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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
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3 | .. _kernel_licensing: |
4 | ||
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5 | Linux kernel licensing rules |
6 | ============================ | |
7 | ||
8 | The Linux Kernel is provided under the terms of the GNU General Public | |
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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. | |
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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:: | |
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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 | |
61ab9fec | 65 | of files this is the first line, except for scripts which require the |
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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. Not recommended 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/other/ | |
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/other/ISC | |
254 | ||
255 | Contains the Internet Systems Consortium license text and the required | |
256 | metatags:: | |
257 | ||
258 | LICENSES/other/ZLib | |
259 | ||
260 | Contains the ZLIB 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. _`Exceptions`: | |
285 | ||
286 | Some licenses can be amended with exceptions which grant certain rights | |
287 | which the original license does not. These exceptions are available | |
288 | from the directory:: | |
289 | ||
290 | LICENSES/exceptions/ | |
291 | ||
292 | in the kernel source tree. The files in this directory contain the full | |
293 | exception text and the required `Exception Metatags`_. | |
294 | ||
295 | Examples:: | |
296 | ||
297 | LICENSES/exceptions/Linux-syscall-note | |
298 | ||
299 | Contains the Linux syscall exception as documented in the COPYING | |
300 | file of the Linux kernel, which is used for UAPI header files. | |
301 | e.g. /\* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note \*/:: | |
302 | ||
303 | LICENSES/exceptions/GCC-exception-2.0 | |
304 | ||
305 | Contains the GCC 'linking exception' which allows to link any binary | |
306 | independent of its license against the compiled version of a file marked | |
307 | with this exception. This is required for creating runnable executables | |
308 | from source code which is not compatible with the GPL. | |
309 | ||
310 | _`Exception Metatags`: | |
311 | ||
312 | The following meta tags must be available in an exception file: | |
313 | ||
314 | - SPDX-Exception-Identifier: | |
315 | ||
316 | One exception identifier which can be used with SPDX license | |
317 | identifiers. | |
318 | ||
319 | - SPDX-URL: | |
320 | ||
321 | The URL of the SPDX page which contains additional information related | |
322 | to the exception. | |
323 | ||
324 | - SPDX-Licenses: | |
325 | ||
326 | A comma separated list of SPDX license identifiers for which the | |
327 | exception can be used. | |
328 | ||
329 | - Usage-Guidance: | |
330 | ||
331 | Freeform text for usage advice. The text must be followed by correct | |
332 | examples for the SPDX license identifiers as they should be put into | |
333 | source files according to the `License identifier syntax`_ guidelines. | |
334 | ||
335 | - Exception-Text: | |
336 | ||
337 | All text after this tag is treated as the original exception text | |
338 | ||
339 | File format examples:: | |
340 | ||
341 | SPDX-Exception-Identifier: Linux-syscall-note | |
342 | SPDX-URL: https://spdx.org/licenses/Linux-syscall-note.html | |
343 | SPDX-Licenses: GPL-2.0, GPL-2.0+, GPL-1.0+, LGPL-2.0, LGPL-2.0+, LGPL-2.1, LGPL-2.1+ | |
344 | Usage-Guidance: | |
345 | This exception is used together with one of the above SPDX-Licenses | |
346 | to mark user-space API (uapi) header files so they can be included | |
347 | into non GPL compliant user-space application code. | |
348 | To use this exception add it with the keyword WITH to one of the | |
349 | identifiers in the SPDX-Licenses tag: | |
350 | SPDX-License-Identifier: <SPDX-License> WITH Linux-syscall-note | |
351 | Exception-Text: | |
352 | Full exception text | |
353 | ||
354 | :: | |
355 | ||
356 | SPDX-Exception-Identifier: GCC-exception-2.0 | |
357 | SPDX-URL: https://spdx.org/licenses/GCC-exception-2.0.html | |
358 | SPDX-Licenses: GPL-2.0, GPL-2.0+ | |
359 | Usage-Guidance: | |
360 | The "GCC Runtime Library exception 2.0" is used together with one | |
361 | of the above SPDX-Licenses for code imported from the GCC runtime | |
362 | library. | |
363 | To use this exception add it with the keyword WITH to one of the | |
364 | identifiers in the SPDX-Licenses tag: | |
365 | SPDX-License-Identifier: <SPDX-License> WITH GCC-exception-2.0 | |
366 | Exception-Text: | |
367 | Full exception text | |
368 | ||
369 | ||
370 | All SPDX license identifiers and exceptions must have a corresponding file | |
61ab9fec | 371 | in the LICENSES subdirectories. This is required to allow tool |
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372 | verification (e.g. checkpatch.pl) and to have the licenses ready to read |
373 | and extract right from the source, which is recommended by various FOSS | |
374 | organizations, e.g. the `FSFE REUSE initiative <https://reuse.software/>`_. | |
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375 | |
376 | _`MODULE_LICENSE` | |
377 | ----------------- | |
378 | ||
379 | Loadable kernel modules also require a MODULE_LICENSE() tag. This tag is | |
380 | neither a replacement for proper source code license information | |
381 | (SPDX-License-Identifier) nor in any way relevant for expressing or | |
382 | determining the exact license under which the source code of the module | |
383 | is provided. | |
384 | ||
385 | The sole purpose of this tag is to provide sufficient information | |
386 | whether the module is free software or proprietary for the kernel | |
387 | module loader and for user space tools. | |
388 | ||
389 | The valid license strings for MODULE_LICENSE() are: | |
390 | ||
391 | ============================= ============================================= | |
392 | "GPL" Module is licensed under GPL version 2. This | |
393 | does not express any distinction between | |
394 | GPL-2.0-only or GPL-2.0-or-later. The exact | |
395 | license information can only be determined | |
396 | via the license information in the | |
397 | corresponding source files. | |
398 | ||
399 | "GPL v2" Same as "GPL". It exists for historic | |
400 | reasons. | |
401 | ||
402 | "GPL and additional rights" Historical variant of expressing that the | |
403 | module source is dual licensed under a | |
404 | GPL v2 variant and MIT license. Please do | |
405 | not use in new code. | |
406 | ||
407 | "Dual MIT/GPL" The correct way of expressing that the | |
408 | module is dual licensed under a GPL v2 | |
409 | variant or MIT license choice. | |
410 | ||
411 | "Dual BSD/GPL" The module is dual licensed under a GPL v2 | |
412 | variant or BSD license choice. The exact | |
413 | variant of the BSD license can only be | |
414 | determined via the license information | |
415 | in the corresponding source files. | |
416 | ||
417 | "Dual MPL/GPL" The module is dual licensed under a GPL v2 | |
418 | variant or Mozilla Public License (MPL) | |
419 | choice. The exact variant of the MPL | |
420 | license can only be determined via the | |
421 | license information in the corresponding | |
422 | source files. | |
423 | ||
424 | "Proprietary" The module is under a proprietary license. | |
425 | This string is solely for proprietary third | |
426 | party modules and cannot be used for modules | |
427 | which have their source code in the kernel | |
428 | tree. Modules tagged that way are tainting | |
429 | the kernel with the 'P' flag when loaded and | |
430 | the kernel module loader refuses to link such | |
431 | modules against symbols which are exported | |
432 | with EXPORT_SYMBOL_GPL(). | |
433 | ============================= ============================================= | |
434 | ||
435 | ||
436 |