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

Add a file to the Documentation directory to describe how file licenses
should be described in all kernel files, using the SPDX identifier, as well
as where all licenses should be in the kernel source tree for people to
refer to (LICENSES/).

Thanks to Kate and Greg for review and editing and Jonas for the
suggestions concerning the meta tags in the licenses files.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Reviewed-by: NJonas Oberg <jonas@fsfe.org>
Reviewed-by: NJonathan Corbet <corbet@lwn.net>
Reviewed-by: NPhilippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: NKate Stewart <kstewart@linuxfoundation.org>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/index.rst

@@ -13,6 +13,18 @@ documents into a coherent whole.  Please note that improvements to the
 documentation are welcome; join the linux-doc list at vger.kernel.org if
 you want to help out.
 
+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
 ---------------------------
 

Documentation/process/license-rules.rst

+.. 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/>`_.