1 .. SPDX-License-Identifier: GPL-2.0
2
3 .. _kernel_licensing:
4
5 Linux kernel licensing rules
6 ============================
7
8 The Linux Kernel is provided under the terms of the GNU General Public
9 License version 2 only (GPL-2.0), as provided in LICENSES/preferred/GPL-2.0,
10 with an explicit syscall exception described in
11 LICENSES/exceptions/Linux-syscall-note, as described in the COPYING file.
12
13 This documentation file provides a description of how each source file
14 should be annotated to make its license clear and unambiguous.
15 It doesn't replace the Kernel's license.
16
17 The license described in the COPYING file applies to the kernel source
18 as a whole, though individual source files can have a different license
19 which is required to be compatible with the GPL-2.0::
20
21 GPL-1.0+ : GNU General Public License v1.0 or later
22 GPL-2.0+ : GNU General Public License v2.0 or later
23 LGPL-2.0 : GNU Library General Public License v2 only
24 LGPL-2.0+ : GNU Library General Public License v2 or later
25 LGPL-2.1 : GNU Lesser General Public License v2.1 only
26 LGPL-2.1+ : GNU Lesser General Public License v2.1 or later
27
28 Aside from that, individual files can be provided under a dual license,
29 e.g. one of the compatible GPL variants and alternatively under a
30 permissive license like BSD, MIT etc.
31
32 The User-space API (UAPI) header files, which describe the interface of
33 user-space programs to the kernel are a special case. According to the
34 note in the kernel COPYING file, the syscall interface is a clear boundary,
35 which does not extend the GPL requirements to any software which uses it to
36 communicate with the kernel. Because the UAPI headers must be includable
37 into any source files which create an executable running on the Linux
38 kernel, the exception must be documented by a special license expression.
39
40 The common way of expressing the license of a source file is to add the
41 matching boilerplate text into the top comment of the file. Due to
42 formatting, typos etc. these "boilerplates" are hard to validate for
43 tools which are used in the context of license compliance.
44
45 An alternative to boilerplate text is the use of Software Package Data
46 Exchange (SPDX) license identifiers in each source file. SPDX license
47 identifiers are machine parsable and precise shorthands for the license
48 under which the content of the file is contributed. SPDX license
49 identifiers are managed by the SPDX Workgroup at the Linux Foundation and
50 have been agreed on by partners throughout the industry, tool vendors, and
51 legal teams. For further information see https://spdx.org/
52
53 The Linux kernel requires the precise SPDX identifier in all source files.
54 The valid identifiers used in the kernel are explained in the section
55 `License identifiers`_ and have been retrieved from the official SPDX
56 license list at https://spdx.org/licenses/ along with the license texts.
57
58 License identifier syntax
59 -------------------------
60
61 1. Placement:
62
63 The SPDX license identifier in kernel files shall be added at the first
64 possible line in a file which can contain a comment. For the majority
65 of files this is the first line, except for scripts which require the
66 '#!PATH_TO_INTERPRETER' in the first line. For those scripts the SPDX
67 identifier goes into the second line.
68
69 |
70
71 2. Style:
72
73 The SPDX license identifier is added in form of a comment. The comment
74 style depends on the file type::
75
76 C source: // SPDX-License-Identifier: <SPDX License Expression>
77 C header: /* SPDX-License-Identifier: <SPDX License Expression> */
78 ASM: /* SPDX-License-Identifier: <SPDX License Expression> */
79 scripts: # SPDX-License-Identifier: <SPDX License Expression>
80 .rst: .. SPDX-License-Identifier: <SPDX License Expression>
81 .dts{i}: // SPDX-License-Identifier: <SPDX License Expression>
82
83 If a specific tool cannot handle the standard comment style, then the
84 appropriate comment mechanism which the tool accepts shall be used. This
85 is the reason for having the "/\* \*/" style comment in C header
86 files. There was build breakage observed with generated .lds files where
87 'ld' failed to parse the C++ comment. This has been fixed by now, but
88 there are still older assembler tools which cannot handle C++ style
89 comments.
90
91 |
92
93 3. Syntax:
94
95 A <SPDX License Expression> is either an SPDX short form license
96 identifier found on the SPDX License List, or the combination of two
97 SPDX short form license identifiers separated by "WITH" when a license
98 exception applies. When multiple licenses apply, an expression consists
99 of keywords "AND", "OR" separating sub-expressions and surrounded by
100 "(", ")" .
101
102 License identifiers for licenses like [L]GPL with the 'or later' option
103 are constructed by using a "+" for indicating the 'or later' option.::
104
105 // SPDX-License-Identifier: GPL-2.0+
106 // SPDX-License-Identifier: LGPL-2.1+
107
108 WITH should be used when there is a modifier to a license needed.
109 For example, the linux kernel UAPI files use the expression::
110
111 // SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note
112 // SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note
113
114 Other examples using WITH exceptions found in the kernel are::
115
116 // SPDX-License-Identifier: GPL-2.0 WITH mif-exception
117 // SPDX-License-Identifier: GPL-2.0+ WITH GCC-exception-2.0
118
119 Exceptions can only be used with particular License identifiers. The
120 valid License identifiers are listed in the tags of the exception text
121 file. For details see the point `Exceptions`_ in the chapter `License
122 identifiers`_.
123
124 OR should be used if the file is dual licensed and only one license is
125 to be selected. For example, some dtsi files are available under dual
126 licenses::
127
128 // SPDX-License-Identifier: GPL-2.0 OR BSD-3-Clause
129
130 Examples from the kernel for license expressions in dual licensed files::
131
132 // SPDX-License-Identifier: GPL-2.0 OR MIT
133 // SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
134 // SPDX-License-Identifier: GPL-2.0 OR Apache-2.0
135 // SPDX-License-Identifier: GPL-2.0 OR MPL-1.1
136 // SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) OR MIT
137 // SPDX-License-Identifier: GPL-1.0+ OR BSD-3-Clause OR OpenSSL
138
139 AND should be used if the file has multiple licenses whose terms all
140 apply to use the file. For example, if code is inherited from another
141 project and permission has been given to put it in the kernel, but the
142 original license terms need to remain in effect::
143
144 // SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) AND MIT
145
146 Another other example where both sets of license terms need to be
147 adhered to is::
148
149 // SPDX-License-Identifier: GPL-1.0+ AND LGPL-2.1+
150
151 License identifiers
152 -------------------
153
154 The licenses currently used, as well as the licenses for code added to the
155 kernel, can be broken down into:
156
157 1. _`Preferred licenses`:
158
159 Whenever possible these licenses should be used as they are known to be
160 fully compatible and widely used. These licenses are available from the
161 directory::
162
163 LICENSES/preferred/
164
165 in the kernel source tree.
166
167 The files in this directory contain the full license text and
168 `Metatags`_. The file names are identical to the SPDX license
169 identifier which shall be used for the license in source files.
170
171 Examples::
172
173 LICENSES/preferred/GPL-2.0
174
175 Contains the GPL version 2 license text and the required metatags::
176
177 LICENSES/preferred/MIT
178
179 Contains the MIT license text and the required metatags
180
181 _`Metatags`:
182
183 The following meta tags must be available in a license file:
184
185 - Valid-License-Identifier:
186
187 One or more lines which declare which License Identifiers are valid
188 inside the project to reference this particular license text. Usually
189 this is a single valid identifier, but e.g. for licenses with the 'or
190 later' options two identifiers are valid.
191
192 - SPDX-URL:
193
194 The URL of the SPDX page which contains additional information related
195 to the license.
196
197 - Usage-Guidance:
198
199 Freeform text for usage advice. The text must include correct examples
200 for the SPDX license identifiers as they should be put into source
201 files according to the `License identifier syntax`_ guidelines.
202
203 - License-Text:
204
205 All text after this tag is treated as the original license text
206
207 File format examples::
208
209 Valid-License-Identifier: GPL-2.0
210 Valid-License-Identifier: GPL-2.0+
211 SPDX-URL: https://spdx.org/licenses/GPL-2.0.html
212 Usage-Guide:
213 To use this license in source code, put one of the following SPDX
214 tag/value pairs into a comment according to the placement
215 guidelines in the licensing rules documentation.
216 For 'GNU General Public License (GPL) version 2 only' use:
217 SPDX-License-Identifier: GPL-2.0
218 For 'GNU General Public License (GPL) version 2 or any later version' use:
219 SPDX-License-Identifier: GPL-2.0+
220 License-Text:
221 Full license text
222
223 ::
224
225 SPDX-License-Identifier: MIT
226 SPDX-URL: https://spdx.org/licenses/MIT.html
227 Usage-Guide:
228 To use this license in source code, put the following SPDX
229 tag/value pair into a comment according to the placement
230 guidelines in the licensing rules documentation.
231 SPDX-License-Identifier: MIT
232 License-Text:
233 Full license text
234
235 |
236
237 2. Deprecated licenses:
238
239 These licenses should only be used for existing code or for importing
240 code from a different project. These licenses are available from the
241 directory::
242
243 LICENSES/deprecated/
244
245 in the kernel source tree.
246
247 The files in this directory contain the full license text and
248 `Metatags`_. The file names are identical to the SPDX license
249 identifier which shall be used for the license in source files.
250
251 Examples::
252
253 LICENSES/deprecated/ISC
254
255 Contains the Internet Systems Consortium license text and the required
256 metatags::
257
258 LICENSES/deprecated/GPL-1.0
259
260 Contains the GPL version 1 license text and the required metatags.
261
262 Metatags:
263
264 The metatag requirements for 'other' licenses are identical to the
265 requirements of the `Preferred licenses`_.
266
267 File format example::
268
269 Valid-License-Identifier: ISC
270 SPDX-URL: https://spdx.org/licenses/ISC.html
271 Usage-Guide:
272 Usage of this license in the kernel for new code is discouraged
273 and it should solely be used for importing code from an already
274 existing project.
275 To use this license in source code, put the following SPDX
276 tag/value pair into a comment according to the placement
277 guidelines in the licensing rules documentation.
278 SPDX-License-Identifier: ISC
279 License-Text:
280 Full license text
281
282 |
283
284 3. Dual Licensing Only
285
286 These licenses should only be used to dual license code with another
287 license in addition to a preferred license. These licenses are available
288 from the directory::
289
290 LICENSES/dual/
291
292 in the kernel source tree.
293
294 The files in this directory contain the full license text and
295 `Metatags`_. The file names are identical to the SPDX license
296 identifier which shall be used for the license in source files.
297
298 Examples::
299
300 LICENSES/dual/MPL-1.1
301
302 Contains the Mozilla Public License version 1.1 license text and the
303 required metatags::
304
305 LICENSES/dual/Apache-2.0
306
307 Contains the Apache License version 2.0 license text and the required
308 metatags.
309
310 Metatags:
311
312 The metatag requirements for 'other' licenses are identical to the
313 requirements of the `Preferred licenses`_.
314
315 File format example::
316
317 Valid-License-Identifier: MPL-1.1
318 SPDX-URL: https://spdx.org/licenses/MPL-1.1.html
319 Usage-Guide:
320 Do NOT use. The MPL-1.1 is not GPL2 compatible. It may only be used for
321 dual-licensed files where the other license is GPL2 compatible.
322 If you end up using this it MUST be used together with a GPL2 compatible
323 license using "OR".
324 To use the Mozilla Public License version 1.1 put the following SPDX
325 tag/value pair into a comment according to the placement guidelines in
326 the licensing rules documentation:
327 SPDX-License-Identifier: MPL-1.1
328 License-Text:
329 Full license text
330
331 |
332
333 4. _`Exceptions`:
334
335 Some licenses can be amended with exceptions which grant certain rights
336 which the original license does not. These exceptions are available
337 from the directory::
338
339 LICENSES/exceptions/
340
341 in the kernel source tree. The files in this directory contain the full
342 exception text and the required `Exception Metatags`_.
343
344 Examples::
345
346 LICENSES/exceptions/Linux-syscall-note
347
348 Contains the Linux syscall exception as documented in the COPYING
349 file of the Linux kernel, which is used for UAPI header files.
350 e.g. /\* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note \*/::
351
352 LICENSES/exceptions/GCC-exception-2.0
353
354 Contains the GCC 'linking exception' which allows to link any binary
355 independent of its license against the compiled version of a file marked
356 with this exception. This is required for creating runnable executables
357 from source code which is not compatible with the GPL.
358
359 _`Exception Metatags`:
360
361 The following meta tags must be available in an exception file:
362
363 - SPDX-Exception-Identifier:
364
365 One exception identifier which can be used with SPDX license
366 identifiers.
367
368 - SPDX-URL:
369
370 The URL of the SPDX page which contains additional information related
371 to the exception.
372
373 - SPDX-Licenses:
374
375 A comma separated list of SPDX license identifiers for which the
376 exception can be used.
377
378 - Usage-Guidance:
379
380 Freeform text for usage advice. The text must be followed by correct
381 examples for the SPDX license identifiers as they should be put into
382 source files according to the `License identifier syntax`_ guidelines.
383
384 - Exception-Text:
385
386 All text after this tag is treated as the original exception text
387
388 File format examples::
389
390 SPDX-Exception-Identifier: Linux-syscall-note
391 SPDX-URL: https://spdx.org/licenses/Linux-syscall-note.html
392 SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-1.0+, LGPL-2.0, LGPL-2.0+, LGPL-2.1, LGPL-2.1+
393 Usage-Guidance:
394 This exception is used together with one of the above SPDX-Licenses
395 to mark user-space API (uapi) header files so they can be included
396 into non GPL compliant user-space application code.
397 To use this exception add it with the keyword WITH to one of the
398 identifiers in the SPDX-Licenses tag:
399 SPDX-License-Identifier: <SPDX-License> WITH Linux-syscall-note
400 Exception-Text:
401 Full exception text
402
403 ::
404
405 SPDX-Exception-Identifier: GCC-exception-2.0
406 SPDX-URL: https://spdx.org/licenses/GCC-exception-2.0.html
407 SPDX-Licenses: GPL-2.0, GPL-2.0+
408 Usage-Guidance:
409 The "GCC Runtime Library exception 2.0" is used together with one
410 of the above SPDX-Licenses for code imported from the GCC runtime
411 library.
412 To use this exception add it with the keyword WITH to one of the
413 identifiers in the SPDX-Licenses tag:
414 SPDX-License-Identifier: <SPDX-License> WITH GCC-exception-2.0
415 Exception-Text:
416 Full exception text
417
418
419 All SPDX license identifiers and exceptions must have a corresponding file
420 in the LICENSES subdirectories. This is required to allow tool
421 verification (e.g. checkpatch.pl) and to have the licenses ready to read
422 and extract right from the source, which is recommended by various FOSS
423 organizations, e.g. the `FSFE REUSE initiative <https://reuse.software/>`_.
424
425 _`MODULE_LICENSE`
426 -----------------
427
428 Loadable kernel modules also require a MODULE_LICENSE() tag. This tag is
429 neither a replacement for proper source code license information
430 (SPDX-License-Identifier) nor in any way relevant for expressing or
431 determining the exact license under which the source code of the module
432 is provided.
433
434 The sole purpose of this tag is to provide sufficient information
435 whether the module is free software or proprietary for the kernel
436 module loader and for user space tools.
437
438 The valid license strings for MODULE_LICENSE() are:
439
440 ============================= =============================================
441 "GPL" Module is licensed under GPL version 2. This
442 does not express any distinction between
443 GPL-2.0-only or GPL-2.0-or-later. The exact
444 license information can only be determined
445 via the license information in the
446 corresponding source files.
447
448 "GPL v2" Same as "GPL". It exists for historic
449 reasons.
450
451 "GPL and additional rights" Historical variant of expressing that the
452 module source is dual licensed under a
453 GPL v2 variant and MIT license. Please do
454 not use in new code.
455
456 "Dual MIT/GPL" The correct way of expressing that the
457 module is dual licensed under a GPL v2
458 variant or MIT license choice.
459
460 "Dual BSD/GPL" The module is dual licensed under a GPL v2
461 variant or BSD license choice. The exact
462 variant of the BSD license can only be
463 determined via the license information
464 in the corresponding source files.
465
466 "Dual MPL/GPL" The module is dual licensed under a GPL v2
467 variant or Mozilla Public License (MPL)
468 choice. The exact variant of the MPL
469 license can only be determined via the
470 license information in the corresponding
471 source files.
472
473 "Proprietary" The module is under a proprietary license.
474 This string is solely for proprietary third
475 party modules and cannot be used for modules
476 which have their source code in the kernel
477 tree. Modules tagged that way are tainting
478 the kernel with the 'P' flag when loaded and
479 the kernel module loader refuses to link such
480 modules against symbols which are exported
481 with EXPORT_SYMBOL_GPL().
482 ============================= =============================================
483
484
485
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