1========================= 2Building External Modules 3========================= 4 5This document describes how to build an out-of-tree kernel module. 6 7.. Table of Contents 8 9 === 1 Introduction 10 === 2 How to Build External Modules 11 --- 2.1 Command Syntax 12 --- 2.2 Options 13 --- 2.3 Targets 14 --- 2.4 Building Separate Files 15 === 3. Creating a Kbuild File for an External Module 16 --- 3.1 Shared Makefile 17 --- 3.2 Separate Kbuild file and Makefile 18 --- 3.3 Binary Blobs 19 --- 3.4 Building Multiple Modules 20 === 4. Include Files 21 --- 4.1 Kernel Includes 22 --- 4.2 Single Subdirectory 23 --- 4.3 Several Subdirectories 24 === 5. Module Installation 25 --- 5.1 INSTALL_MOD_PATH 26 --- 5.2 INSTALL_MOD_DIR 27 === 6. Module Versioning 28 --- 6.1 Symbols From the Kernel (vmlinux + modules) 29 --- 6.2 Symbols and External Modules 30 --- 6.3 Symbols From Another External Module 31 === 7. Tips & Tricks 32 --- 7.1 Testing for CONFIG_FOO_BAR 33 34 35 361. Introduction 37=============== 38 39"kbuild" is the build system used by the Linux kernel. Modules must use 40kbuild to stay compatible with changes in the build infrastructure and 41to pick up the right flags to "gcc." Functionality for building modules 42both in-tree and out-of-tree is provided. The method for building 43either is similar, and all modules are initially developed and built 44out-of-tree. 45 46Covered in this document is information aimed at developers interested 47in building out-of-tree (or "external") modules. The author of an 48external module should supply a makefile that hides most of the 49complexity, so one only has to type "make" to build the module. This is 50easily accomplished, and a complete example will be presented in 51section 3. 52 53 542. How to Build External Modules 55================================ 56 57To build external modules, you must have a prebuilt kernel available 58that contains the configuration and header files used in the build. 59Also, the kernel must have been built with modules enabled. If you are 60using a distribution kernel, there will be a package for the kernel you 61are running provided by your distribution. 62 63An alternative is to use the "make" target "modules_prepare." This will 64make sure the kernel contains the information required. The target 65exists solely as a simple way to prepare a kernel source tree for 66building external modules. 67 68NOTE: "modules_prepare" will not build Module.symvers even if 69CONFIG_MODVERSIONS is set; therefore, a full kernel build needs to be 70executed to make module versioning work. 71 722.1 Command Syntax 73================== 74 75 The command to build an external module is:: 76 77 $ make -C <path_to_kernel_src> M=$PWD 78 79 The kbuild system knows that an external module is being built 80 due to the "M=<dir>" option given in the command. 81 82 To build against the running kernel use:: 83 84 $ make -C /lib/modules/`uname -r`/build M=$PWD 85 86 Then to install the module(s) just built, add the target 87 "modules_install" to the command:: 88 89 $ make -C /lib/modules/`uname -r`/build M=$PWD modules_install 90 912.2 Options 92=========== 93 94 ($KDIR refers to the path of the kernel source directory.) 95 96 make -C $KDIR M=$PWD 97 98 -C $KDIR 99 The directory where the kernel source is located. 100 "make" will actually change to the specified directory 101 when executing and will change back when finished. 102 103 M=$PWD 104 Informs kbuild that an external module is being built. 105 The value given to "M" is the absolute path of the 106 directory where the external module (kbuild file) is 107 located. 108 1092.3 Targets 110=========== 111 112 When building an external module, only a subset of the "make" 113 targets are available. 114 115 make -C $KDIR M=$PWD [target] 116 117 The default will build the module(s) located in the current 118 directory, so a target does not need to be specified. All 119 output files will also be generated in this directory. No 120 attempts are made to update the kernel source, and it is a 121 precondition that a successful "make" has been executed for the 122 kernel. 123 124 modules 125 The default target for external modules. It has the 126 same functionality as if no target was specified. See 127 description above. 128 129 modules_install 130 Install the external module(s). The default location is 131 /lib/modules/<kernel_release>/updates/, but a prefix may 132 be added with INSTALL_MOD_PATH (discussed in section 5). 133 134 clean 135 Remove all generated files in the module directory only. 136 137 help 138 List the available targets for external modules. 139 1402.4 Building Separate Files 141=========================== 142 143 It is possible to build single files that are part of a module. 144 This works equally well for the kernel, a module, and even for 145 external modules. 146 147 Example (The module foo.ko, consist of bar.o and baz.o):: 148 149 make -C $KDIR M=$PWD bar.lst 150 make -C $KDIR M=$PWD baz.o 151 make -C $KDIR M=$PWD foo.ko 152 make -C $KDIR M=$PWD ./ 153 154 1553. Creating a Kbuild File for an External Module 156================================================ 157 158In the last section we saw the command to build a module for the 159running kernel. The module is not actually built, however, because a 160build file is required. Contained in this file will be the name of 161the module(s) being built, along with the list of requisite source 162files. The file may be as simple as a single line:: 163 164 obj-m := <module_name>.o 165 166The kbuild system will build <module_name>.o from <module_name>.c, 167and, after linking, will result in the kernel module <module_name>.ko. 168The above line can be put in either a "Kbuild" file or a "Makefile." 169When the module is built from multiple sources, an additional line is 170needed listing the files:: 171 172 <module_name>-y := <src1>.o <src2>.o ... 173 174NOTE: Further documentation describing the syntax used by kbuild is 175located in Documentation/kbuild/makefiles.rst. 176 177The examples below demonstrate how to create a build file for the 178module 8123.ko, which is built from the following files:: 179 180 8123_if.c 181 8123_if.h 182 8123_pci.c 183 8123_bin.o_shipped <= Binary blob 184 1853.1 Shared Makefile 186------------------- 187 188 An external module always includes a wrapper makefile that 189 supports building the module using "make" with no arguments. 190 This target is not used by kbuild; it is only for convenience. 191 Additional functionality, such as test targets, can be included 192 but should be filtered out from kbuild due to possible name 193 clashes. 194 195 Example 1:: 196 197 --> filename: Makefile 198 ifneq ($(KERNELRELEASE),) 199 # kbuild part of makefile 200 obj-m := 8123.o 201 8123-y := 8123_if.o 8123_pci.o 8123_bin.o 202 203 else 204 # normal makefile 205 KDIR ?= /lib/modules/`uname -r`/build 206 207 default: 208 $(MAKE) -C $(KDIR) M=$$PWD 209 210 # Module specific targets 211 genbin: 212 echo "X" > 8123_bin.o_shipped 213 214 endif 215 216 The check for KERNELRELEASE is used to separate the two parts 217 of the makefile. In the example, kbuild will only see the two 218 assignments, whereas "make" will see everything except these 219 two assignments. This is due to two passes made on the file: 220 the first pass is by the "make" instance run on the command 221 line; the second pass is by the kbuild system, which is 222 initiated by the parameterized "make" in the default target. 223 2243.2 Separate Kbuild File and Makefile 225------------------------------------- 226 227 In newer versions of the kernel, kbuild will first look for a 228 file named "Kbuild," and only if that is not found, will it 229 then look for a makefile. Utilizing a "Kbuild" file allows us 230 to split up the makefile from example 1 into two files: 231 232 Example 2:: 233 234 --> filename: Kbuild 235 obj-m := 8123.o 236 8123-y := 8123_if.o 8123_pci.o 8123_bin.o 237 238 --> filename: Makefile 239 KDIR ?= /lib/modules/`uname -r`/build 240 241 default: 242 $(MAKE) -C $(KDIR) M=$$PWD 243 244 # Module specific targets 245 genbin: 246 echo "X" > 8123_bin.o_shipped 247 248 The split in example 2 is questionable due to the simplicity of 249 each file; however, some external modules use makefiles 250 consisting of several hundred lines, and here it really pays 251 off to separate the kbuild part from the rest. 252 253 The next example shows a backward compatible version. 254 255 Example 3:: 256 257 --> filename: Kbuild 258 obj-m := 8123.o 259 8123-y := 8123_if.o 8123_pci.o 8123_bin.o 260 261 --> filename: Makefile 262 ifneq ($(KERNELRELEASE),) 263 # kbuild part of makefile 264 include Kbuild 265 266 else 267 # normal makefile 268 KDIR ?= /lib/modules/`uname -r`/build 269 270 default: 271 $(MAKE) -C $(KDIR) M=$$PWD 272 273 # Module specific targets 274 genbin: 275 echo "X" > 8123_bin.o_shipped 276 277 endif 278 279 Here the "Kbuild" file is included from the makefile. This 280 allows an older version of kbuild, which only knows of 281 makefiles, to be used when the "make" and kbuild parts are 282 split into separate files. 283 2843.3 Binary Blobs 285---------------- 286 287 Some external modules need to include an object file as a blob. 288 kbuild has support for this, but requires the blob file to be 289 named <filename>_shipped. When the kbuild rules kick in, a copy 290 of <filename>_shipped is created with _shipped stripped off, 291 giving us <filename>. This shortened filename can be used in 292 the assignment to the module. 293 294 Throughout this section, 8123_bin.o_shipped has been used to 295 build the kernel module 8123.ko; it has been included as 296 8123_bin.o:: 297 298 8123-y := 8123_if.o 8123_pci.o 8123_bin.o 299 300 Although there is no distinction between the ordinary source 301 files and the binary file, kbuild will pick up different rules 302 when creating the object file for the module. 303 3043.4 Building Multiple Modules 305============================= 306 307 kbuild supports building multiple modules with a single build 308 file. For example, if you wanted to build two modules, foo.ko 309 and bar.ko, the kbuild lines would be:: 310 311 obj-m := foo.o bar.o 312 foo-y := <foo_srcs> 313 bar-y := <bar_srcs> 314 315 It is that simple! 316 317 3184. Include Files 319================ 320 321Within the kernel, header files are kept in standard locations 322according to the following rule: 323 324 * If the header file only describes the internal interface of a 325 module, then the file is placed in the same directory as the 326 source files. 327 * If the header file describes an interface used by other parts 328 of the kernel that are located in different directories, then 329 the file is placed in include/linux/. 330 331 NOTE: 332 There are two notable exceptions to this rule: larger 333 subsystems have their own directory under include/, such as 334 include/scsi; and architecture specific headers are located 335 under arch/$(SRCARCH)/include/. 336 3374.1 Kernel Includes 338------------------- 339 340 To include a header file located under include/linux/, simply 341 use:: 342 343 #include <linux/module.h> 344 345 kbuild will add options to "gcc" so the relevant directories 346 are searched. 347 3484.2 Single Subdirectory 349----------------------- 350 351 External modules tend to place header files in a separate 352 include/ directory where their source is located, although this 353 is not the usual kernel style. To inform kbuild of the 354 directory, use either ccflags-y or CFLAGS_<filename>.o. 355 356 Using the example from section 3, if we moved 8123_if.h to a 357 subdirectory named include, the resulting kbuild file would 358 look like:: 359 360 --> filename: Kbuild 361 obj-m := 8123.o 362 363 ccflags-y := -Iinclude 364 8123-y := 8123_if.o 8123_pci.o 8123_bin.o 365 366 Note that in the assignment there is no space between -I and 367 the path. This is a limitation of kbuild: there must be no 368 space present. 369 3704.3 Several Subdirectories 371-------------------------- 372 373 kbuild can handle files that are spread over several directories. 374 Consider the following example:: 375 376 . 377 |__ src 378 | |__ complex_main.c 379 | |__ hal 380 | |__ hardwareif.c 381 | |__ include 382 | |__ hardwareif.h 383 |__ include 384 |__ complex.h 385 386 To build the module complex.ko, we then need the following 387 kbuild file:: 388 389 --> filename: Kbuild 390 obj-m := complex.o 391 complex-y := src/complex_main.o 392 complex-y += src/hal/hardwareif.o 393 394 ccflags-y := -I$(src)/include 395 ccflags-y += -I$(src)/src/hal/include 396 397 As you can see, kbuild knows how to handle object files located 398 in other directories. The trick is to specify the directory 399 relative to the kbuild file's location. That being said, this 400 is NOT recommended practice. 401 402 For the header files, kbuild must be explicitly told where to 403 look. When kbuild executes, the current directory is always the 404 root of the kernel tree (the argument to "-C") and therefore an 405 absolute path is needed. $(src) provides the absolute path by 406 pointing to the directory where the currently executing kbuild 407 file is located. 408 409 4105. Module Installation 411====================== 412 413Modules which are included in the kernel are installed in the 414directory: 415 416 /lib/modules/$(KERNELRELEASE)/kernel/ 417 418And external modules are installed in: 419 420 /lib/modules/$(KERNELRELEASE)/updates/ 421 4225.1 INSTALL_MOD_PATH 423-------------------- 424 425 Above are the default directories but as always some level of 426 customization is possible. A prefix can be added to the 427 installation path using the variable INSTALL_MOD_PATH:: 428 429 $ make INSTALL_MOD_PATH=/frodo modules_install 430 => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/ 431 432 INSTALL_MOD_PATH may be set as an ordinary shell variable or, 433 as shown above, can be specified on the command line when 434 calling "make." This has effect when installing both in-tree 435 and out-of-tree modules. 436 4375.2 INSTALL_MOD_DIR 438------------------- 439 440 External modules are by default installed to a directory under 441 /lib/modules/$(KERNELRELEASE)/updates/, but you may wish to 442 locate modules for a specific functionality in a separate 443 directory. For this purpose, use INSTALL_MOD_DIR to specify an 444 alternative name to "updates.":: 445 446 $ make INSTALL_MOD_DIR=gandalf -C $KDIR \ 447 M=$PWD modules_install 448 => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf/ 449 450 4516. Module Versioning 452==================== 453 454Module versioning is enabled by the CONFIG_MODVERSIONS tag, and is used 455as a simple ABI consistency check. A CRC value of the full prototype 456for an exported symbol is created. When a module is loaded/used, the 457CRC values contained in the kernel are compared with similar values in 458the module; if they are not equal, the kernel refuses to load the 459module. 460 461Module.symvers contains a list of all exported symbols from a kernel 462build. 463 4646.1 Symbols From the Kernel (vmlinux + modules) 465----------------------------------------------- 466 467 During a kernel build, a file named Module.symvers will be 468 generated. Module.symvers contains all exported symbols from 469 the kernel and compiled modules. For each symbol, the 470 corresponding CRC value is also stored. 471 472 The syntax of the Module.symvers file is:: 473 474 <CRC> <Symbol> <Module> <Export Type> <Namespace> 475 476 0xe1cc2a05 usb_stor_suspend drivers/usb/storage/usb-storage EXPORT_SYMBOL_GPL USB_STORAGE 477 478 The fields are separated by tabs and values may be empty (e.g. 479 if no namespace is defined for an exported symbol). 480 481 For a kernel build without CONFIG_MODVERSIONS enabled, the CRC 482 would read 0x00000000. 483 484 Module.symvers serves two purposes: 485 486 1) It lists all exported symbols from vmlinux and all modules. 487 2) It lists the CRC if CONFIG_MODVERSIONS is enabled. 488 4896.2 Symbols and External Modules 490-------------------------------- 491 492 When building an external module, the build system needs access 493 to the symbols from the kernel to check if all external symbols 494 are defined. This is done in the MODPOST step. modpost obtains 495 the symbols by reading Module.symvers from the kernel source 496 tree. During the MODPOST step, a new Module.symvers file will be 497 written containing all exported symbols from that external module. 498 4996.3 Symbols From Another External Module 500---------------------------------------- 501 502 Sometimes, an external module uses exported symbols from 503 another external module. Kbuild needs to have full knowledge of 504 all symbols to avoid spitting out warnings about undefined 505 symbols. Two solutions exist for this situation. 506 507 NOTE: The method with a top-level kbuild file is recommended 508 but may be impractical in certain situations. 509 510 Use a top-level kbuild file 511 If you have two modules, foo.ko and bar.ko, where 512 foo.ko needs symbols from bar.ko, you can use a 513 common top-level kbuild file so both modules are 514 compiled in the same build. Consider the following 515 directory layout:: 516 517 ./foo/ <= contains foo.ko 518 ./bar/ <= contains bar.ko 519 520 The top-level kbuild file would then look like:: 521 522 #./Kbuild (or ./Makefile): 523 obj-m := foo/ bar/ 524 525 And executing:: 526 527 $ make -C $KDIR M=$PWD 528 529 will then do the expected and compile both modules with 530 full knowledge of symbols from either module. 531 532 Use "make" variable KBUILD_EXTRA_SYMBOLS 533 If it is impractical to add a top-level kbuild file, 534 you can assign a space separated list 535 of files to KBUILD_EXTRA_SYMBOLS in your build file. 536 These files will be loaded by modpost during the 537 initialization of its symbol tables. 538 539 5407. Tips & Tricks 541================ 542 5437.1 Testing for CONFIG_FOO_BAR 544------------------------------ 545 546 Modules often need to check for certain `CONFIG_` options to 547 decide if a specific feature is included in the module. In 548 kbuild this is done by referencing the `CONFIG_` variable 549 directly:: 550 551 #fs/ext2/Makefile 552 obj-$(CONFIG_EXT2_FS) += ext2.o 553 554 ext2-y := balloc.o bitmap.o dir.o 555 ext2-$(CONFIG_EXT2_FS_XATTR) += xattr.o 556 557 External modules have traditionally used "grep" to check for 558 specific `CONFIG_` settings directly in .config. This usage is 559 broken. As introduced before, external modules should use 560 kbuild for building and can therefore use the same methods as 561 in-tree modules when testing for `CONFIG_` definitions. 562