Linux license source code

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Linux kernel licensing rules¶
License identifier syntax¶
License identifiers¶

Linux kernel licensing rules¶

The Linux Kernel is provided under the terms of the GNU General Public License version 2 only (GPL-2.0), as provided in LICENSES/preferred/GPL-2.0, with an explicit syscall exception described in LICENSES/exceptions/Linux-syscall-note, as described in the COPYING file.

This documentation file provides a description of how each source file should be annotated to make its license clear and unambiguous. It doesn’t replace the Kernel’s license.

The license described in the COPYING file applies to the kernel source as a whole, though individual source files can have a different license which is required to be compatible with the GPL-2.0:

GPL-1.0+ : GNU General Public License v1.0 or later GPL-2.0+ : GNU General Public License v2.0 or later LGPL-2.0 : GNU Library General Public License v2 only LGPL-2.0+ : GNU Library General Public License v2 or later LGPL-2.1 : GNU Lesser General Public License v2.1 only LGPL-2.1+ : GNU Lesser General Public License v2.1 or later

Aside from that, individual files can be provided under a dual license, e.g. one of the compatible GPL variants and alternatively under a permissive license like BSD, MIT etc.

The User-space API (UAPI) header files, which describe the interface of user-space programs to the kernel are a special case. According to the note in the kernel COPYING file, the syscall interface is a clear boundary, which does not extend the GPL requirements to any software which uses it to communicate with the kernel. Because the UAPI headers must be includable into any source files which create an executable running on the Linux kernel, the exception must be documented by a special license expression.

The common way of expressing the license of a source file is to add the matching boilerplate text into the top comment of the file. Due to formatting, typos etc. these “boilerplates” are hard to validate for tools which are used in the context of license compliance.

An alternative to boilerplate text is the use of Software Package Data Exchange (SPDX) license identifiers in each source file. SPDX license identifiers are machine parsable and precise shorthands for the license under which the content of the file is contributed. SPDX license identifiers are managed by the SPDX Workgroup at the Linux Foundation and have been agreed on by partners throughout the industry, tool vendors, and legal teams. For further information see https://spdx.org/

The Linux kernel requires the precise SPDX identifier in all source files. The valid identifiers used in the kernel are explained in the section License identifiers and have been retrieved from the official SPDX license list at https://spdx.org/licenses/ along with the license texts.

License identifier syntax¶

Placement: The SPDX license identifier in kernel files shall be added at the first possible line in a file which can contain a comment. For the majority or files this is the first line, except for scripts which require the ‘#!PATH_TO_INTERPRETER’ in the first line. For those scripts the SPDX identifier goes into the second line.

Style: The SPDX license identifier is added in form of a comment. The comment style depends on the file type:

C source: // SPDX-License-Identifier: C header: /* SPDX-License-Identifier: */ ASM: /* SPDX-License-Identifier: */ scripts: # SPDX-License-Identifier: .rst: .. SPDX-License-Identifier: .dts: // SPDX-License-Identifier:

Syntax: A is either an SPDX short form license identifier found on the SPDX License List, or the combination of two SPDX short form license identifiers separated by “WITH” when a license exception applies. When multiple licenses apply, an expression consists of keywords “AND”, “OR” separating sub-expressions and surrounded by “(”, ”)” . License identifiers for licenses like [L]GPL with the ‘or later’ option are constructed by using a “+” for indicating the ‘or later’ option.:

// SPDX-License-Identifier: GPL-2.0+ // SPDX-License-Identifier: LGPL-2.1+

WITH should be used when there is a modifier to a license needed. For example, the linux kernel UAPI files use the expression:

// SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note // SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note

// SPDX-License-Identifier: GPL-2.0 WITH mif-exception // SPDX-License-Identifier: GPL-2.0+ WITH GCC-exception-2.0

Exceptions can only be used with particular License identifiers. The valid License identifiers are listed in the tags of the exception text file. For details see the point Exceptions in the chapter License identifiers. OR should be used if the file is dual licensed and only one license is to be selected. For example, some dtsi files are available under dual licenses:

// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause

// SPDX-License-Identifier: GPL-2.0 OR MIT // SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause // SPDX-License-Identifier: GPL-2.0 OR Apache-2.0 // SPDX-License-Identifier: GPL-2.0 OR MPL-1.1 // SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) OR MIT // SPDX-License-Identifier: GPL-1.0+ OR BSD-3-Clause OR OpenSSL

AND should be used if the file has multiple licenses whose terms all apply to use the file. For example, if code is inherited from another project and permission has been given to put it in the kernel, but the original license terms need to remain in effect:

// SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) AND MIT

// SPDX-License-Identifier: GPL-1.0+ AND LGPL-2.1+

License identifiers¶

The licenses currently used, as well as the licenses for code added to the kernel, can be broken down into:

in the kernel source tree. The files in this directory contain the full license text and Metatags. The file names are identical to the SPDX license identifier which shall be used for the license in source files. Examples:

Valid-License-Identifier: One or more lines which declare which License Identifiers are valid inside the project to reference this particular license text. Usually this is a single valid identifier, but e.g. for licenses with the ‘or later’ options two identifiers are valid.
SPDX-URL: The URL of the SPDX page which contains additional information related to the license.
Usage-Guidance: Freeform text for usage advice. The text must include correct examples for the SPDX license identifiers as they should be put into source files according to the License identifier syntax guidelines.
License-Text: All text after this tag is treated as the original license text

Valid-License-Identifier: GPL-2.0 Valid-License-Identifier: GPL-2.0+ SPDX-URL: https://spdx.org/licenses/GPL-2.0.html Usage-Guide: To use this license in source code, put one of the following SPDX tag/value pairs into a comment according to the placement guidelines in the licensing rules documentation. For 'GNU General Public License (GPL) version 2 only' use: SPDX-License-Identifier: GPL-2.0 For 'GNU General Public License (GPL) version 2 or any later version' use: SPDX-License-Identifier: GPL-2.0+ License-Text: Full license text

SPDX-License-Identifier: MIT SPDX-URL: https://spdx.org/licenses/MIT.html Usage-Guide: To use this license in source code, put the following SPDX tag/value pair into a comment according to the placement guidelines in the licensing rules documentation. SPDX-License-Identifier: MIT License-Text: Full license text

Not recommended licenses: These licenses should only be used for existing code or for importing code from a different project. These licenses are available from the directory:

Contains the ZLIB license text and the required metatags. Metatags: The metatag requirements for ‘other’ licenses are identical to the requirements of the Preferred licenses. File format example:

Valid-License-Identifier: ISC SPDX-URL: https://spdx.org/licenses/ISC.html Usage-Guide: Usage of this license in the kernel for new code is discouraged and it should solely be used for importing code from an already existing project. To use this license in source code, put the following SPDX tag/value pair into a comment according to the placement guidelines in the licensing rules documentation. SPDX-License-Identifier: ISC License-Text: Full license text

Exceptions : Some licenses can be amended with exceptions which grant certain rights which the original license does not. These exceptions are available from the directory:

in the kernel source tree. The files in this directory contain the full exception text and the required Exception Metatags. Examples:

LICENSES/exceptions/Linux-syscall-note

Contains the Linux syscall exception as documented in the COPYING file of the Linux kernel, which is used for UAPI header files. e.g. /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */:

LICENSES/exceptions/GCC-exception-2.0

SPDX-Exception-Identifier: One exception identifier which can be used with SPDX license identifiers.
SPDX-URL: The URL of the SPDX page which contains additional information related to the exception.
SPDX-Licenses: A comma separated list of SPDX license identifiers for which the exception can be used.
Usage-Guidance: Freeform text for usage advice. The text must be followed by correct examples for the SPDX license identifiers as they should be put into source files according to the License identifier syntax guidelines.
Exception-Text: All text after this tag is treated as the original exception text

SPDX-Exception-Identifier: Linux-syscall-note SPDX-URL: https://spdx.org/licenses/Linux-syscall-note.html SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-1.0+, LGPL-2.0, LGPL-2.0+, LGPL-2.1, LGPL-2.1+ Usage-Guidance: This exception is used together with one of the above SPDX-Licenses to mark user-space API (uapi) header files so they can be included into non GPL compliant user-space application code. To use this exception add it with the keyword WITH to one of the identifiers in the SPDX-Licenses tag: SPDX-License-Identifier: WITH Linux-syscall-note Exception-Text: Full exception text

SPDX-Exception-Identifier: GCC-exception-2.0 SPDX-URL: https://spdx.org/licenses/GCC-exception-2.0.html SPDX-Licenses: GPL-2.0, GPL-2.0+ Usage-Guidance: The "GCC Runtime Library exception 2.0" is used together with one of the above SPDX-Licenses for code imported from the GCC runtime library. To use this exception add it with the keyword WITH to one of the identifiers in the SPDX-Licenses tag: SPDX-License-Identifier: WITH GCC-exception-2.0 Exception-Text: Full exception text

All SPDX license identifiers and exceptions must have a corresponding file in the LICENSE subdirectories. This is required to allow tool verification (e.g. checkpatch.pl) and to have the licenses ready to read and extract right from the source, which is recommended by various FOSS organizations, e.g. the FSFE REUSE initiative.

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