Documentation: Add license-rules.rst to describe how to properly identify file licenses

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Licensing documentation

-----------------------

The following describes the license of the Linux kernel source code

(GPLv2), how to properly mark the license of individual files in the source

tree, as well as links to the full license text.

.. toctree::

   :maxdepth: 2

   process/license-rules.rst

User-oriented documentation

---------------------------

.. SPDX-License-Identifier: GPL-2.0

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 published by the Free Software

Foundation, and provided in the COPYING file.  This documentation file is

not meant to replace the COPYING file, but provides a description of how

each source file should be annotated to make the licensing it is governed

under clear and unambiguous.

The license 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

-------------------------

1. 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.

|

2. 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: <SPDX License Expression>

      C header:	/* SPDX-License-Identifier: <SPDX License Expression> */

      ASM:	/* SPDX-License-Identifier: <SPDX License Expression> */

      scripts:	# SPDX-License-Identifier: <SPDX License Expression>

      .rst:	.. SPDX-License-Identifier: <SPDX License Expression>

      .dts{i}:	// SPDX-License-Identifier: <SPDX License Expression>

   If a specific tool cannot handle the standard comment style, then the

   appropriate comment mechanism which the tool accepts shall be used. This

   is the reason for having the "/\* \*/" style comment in C header

   files. There was build breakage observed with generated .lds files where

   'ld' failed to parse the C++ comment. This has been fixed by now, but

   there are still older assembler tools which cannot handle C++ style

   comments.

|

3. Syntax:

   A <SPDX License Expression> 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

   Other examples using WITH exceptions found in the kernel are::

      // 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

   Examples from the kernel for license expressions in dual licensed files::

      // 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

   Another other example where both sets of license terms need to be

   adhered to is::

      // 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:

1. _`Preferred licenses`:

   Whenever possible these licenses should be used as they are known to be

   fully compatible and widely used.  These licenses are available from the

   directory::

      LICENSES/preferred/

   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::

      LICENSES/preferred/GPL-2.0

   Contains the GPL version 2 license text and the required metatags::

      LICENSES/preferred/MIT

   Contains the MIT license text and the required metatags

   _`Metatags`:

   The following meta tags must be available in a license file:

   - 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

   File format examples::

      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

|

2. 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::

      LICENSES/other/

   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::

      LICENSES/other/ISC

   Contains the Internet Systems Consortium license text and the required

   metatags::

      LICENSES/other/ZLib

   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

|

3. _`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::

      LICENSES/exceptions/

   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

   Contains the GCC 'linking exception' which allows to link any binary

   independent of its license against the compiled version of a file marked

   with this exception. This is required for creating runnable executables

   from source code which is not compatible with the GPL.

   _`Exception Metatags`:

   The following meta tags must be available in an exception file:

   - 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

   File format examples::

      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: <SPDX-License> 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: <SPDX-License> 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 <https://reuse.software/>`_.