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ReStructuredText
373 lines
14 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Linux kernel licensing rules
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============================
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The Linux Kernel is provided under the terms of the GNU General Public
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License version 2 only (GPL-2.0), as provided in LICENSES/preferred/GPL-2.0,
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with an explicit syscall exception described in
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LICENSES/exceptions/Linux-syscall-note, as described in the COPYING file.
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This documentation file provides a description of how each source file
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should be annotated to make its license clear and unambiguous.
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It doesn't replace the Kernel's license.
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The license described in the COPYING file applies to the kernel source
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as a whole, though individual source files can have a different license
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which is required to be compatible with the GPL-2.0::
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GPL-1.0+ : GNU General Public License v1.0 or later
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GPL-2.0+ : GNU General Public License v2.0 or later
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LGPL-2.0 : GNU Library General Public License v2 only
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LGPL-2.0+ : GNU Library General Public License v2 or later
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LGPL-2.1 : GNU Lesser General Public License v2.1 only
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LGPL-2.1+ : GNU Lesser General Public License v2.1 or later
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Aside from that, individual files can be provided under a dual license,
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e.g. one of the compatible GPL variants and alternatively under a
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permissive license like BSD, MIT etc.
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The User-space API (UAPI) header files, which describe the interface of
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user-space programs to the kernel are a special case. According to the
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note in the kernel COPYING file, the syscall interface is a clear boundary,
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which does not extend the GPL requirements to any software which uses it to
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communicate with the kernel. Because the UAPI headers must be includable
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into any source files which create an executable running on the Linux
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kernel, the exception must be documented by a special license expression.
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The common way of expressing the license of a source file is to add the
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matching boilerplate text into the top comment of the file. Due to
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formatting, typos etc. these "boilerplates" are hard to validate for
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tools which are used in the context of license compliance.
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An alternative to boilerplate text is the use of Software Package Data
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Exchange (SPDX) license identifiers in each source file. SPDX license
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identifiers are machine parsable and precise shorthands for the license
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under which the content of the file is contributed. SPDX license
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identifiers are managed by the SPDX Workgroup at the Linux Foundation and
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have been agreed on by partners throughout the industry, tool vendors, and
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legal teams. For further information see https://spdx.org/
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The Linux kernel requires the precise SPDX identifier in all source files.
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The valid identifiers used in the kernel are explained in the section
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`License identifiers`_ and have been retrieved from the official SPDX
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license list at https://spdx.org/licenses/ along with the license texts.
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License identifier syntax
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-------------------------
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1. Placement:
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The SPDX license identifier in kernel files shall be added at the first
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possible line in a file which can contain a comment. For the majority
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or files this is the first line, except for scripts which require the
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'#!PATH_TO_INTERPRETER' in the first line. For those scripts the SPDX
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identifier goes into the second line.
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2. Style:
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The SPDX license identifier is added in form of a comment. The comment
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style depends on the file type::
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C source: // SPDX-License-Identifier: <SPDX License Expression>
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C header: /* SPDX-License-Identifier: <SPDX License Expression> */
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ASM: /* SPDX-License-Identifier: <SPDX License Expression> */
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scripts: # SPDX-License-Identifier: <SPDX License Expression>
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.rst: .. SPDX-License-Identifier: <SPDX License Expression>
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.dts{i}: // SPDX-License-Identifier: <SPDX License Expression>
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If a specific tool cannot handle the standard comment style, then the
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appropriate comment mechanism which the tool accepts shall be used. This
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is the reason for having the "/\* \*/" style comment in C header
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files. There was build breakage observed with generated .lds files where
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'ld' failed to parse the C++ comment. This has been fixed by now, but
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there are still older assembler tools which cannot handle C++ style
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comments.
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3. Syntax:
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A <SPDX License Expression> is either an SPDX short form license
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identifier found on the SPDX License List, or the combination of two
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SPDX short form license identifiers separated by "WITH" when a license
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exception applies. When multiple licenses apply, an expression consists
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of keywords "AND", "OR" separating sub-expressions and surrounded by
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"(", ")" .
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License identifiers for licenses like [L]GPL with the 'or later' option
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are constructed by using a "+" for indicating the 'or later' option.::
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// SPDX-License-Identifier: GPL-2.0+
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// SPDX-License-Identifier: LGPL-2.1+
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WITH should be used when there is a modifier to a license needed.
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For example, the linux kernel UAPI files use the expression::
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// SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note
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// SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note
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Other examples using WITH exceptions found in the kernel are::
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// SPDX-License-Identifier: GPL-2.0 WITH mif-exception
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// SPDX-License-Identifier: GPL-2.0+ WITH GCC-exception-2.0
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Exceptions can only be used with particular License identifiers. The
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valid License identifiers are listed in the tags of the exception text
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file. For details see the point `Exceptions`_ in the chapter `License
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identifiers`_.
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OR should be used if the file is dual licensed and only one license is
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to be selected. For example, some dtsi files are available under dual
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licenses::
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// SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
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Examples from the kernel for license expressions in dual licensed files::
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// SPDX-License-Identifier: GPL-2.0 OR MIT
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// SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
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// SPDX-License-Identifier: GPL-2.0 OR Apache-2.0
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// SPDX-License-Identifier: GPL-2.0 OR MPL-1.1
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// SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) OR MIT
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// SPDX-License-Identifier: GPL-1.0+ OR BSD-3-Clause OR OpenSSL
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AND should be used if the file has multiple licenses whose terms all
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apply to use the file. For example, if code is inherited from another
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project and permission has been given to put it in the kernel, but the
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original license terms need to remain in effect::
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// SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) AND MIT
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Another other example where both sets of license terms need to be
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adhered to is::
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// SPDX-License-Identifier: GPL-1.0+ AND LGPL-2.1+
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License identifiers
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-------------------
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The licenses currently used, as well as the licenses for code added to the
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kernel, can be broken down into:
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1. _`Preferred licenses`:
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Whenever possible these licenses should be used as they are known to be
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fully compatible and widely used. These licenses are available from the
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directory::
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LICENSES/preferred/
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in the kernel source tree.
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The files in this directory contain the full license text and
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`Metatags`_. The file names are identical to the SPDX license
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identifier which shall be used for the license in source files.
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Examples::
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LICENSES/preferred/GPL-2.0
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Contains the GPL version 2 license text and the required metatags::
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LICENSES/preferred/MIT
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Contains the MIT license text and the required metatags
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_`Metatags`:
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The following meta tags must be available in a license file:
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- Valid-License-Identifier:
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One or more lines which declare which License Identifiers are valid
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inside the project to reference this particular license text. Usually
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this is a single valid identifier, but e.g. for licenses with the 'or
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later' options two identifiers are valid.
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- SPDX-URL:
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The URL of the SPDX page which contains additional information related
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to the license.
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- Usage-Guidance:
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Freeform text for usage advice. The text must include correct examples
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for the SPDX license identifiers as they should be put into source
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files according to the `License identifier syntax`_ guidelines.
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- License-Text:
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All text after this tag is treated as the original license text
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File format examples::
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Valid-License-Identifier: GPL-2.0
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Valid-License-Identifier: GPL-2.0+
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SPDX-URL: https://spdx.org/licenses/GPL-2.0.html
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Usage-Guide:
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To use this license in source code, put one of the following SPDX
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tag/value pairs into a comment according to the placement
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guidelines in the licensing rules documentation.
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For 'GNU General Public License (GPL) version 2 only' use:
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SPDX-License-Identifier: GPL-2.0
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For 'GNU General Public License (GPL) version 2 or any later version' use:
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SPDX-License-Identifier: GPL-2.0+
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License-Text:
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Full license text
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::
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SPDX-License-Identifier: MIT
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SPDX-URL: https://spdx.org/licenses/MIT.html
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Usage-Guide:
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To use this license in source code, put the following SPDX
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tag/value pair into a comment according to the placement
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guidelines in the licensing rules documentation.
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SPDX-License-Identifier: MIT
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License-Text:
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Full license text
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2. Not recommended licenses:
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These licenses should only be used for existing code or for importing
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code from a different project. These licenses are available from the
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directory::
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LICENSES/other/
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in the kernel source tree.
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The files in this directory contain the full license text and
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`Metatags`_. The file names are identical to the SPDX license
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identifier which shall be used for the license in source files.
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Examples::
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LICENSES/other/ISC
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Contains the Internet Systems Consortium license text and the required
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metatags::
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LICENSES/other/ZLib
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Contains the ZLIB license text and the required metatags.
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Metatags:
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The metatag requirements for 'other' licenses are identical to the
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requirements of the `Preferred licenses`_.
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File format example::
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Valid-License-Identifier: ISC
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SPDX-URL: https://spdx.org/licenses/ISC.html
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Usage-Guide:
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Usage of this license in the kernel for new code is discouraged
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and it should solely be used for importing code from an already
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existing project.
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To use this license in source code, put the following SPDX
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tag/value pair into a comment according to the placement
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guidelines in the licensing rules documentation.
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SPDX-License-Identifier: ISC
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License-Text:
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Full license text
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3. _`Exceptions`:
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Some licenses can be amended with exceptions which grant certain rights
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which the original license does not. These exceptions are available
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from the directory::
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LICENSES/exceptions/
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in the kernel source tree. The files in this directory contain the full
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exception text and the required `Exception Metatags`_.
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Examples::
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LICENSES/exceptions/Linux-syscall-note
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Contains the Linux syscall exception as documented in the COPYING
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file of the Linux kernel, which is used for UAPI header files.
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e.g. /\* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note \*/::
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LICENSES/exceptions/GCC-exception-2.0
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Contains the GCC 'linking exception' which allows to link any binary
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independent of its license against the compiled version of a file marked
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with this exception. This is required for creating runnable executables
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from source code which is not compatible with the GPL.
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_`Exception Metatags`:
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The following meta tags must be available in an exception file:
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- SPDX-Exception-Identifier:
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One exception identifier which can be used with SPDX license
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identifiers.
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- SPDX-URL:
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The URL of the SPDX page which contains additional information related
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to the exception.
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- SPDX-Licenses:
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A comma separated list of SPDX license identifiers for which the
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exception can be used.
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- Usage-Guidance:
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Freeform text for usage advice. The text must be followed by correct
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examples for the SPDX license identifiers as they should be put into
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source files according to the `License identifier syntax`_ guidelines.
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- Exception-Text:
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All text after this tag is treated as the original exception text
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File format examples::
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SPDX-Exception-Identifier: Linux-syscall-note
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SPDX-URL: https://spdx.org/licenses/Linux-syscall-note.html
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SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-1.0+, LGPL-2.0, LGPL-2.0+, LGPL-2.1, LGPL-2.1+
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Usage-Guidance:
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This exception is used together with one of the above SPDX-Licenses
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to mark user-space API (uapi) header files so they can be included
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into non GPL compliant user-space application code.
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To use this exception add it with the keyword WITH to one of the
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identifiers in the SPDX-Licenses tag:
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SPDX-License-Identifier: <SPDX-License> WITH Linux-syscall-note
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Exception-Text:
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Full exception text
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SPDX-Exception-Identifier: GCC-exception-2.0
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SPDX-URL: https://spdx.org/licenses/GCC-exception-2.0.html
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SPDX-Licenses: GPL-2.0, GPL-2.0+
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Usage-Guidance:
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The "GCC Runtime Library exception 2.0" is used together with one
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of the above SPDX-Licenses for code imported from the GCC runtime
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library.
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To use this exception add it with the keyword WITH to one of the
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identifiers in the SPDX-Licenses tag:
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SPDX-License-Identifier: <SPDX-License> WITH GCC-exception-2.0
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Exception-Text:
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Full exception text
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All SPDX license identifiers and exceptions must have a corresponding file
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in the LICENSE subdirectories. This is required to allow tool
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verification (e.g. checkpatch.pl) and to have the licenses ready to read
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and extract right from the source, which is recommended by various FOSS
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organizations, e.g. the `FSFE REUSE initiative <https://reuse.software/>`_.
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