1.. SPDX-License-Identifier: GPL-2.0
2
3Linux kernel licensing rules
4============================
5
6The Linux Kernel is provided under the terms of the GNU General Public
7License version 2 only (GPL-2.0), as provided in LICENSES/preferred/GPL-2.0,
8with an explicit syscall exception described in
9LICENSES/exceptions/Linux-syscall-note, as described in the COPYING file.
10
11This documentation file provides a description of how each source file
12should be annotated to make its license clear and unambiguous.
13It doesn't replace the Kernel's license.
14
15The license described in the COPYING file applies to the kernel source
16as a whole, though individual source files can have a different license
17which is required to be compatible with the GPL-2.0::
18
19    GPL-1.0+  :  GNU General Public License v1.0 or later
20    GPL-2.0+  :  GNU General Public License v2.0 or later
21    LGPL-2.0  :  GNU Library General Public License v2 only
22    LGPL-2.0+ :  GNU Library General Public License v2 or later
23    LGPL-2.1  :  GNU Lesser General Public License v2.1 only
24    LGPL-2.1+ :  GNU Lesser General Public License v2.1 or later
25
26Aside from that, individual files can be provided under a dual license,
27e.g. one of the compatible GPL variants and alternatively under a
28permissive license like BSD, MIT etc.
29
30The User-space API (UAPI) header files, which describe the interface of
31user-space programs to the kernel are a special case.  According to the
32note in the kernel COPYING file, the syscall interface is a clear boundary,
33which does not extend the GPL requirements to any software which uses it to
34communicate with the kernel.  Because the UAPI headers must be includable
35into any source files which create an executable running on the Linux
36kernel, the exception must be documented by a special license expression.
37
38The common way of expressing the license of a source file is to add the
39matching boilerplate text into the top comment of the file.  Due to
40formatting, typos etc. these "boilerplates" are hard to validate for
41tools which are used in the context of license compliance.
42
43An alternative to boilerplate text is the use of Software Package Data
44Exchange (SPDX) license identifiers in each source file.  SPDX license
45identifiers are machine parsable and precise shorthands for the license
46under which the content of the file is contributed.  SPDX license
47identifiers are managed by the SPDX Workgroup at the Linux Foundation and
48have been agreed on by partners throughout the industry, tool vendors, and
49legal teams.  For further information see https://spdx.org/
50
51The Linux kernel requires the precise SPDX identifier in all source files.
52The valid identifiers used in the kernel are explained in the section
53`License identifiers`_ and have been retrieved from the official SPDX
54license list at https://spdx.org/licenses/ along with the license texts.
55
56License identifier syntax
57-------------------------
58
591. Placement:
60
61   The SPDX license identifier in kernel files shall be added at the first
62   possible line in a file which can contain a comment.  For the majority
63   or files this is the first line, except for scripts which require the
64   '#!PATH_TO_INTERPRETER' in the first line.  For those scripts the SPDX
65   identifier goes into the second line.
66
67|
68
692. Style:
70
71   The SPDX license identifier is added in form of a comment.  The comment
72   style depends on the file type::
73
74      C source:	// SPDX-License-Identifier: <SPDX License Expression>
75      C header:	/* SPDX-License-Identifier: <SPDX License Expression> */
76      ASM:	/* SPDX-License-Identifier: <SPDX License Expression> */
77      scripts:	# SPDX-License-Identifier: <SPDX License Expression>
78      .rst:	.. SPDX-License-Identifier: <SPDX License Expression>
79      .dts{i}:	// SPDX-License-Identifier: <SPDX License Expression>
80
81   If a specific tool cannot handle the standard comment style, then the
82   appropriate comment mechanism which the tool accepts shall be used. This
83   is the reason for having the "/\* \*/" style comment in C header
84   files. There was build breakage observed with generated .lds files where
85   'ld' failed to parse the C++ comment. This has been fixed by now, but
86   there are still older assembler tools which cannot handle C++ style
87   comments.
88
89|
90
913. Syntax:
92
93   A <SPDX License Expression> is either an SPDX short form license
94   identifier found on the SPDX License List, or the combination of two
95   SPDX short form license identifiers separated by "WITH" when a license
96   exception applies. When multiple licenses apply, an expression consists
97   of keywords "AND", "OR" separating sub-expressions and surrounded by
98   "(", ")" .
99
100   License identifiers for licenses like [L]GPL with the 'or later' option
101   are constructed by using a "+" for indicating the 'or later' option.::
102
103      // SPDX-License-Identifier: GPL-2.0+
104      // SPDX-License-Identifier: LGPL-2.1+
105
106   WITH should be used when there is a modifier to a license needed.
107   For example, the linux kernel UAPI files use the expression::
108
109      // SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note
110      // SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note
111
112   Other examples using WITH exceptions found in the kernel are::
113
114      // SPDX-License-Identifier: GPL-2.0 WITH mif-exception
115      // SPDX-License-Identifier: GPL-2.0+ WITH GCC-exception-2.0
116
117   Exceptions can only be used with particular License identifiers. The
118   valid License identifiers are listed in the tags of the exception text
119   file. For details see the point `Exceptions`_ in the chapter `License
120   identifiers`_.
121
122   OR should be used if the file is dual licensed and only one license is
123   to be selected.  For example, some dtsi files are available under dual
124   licenses::
125
126      // SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
127
128   Examples from the kernel for license expressions in dual licensed files::
129
130      // SPDX-License-Identifier: GPL-2.0 OR MIT
131      // SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
132      // SPDX-License-Identifier: GPL-2.0 OR Apache-2.0
133      // SPDX-License-Identifier: GPL-2.0 OR MPL-1.1
134      // SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) OR MIT
135      // SPDX-License-Identifier: GPL-1.0+ OR BSD-3-Clause OR OpenSSL
136
137   AND should be used if the file has multiple licenses whose terms all
138   apply to use the file. For example, if code is inherited from another
139   project and permission has been given to put it in the kernel, but the
140   original license terms need to remain in effect::
141
142      // SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) AND MIT
143
144   Another other example where both sets of license terms need to be
145   adhered to is::
146
147      // SPDX-License-Identifier: GPL-1.0+ AND LGPL-2.1+
148
149License identifiers
150-------------------
151
152The licenses currently used, as well as the licenses for code added to the
153kernel, can be broken down into:
154
1551. _`Preferred licenses`:
156
157   Whenever possible these licenses should be used as they are known to be
158   fully compatible and widely used.  These licenses are available from the
159   directory::
160
161      LICENSES/preferred/
162
163   in the kernel source tree.
164
165   The files in this directory contain the full license text and
166   `Metatags`_.  The file names are identical to the SPDX license
167   identifier which shall be used for the license in source files.
168
169   Examples::
170
171      LICENSES/preferred/GPL-2.0
172
173   Contains the GPL version 2 license text and the required metatags::
174
175      LICENSES/preferred/MIT
176
177   Contains the MIT license text and the required metatags
178
179   _`Metatags`:
180
181   The following meta tags must be available in a license file:
182
183   - Valid-License-Identifier:
184
185     One or more lines which declare which License Identifiers are valid
186     inside the project to reference this particular license text.  Usually
187     this is a single valid identifier, but e.g. for licenses with the 'or
188     later' options two identifiers are valid.
189
190   - SPDX-URL:
191
192     The URL of the SPDX page which contains additional information related
193     to the license.
194
195   - Usage-Guidance:
196
197     Freeform text for usage advice. The text must include correct examples
198     for the SPDX license identifiers as they should be put into source
199     files according to the `License identifier syntax`_ guidelines.
200
201   - License-Text:
202
203     All text after this tag is treated as the original license text
204
205   File format examples::
206
207      Valid-License-Identifier: GPL-2.0
208      Valid-License-Identifier: GPL-2.0+
209      SPDX-URL: https://spdx.org/licenses/GPL-2.0.html
210      Usage-Guide:
211        To use this license in source code, put one of the following SPDX
212	tag/value pairs into a comment according to the placement
213	guidelines in the licensing rules documentation.
214	For 'GNU General Public License (GPL) version 2 only' use:
215	  SPDX-License-Identifier: GPL-2.0
216	For 'GNU General Public License (GPL) version 2 or any later version' use:
217	  SPDX-License-Identifier: GPL-2.0+
218      License-Text:
219        Full license text
220
221   ::
222
223      SPDX-License-Identifier: MIT
224      SPDX-URL: https://spdx.org/licenses/MIT.html
225      Usage-Guide:
226	To use this license in source code, put the following SPDX
227	tag/value pair into a comment according to the placement
228	guidelines in the licensing rules documentation.
229	  SPDX-License-Identifier: MIT
230      License-Text:
231        Full license text
232
233|
234
2352. Not recommended licenses:
236
237   These licenses should only be used for existing code or for importing
238   code from a different project.  These licenses are available from the
239   directory::
240
241      LICENSES/other/
242
243   in the kernel source tree.
244
245   The files in this directory contain the full license text and
246   `Metatags`_.  The file names are identical to the SPDX license
247   identifier which shall be used for the license in source files.
248
249   Examples::
250
251      LICENSES/other/ISC
252
253   Contains the Internet Systems Consortium license text and the required
254   metatags::
255
256      LICENSES/other/ZLib
257
258   Contains the ZLIB license text and the required metatags.
259
260   Metatags:
261
262   The metatag requirements for 'other' licenses are identical to the
263   requirements of the `Preferred licenses`_.
264
265   File format example::
266
267      Valid-License-Identifier: ISC
268      SPDX-URL: https://spdx.org/licenses/ISC.html
269      Usage-Guide:
270        Usage of this license in the kernel for new code is discouraged
271	and it should solely be used for importing code from an already
272	existing project.
273        To use this license in source code, put the following SPDX
274	tag/value pair into a comment according to the placement
275	guidelines in the licensing rules documentation.
276	  SPDX-License-Identifier: ISC
277      License-Text:
278        Full license text
279
280|
281
2823. _`Exceptions`:
283
284   Some licenses can be amended with exceptions which grant certain rights
285   which the original license does not.  These exceptions are available
286   from the directory::
287
288      LICENSES/exceptions/
289
290   in the kernel source tree.  The files in this directory contain the full
291   exception text and the required `Exception Metatags`_.
292
293   Examples::
294
295      LICENSES/exceptions/Linux-syscall-note
296
297   Contains the Linux syscall exception as documented in the COPYING
298   file of the Linux kernel, which is used for UAPI header files.
299   e.g. /\* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note \*/::
300
301      LICENSES/exceptions/GCC-exception-2.0
302
303   Contains the GCC 'linking exception' which allows to link any binary
304   independent of its license against the compiled version of a file marked
305   with this exception. This is required for creating runnable executables
306   from source code which is not compatible with the GPL.
307
308   _`Exception Metatags`:
309
310   The following meta tags must be available in an exception file:
311
312   - SPDX-Exception-Identifier:
313
314     One exception identifier which can be used with SPDX license
315     identifiers.
316
317   - SPDX-URL:
318
319     The URL of the SPDX page which contains additional information related
320     to the exception.
321
322   - SPDX-Licenses:
323
324     A comma separated list of SPDX license identifiers for which the
325     exception can be used.
326
327   - Usage-Guidance:
328
329     Freeform text for usage advice. The text must be followed by correct
330     examples for the SPDX license identifiers as they should be put into
331     source files according to the `License identifier syntax`_ guidelines.
332
333   - Exception-Text:
334
335     All text after this tag is treated as the original exception text
336
337   File format examples::
338
339      SPDX-Exception-Identifier: Linux-syscall-note
340      SPDX-URL: https://spdx.org/licenses/Linux-syscall-note.html
341      SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-1.0+, LGPL-2.0, LGPL-2.0+, LGPL-2.1, LGPL-2.1+
342      Usage-Guidance:
343        This exception is used together with one of the above SPDX-Licenses
344	to mark user-space API (uapi) header files so they can be included
345	into non GPL compliant user-space application code.
346        To use this exception add it with the keyword WITH to one of the
347	identifiers in the SPDX-Licenses tag:
348	  SPDX-License-Identifier: <SPDX-License> WITH Linux-syscall-note
349      Exception-Text:
350        Full exception text
351
352   ::
353
354      SPDX-Exception-Identifier: GCC-exception-2.0
355      SPDX-URL: https://spdx.org/licenses/GCC-exception-2.0.html
356      SPDX-Licenses: GPL-2.0, GPL-2.0+
357      Usage-Guidance:
358        The "GCC Runtime Library exception 2.0" is used together with one
359	of the above SPDX-Licenses for code imported from the GCC runtime
360	library.
361        To use this exception add it with the keyword WITH to one of the
362	identifiers in the SPDX-Licenses tag:
363	  SPDX-License-Identifier: <SPDX-License> WITH GCC-exception-2.0
364      Exception-Text:
365        Full exception text
366
367
368All SPDX license identifiers and exceptions must have a corresponding file
369in the LICENSE subdirectories. This is required to allow tool
370verification (e.g. checkpatch.pl) and to have the licenses ready to read
371and extract right from the source, which is recommended by various FOSS
372organizations, e.g. the `FSFE REUSE initiative <https://reuse.software/>`_.
373