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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