference Europe. The issues brought up were a few days later discussed at the Maintainers Summit. Concerns over the lack of consistency in how maintainers responded to patches submitted by developers were echoed by Shuah Khan, the maintainer of the kernel self-test framework. Torvalds contended that there would never be consistency in the handling of patches because different kernel subsystems have over time adopted different development processes. Therefore, it was agreed upon that each kernel subsystem maintainer would document the rules for patch acceptance.[172]

Development community conflicts

There have been several notable conflicts among Linux kernel developers. Examples of such conflicts are:

  • In July 2007, Con Kolivas announced that he would cease developing for the Linux kernel. Discussing his reasons in an interview, he expressed frustration with aspects of the mainline kernel development process, which he felt did not give sufficient priority to desktop interactivity, in addition to hacking taking a toll on his health, work and family.[173][174]
  • In July 2009, Alan Cox quit his role as the TTY layer maintainer after disagreement with Linus Torvalds about the scope of work required to fix an error in that subsystem.[175]
  • In December 2010, there was a discussion between Linux SCSI maintainer James Bottomley and SCST maintainer Vladislav Bolkhovitin about which SCSI target stack should be included in the Linux kernel – SCST or LIO. Although at that time SCST was considered technically superior, LIO was merged upstream.[176] This made some Linux users upset.[177]
  • In June 2012, Torvalds made it very clear that he did not agree with NVIDIA releasing its drivers as closed source drivers by showing the middle finger gesture.[178]
  • In April 2014, Torvalds banned Kay Sievers from submitting patches to the Linux kernel for failing to deal with bugs that caused systemd to negatively interact with the kernel.[179]
  • In October 2014, Lennart Poettering accused Torvalds of tolerating the rough discussion style on Linux kernel related mailing lists and of being a bad role model.[180]
  • In March 2015, Christoph Hellwig filed a lawsuit against VMware for infringement of the copyright on the Linux kernel.[181] Linus Torvalds made it clear that he did not agree with this and similar initiatives by calling lawyers a festering disease.[182]

Prominent Linux kernel developers have been aware of the importance of avoiding conflicts between developers.[183] For a long time there has been no code of conduct for kernel developers due to opposition by Linus Torvalds.[184] However, a Linux Kernel Code of Conflict was introduced on 8 March 2015.[185] It was replaced on 16 September 2018 by a new Code of Conduct based on the Contributor Covenant. This coincided with a public apology by Linus and a brief break from kernel development.[186][187] On 30 November 2018, complying with the Code of Conduct, Jarkko Sakkinen of Intel sent out patches replacing instances of "fuck" appearing in source code comments with suitable versions focused on the word 'hug'.[188]

Codebase

As of 2020, the 5.6 release of the Linux kernel had around 33 million lines of code, roughly 14% of the code is part of the "core" (arch, kernel and mm directories) while 60% is drivers.

Linux is evolution, not intelligent design!

— Linus Torvalds, 2005[189][190][191]

Estimated cost to redevelop
Redevelopment costs of Linux kernel

The cost to redevelop the Linux kernel version 2.6.0 in a traditional proprietary development setting has been estimated to be US$612 million (€467M, £394M) in 2004 prices using the COCOMO man-month estimation model.[192] In 2006, a study funded by the European Union put the redevelopment cost of kernel version 2.6.8 higher, at €882M ($1.14bn, £744M).[193]

This topic was revisited in October 2008 by Amanda McPherson, Brian Proffitt, and Ron Hale-Evans. Using David A. Wheeler's methodology, they estimated redevelopment of the 2.6.25 kernel now costs $1.3bn (part of a total $10.8bn to redevelop Fedora 9).[194] Again, Garcia-Garcia and Alonso de Magdaleno from University of Oviedo (Spain) estimate that the value annually added to kernel was about €100M between 2005 and 2007 and €225M in 2008, it would cost also more than €1bn (about $1.4bn as of February 2010) to develop in the European Union.[195]

As of 7 March 2011, using then-current LOC (lines of code) of a 2.6.x Linux kernel and wage numbers with David A. Wheeler's calculations it would cost approximately $3bn (about €2.2bn) to redevelop the Linux kernel as it keeps getting bigger. An updated calculation As of 26 September 2018, using then-current 20,088,609 LOC (lines of code) for the 4.14.14 Linux kernel and the current US National average programmer salary of $75,506 show it would cost approximately $14,725,449,000 dollars (£11,191,341,000 pounds) to rewrite the existing code.[196]

Maintenance and long-term support
Boot messages of a Linux kernel 2.6.25.17

The latest kernel version and older kernel versions are maintained separately. Most latest kernel releases were supervised by Linus Torvalds.[197] Current versions are released by Greg Kroah-Hartman.[198]

The Linux kernel developer community maintains a stable kernel by applying fixes for software bugs that have been discovered during the development of the subsequent stable kernel. Therefore, www.kernel.org will always list two stable kernels. The next stable Linux kernel is now released only 8 to 12 weeks later. Therefore, the Linux kernel maintainers have designated some stable kernel releases as longterm, these long-term support Linux kernels are updated with bug fixes for two or more years.[199] In November 2019 there were five longterm Linux kernels: 4.19.84, 4.14.154, 4.9.201, 4.4.201 and 3.16.76.[200] The full list of releases is at Linux kernel version history.

Relation with Linux distributions

Most Linux users run a kernel supplied by their Linux distribution. Some distributions ship the "vanilla" or "stable" kernels. However, several Linux distribution vendors (such as Red Hat and Debian) maintain another set of Linux kernel branches which are integrated into their products. These are usually updated at a slower pace compared to the "vanilla" branch, and they usually include all fixes from the relevant "stable" branch, but at the same time they can also add support for drivers or features which had not been released in the "vanilla" version the distribution vendor started basing their branch from.

Source code management

The Linux kernel development community uses Git to manage the kernel source code. Linus Torvalds initially developed this version control system with speed in mind and as a distributed system. Git users can obtain the latest pushed version of Torvalds' tree and keep up to date with the official kernel tree using the git pull.[201] The kernel source code is distributed in GNU zip (gzip) and bzip2 format. Source code contributions by developers are submitted as patches and incremental changes to the kernel source code means developers can seamlessly move from one Linux kernel version to the next.[202]

GPLv2 licensing terms

The Linux kernel is released under the GNU General Public License version 2 (GPLv2),[12][203] with some firmware images released under various non-free licenses.[204] Initially, Torvalds released Linux under a license which forbade any commercial use.[205] This was changed in version 0.12 by a switch to the GNU General Public License version 2 (GPLv2).[21] This license allows distribution and sale of possibly modified and unmodified versions of Linux but requires that all those copies be released under the same license and be accompanied by the complete corresponding source code.[206] Torvalds has described licensing Linux under the GPLv2 as the "best thing I ever did".[205]

The Linux kernel is licensed explicitly only under version 2 of the GPL,[12] without offering the licensee the option to choose "any later version", which is a common GPL extension. There was considerable debate about how easily the license could be changed to use later GPL versions (including version 3), and whether this change is even desirable.[207] Torvalds himself specifically indicated upon the release of version 2.4.0 that his own code is released only under version 2.[208] However, the terms of the GPL state that if no version is specified, then any version may be used,[209] and Alan Cox pointed out that very few other Linux contributors had specified a particular version of the GPL.[210]

In September 2006, a survey of 29 key kernel programmers indicated that 28 preferred GPLv2 to the then-current GPLv3 draft. Torvalds commented, "I think a number of outsiders... believed that I personally was just the odd man out because I've been so publicly not a huge fan of the GPLv3."[211] This group of high-profile kernel developers, including Torvalds, Greg Kroah-Hartman and Andrew Morton, commented on mass media about their objections to the GPLv3.[212] They referred to clauses regarding DRM/tivoization, patents, "additional restrictions" and warned a Balkanisation of the "Open Source Universe" by the GPLv3.[212][213] Linus Torvalds, who decided not to adopt the GPLv3 for the Linux kernel, reiterated his criticism even years later.[214]

Loadable kernel modules

It is debated whether loadable kernel modules (LKMs) are to be considered derivative works under copyright law, and thereby fall under the terms of the GPL.

Torvalds has stated his belief that LKMs using only a limited, "public" subset of the kernel interfaces can sometimes be non-derived works, thus allowing some binary-only drivers and other LKMs that are not licensed under the GPL. A good example for this is the usage of dma_buf by the proprietary Nvidia graphics drivers. dma_buf is a recent kernel feature (like the rest of the kernel, it is licensed under the GPL), which allows multiple GPUs to quickly copy data into each other's framebuffers.[215] One possible use case would be Nvidia Optimus that pairs a fast GPU with an Intel integrated GPU, where the Nvidia GPU writes into the Intel framebuffer when it is active. But, Nvidia cannot use this infrastructure because it uses a technical means to enforce the rule that it can only be used by LKMs that are also GPL. Alan Cox replied on LKML, rejecting a request from one of their engineers to remove this technical enforcement from the API.[216] Not all Linux kernel contributors agree with this interpretation, however, and even Torvalds agrees that many LKMs are clearly derived works, and indeed he writes that "kernel modules ARE derivative 'by default'".[217]

On the other hand, Torvalds has also said that "one gray area in particular is something like a driver that was originally written for another operating system (i.e. clearly not a derived work of Linux in origin). THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour".[218] Proprietary graphics drivers, in particular, are heavily discussed. Ultimately, it is likely that such questions can only be resolved by a court.

Firmware binary blobs

One point of licensing controversy is the use of firmware "binary blobs" in the Linux kernel to support several hardware devices. These files are under a variety of licenses, out of which many are restrictive and their exact underlying source code is usually unknown.[204]

The official kernel, that is the Linus git branch at the kernel.org repository, doesn't contain any kind of proprietary code;[12][5] however Linux can search the filesystems to locate proprietary firmware, drivers, and other executable modules, then it can load and link them into the kernel space.[219] Whenever proprietary modules are loaded into Linux, the kernel marks itself as being "tainted"[220] and therefore bug reports from tainted kernels will often be ignored by developers.

In 2002, Richard Stallman stated why, in his point of view, such blobs make the Linux kernel partially non-free software, and that distributing Linux kernel "violates the GPL", which requires "complete corresponding source code" to be available.[221] In 2008, Free Software Foundation Latin America started Linux-libre as a project that creates a completely free variant of the Linux kernel without proprietary objects; it is used by certain completely free Linux distributions, such as those endorsed by the Free Software Foundation, while it can also be used on most distributions.[222]

On 15 December 2010, the Debian Project announced that the next Debian stable version "6.0 Squeeze" would come with a kernel "stripped of all non-free firmware bits".[223] This policy continued to be applied in later stable Debian releases.

Trademark
See also: Linux: Copyright, trademark, and naming

Linux is a registered trademark of Linus Torvalds in the United States, the European Union, and some other countries.[224][225] This is the result of an incident in which William Della Croce, Jr., who was not involved in the Linux project, trademarked the name and subsequently demanded royalties for its use.[226] Several Linux backers retained legal counsel and filed suit against Della Croce. The issue was settled in August 1997 when the trademark was assigned to Linus Torvalds.[227][228]

SCO litigation

In early 2007, SCO filed the specific details of a purported copyright infringement. Despite previous claims that SCO was the rightful owner of 1 million lines of code, they specified only 326 lines of code, most of which were uncopyrightable.[229] In August 2007, the court in the Novell case ruled that SCO did not actually own the Unix copyrights, to begin with,[230] though the Tenth Circuit Court of Appeals ruled in August 2009 that the question of who owned the copyright properly remained for a jury to answer.[231] The jury case was decided on 30 March 2010 in Novell's favour.[232]

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