license-rules.rst (18024B)
1.. SPDX-License-Identifier: GPL-2.0 2 3.. _kernel_licensing: 4 5Linux kernel licensing rules 6============================ 7 8The Linux Kernel is provided under the terms of the GNU General Public 9License version 2 only (GPL-2.0), as provided in LICENSES/preferred/GPL-2.0, 10with an explicit syscall exception described in 11LICENSES/exceptions/Linux-syscall-note, as described in the COPYING file. 12 13This documentation file provides a description of how each source file 14should be annotated to make its license clear and unambiguous. 15It doesn't replace the Kernel's license. 16 17The license described in the COPYING file applies to the kernel source 18as a whole, though individual source files can have a different license 19which 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 28Aside from that, individual files can be provided under a dual license, 29e.g. one of the compatible GPL variants and alternatively under a 30permissive license like BSD, MIT etc. 31 32The User-space API (UAPI) header files, which describe the interface of 33user-space programs to the kernel are a special case. According to the 34note in the kernel COPYING file, the syscall interface is a clear boundary, 35which does not extend the GPL requirements to any software which uses it to 36communicate with the kernel. Because the UAPI headers must be includable 37into any source files which create an executable running on the Linux 38kernel, the exception must be documented by a special license expression. 39 40The common way of expressing the license of a source file is to add the 41matching boilerplate text into the top comment of the file. Due to 42formatting, typos etc. these "boilerplates" are hard to validate for 43tools which are used in the context of license compliance. 44 45An alternative to boilerplate text is the use of Software Package Data 46Exchange (SPDX) license identifiers in each source file. SPDX license 47identifiers are machine parsable and precise shorthands for the license 48under which the content of the file is contributed. SPDX license 49identifiers are managed by the SPDX Workgroup at the Linux Foundation and 50have been agreed on by partners throughout the industry, tool vendors, and 51legal teams. For further information see https://spdx.org/ 52 53The Linux kernel requires the precise SPDX identifier in all source files. 54The valid identifiers used in the kernel are explained in the section 55`License identifiers`_ and have been retrieved from the official SPDX 56license list at https://spdx.org/licenses/ along with the license texts. 57 58License identifier syntax 59------------------------- 60 611. 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| 70 712. 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| 92 933. 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 151License identifiers 152------------------- 153 154The licenses currently used, as well as the licenses for code added to the 155kernel, can be broken down into: 156 1571. _`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 2372. 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 2843. 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 3334. _`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 419All SPDX license identifiers and exceptions must have a corresponding file 420in the LICENSES subdirectories. This is required to allow tool 421verification (e.g. checkpatch.pl) and to have the licenses ready to read 422and extract right from the source, which is recommended by various FOSS 423organizations, 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