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. 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 2843. _`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 370All SPDX license identifiers and exceptions must have a corresponding file 371in the LICENSES subdirectories. This is required to allow tool 372verification (e.g. checkpatch.pl) and to have the licenses ready to read 373and extract right from the source, which is recommended by various FOSS 374organizations, e.g. the `FSFE REUSE initiative <https://reuse.software/>`_. 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 ============================= ============================================= 434 435 436 437