1============================================ 2Unreliable Guide To Hacking The Linux Kernel 3============================================ 4 5:Author: Rusty Russell 6 7Introduction 8============ 9 10Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux 11Kernel Hacking. This document describes the common routines and general 12requirements for kernel code: its goal is to serve as a primer for Linux 13kernel development for experienced C programmers. I avoid implementation 14details: that's what the code is for, and I ignore whole tracts of 15useful routines. 16 17Before you read this, please understand that I never wanted to write 18this document, being grossly under-qualified, but I always wanted to 19read it, and this was the only way. I hope it will grow into a 20compendium of best practice, common starting points and random 21information. 22 23The Players 24=========== 25 26At any time each of the CPUs in a system can be: 27 28- not associated with any process, serving a hardware interrupt; 29 30- not associated with any process, serving a softirq or tasklet; 31 32- running in kernel space, associated with a process (user context); 33 34- running a process in user space. 35 36There is an ordering between these. The bottom two can preempt each 37other, but above that is a strict hierarchy: each can only be preempted 38by the ones above it. For example, while a softirq is running on a CPU, 39no other softirq will preempt it, but a hardware interrupt can. However, 40any other CPUs in the system execute independently. 41 42We'll see a number of ways that the user context can block interrupts, 43to become truly non-preemptable. 44 45User Context 46------------ 47 48User context is when you are coming in from a system call or other trap: 49like userspace, you can be preempted by more important tasks and by 50interrupts. You can sleep, by calling :c:func:`schedule()`. 51 52.. note:: 53 54 You are always in user context on module load and unload, and on 55 operations on the block device layer. 56 57In user context, the ``current`` pointer (indicating the task we are 58currently executing) is valid, and :c:func:`in_interrupt()` 59(``include/linux/preempt.h``) is false. 60 61.. warning:: 62 63 Beware that if you have preemption or softirqs disabled (see below), 64 :c:func:`in_interrupt()` will return a false positive. 65 66Hardware Interrupts (Hard IRQs) 67------------------------------- 68 69Timer ticks, network cards and keyboard are examples of real hardware 70which produce interrupts at any time. The kernel runs interrupt 71handlers, which services the hardware. The kernel guarantees that this 72handler is never re-entered: if the same interrupt arrives, it is queued 73(or dropped). Because it disables interrupts, this handler has to be 74fast: frequently it simply acknowledges the interrupt, marks a 'software 75interrupt' for execution and exits. 76 77You can tell you are in a hardware interrupt, because 78:c:func:`in_irq()` returns true. 79 80.. warning:: 81 82 Beware that this will return a false positive if interrupts are 83 disabled (see below). 84 85Software Interrupt Context: Softirqs and Tasklets 86------------------------------------------------- 87 88Whenever a system call is about to return to userspace, or a hardware 89interrupt handler exits, any 'software interrupts' which are marked 90pending (usually by hardware interrupts) are run (``kernel/softirq.c``). 91 92Much of the real interrupt handling work is done here. Early in the 93transition to SMP, there were only 'bottom halves' (BHs), which didn't 94take advantage of multiple CPUs. Shortly after we switched from wind-up 95computers made of match-sticks and snot, we abandoned this limitation 96and switched to 'softirqs'. 97 98``include/linux/interrupt.h`` lists the different softirqs. A very 99important softirq is the timer softirq (``include/linux/timer.h``): you 100can register to have it call functions for you in a given length of 101time. 102 103Softirqs are often a pain to deal with, since the same softirq will run 104simultaneously on more than one CPU. For this reason, tasklets 105(``include/linux/interrupt.h``) are more often used: they are 106dynamically-registrable (meaning you can have as many as you want), and 107they also guarantee that any tasklet will only run on one CPU at any 108time, although different tasklets can run simultaneously. 109 110.. warning:: 111 112 The name 'tasklet' is misleading: they have nothing to do with 113 'tasks', and probably more to do with some bad vodka Alexey 114 Kuznetsov had at the time. 115 116You can tell you are in a softirq (or tasklet) using the 117:c:func:`in_softirq()` macro (``include/linux/preempt.h``). 118 119.. warning:: 120 121 Beware that this will return a false positive if a 122 :ref:`botton half lock <local_bh_disable>` is held. 123 124Some Basic Rules 125================ 126 127No memory protection 128 If you corrupt memory, whether in user context or interrupt context, 129 the whole machine will crash. Are you sure you can't do what you 130 want in userspace? 131 132No floating point or MMX 133 The FPU context is not saved; even in user context the FPU state 134 probably won't correspond with the current process: you would mess 135 with some user process' FPU state. If you really want to do this, 136 you would have to explicitly save/restore the full FPU state (and 137 avoid context switches). It is generally a bad idea; use fixed point 138 arithmetic first. 139 140A rigid stack limit 141 Depending on configuration options the kernel stack is about 3K to 142 6K for most 32-bit architectures: it's about 14K on most 64-bit 143 archs, and often shared with interrupts so you can't use it all. 144 Avoid deep recursion and huge local arrays on the stack (allocate 145 them dynamically instead). 146 147The Linux kernel is portable 148 Let's keep it that way. Your code should be 64-bit clean, and 149 endian-independent. You should also minimize CPU specific stuff, 150 e.g. inline assembly should be cleanly encapsulated and minimized to 151 ease porting. Generally it should be restricted to the 152 architecture-dependent part of the kernel tree. 153 154ioctls: Not writing a new system call 155===================================== 156 157A system call generally looks like this:: 158 159 asmlinkage long sys_mycall(int arg) 160 { 161 return 0; 162 } 163 164 165First, in most cases you don't want to create a new system call. You 166create a character device and implement an appropriate ioctl for it. 167This is much more flexible than system calls, doesn't have to be entered 168in every architecture's ``include/asm/unistd.h`` and 169``arch/kernel/entry.S`` file, and is much more likely to be accepted by 170Linus. 171 172If all your routine does is read or write some parameter, consider 173implementing a :c:func:`sysfs()` interface instead. 174 175Inside the ioctl you're in user context to a process. When a error 176occurs you return a negated errno (see 177``include/uapi/asm-generic/errno-base.h``, 178``include/uapi/asm-generic/errno.h`` and ``include/linux/errno.h``), 179otherwise you return 0. 180 181After you slept you should check if a signal occurred: the Unix/Linux 182way of handling signals is to temporarily exit the system call with the 183``-ERESTARTSYS`` error. The system call entry code will switch back to 184user context, process the signal handler and then your system call will 185be restarted (unless the user disabled that). So you should be prepared 186to process the restart, e.g. if you're in the middle of manipulating 187some data structure. 188 189:: 190 191 if (signal_pending(current)) 192 return -ERESTARTSYS; 193 194 195If you're doing longer computations: first think userspace. If you 196**really** want to do it in kernel you should regularly check if you need 197to give up the CPU (remember there is cooperative multitasking per CPU). 198Idiom:: 199 200 cond_resched(); /* Will sleep */ 201 202 203A short note on interface design: the UNIX system call motto is "Provide 204mechanism not policy". 205 206Recipes for Deadlock 207==================== 208 209You cannot call any routines which may sleep, unless: 210 211- You are in user context. 212 213- You do not own any spinlocks. 214 215- You have interrupts enabled (actually, Andi Kleen says that the 216 scheduling code will enable them for you, but that's probably not 217 what you wanted). 218 219Note that some functions may sleep implicitly: common ones are the user 220space access functions (\*_user) and memory allocation functions 221without ``GFP_ATOMIC``. 222 223You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on, 224and it will warn you if you break these rules. If you **do** break the 225rules, you will eventually lock up your box. 226 227Really. 228 229Common Routines 230=============== 231 232:c:func:`printk()` 233------------------ 234 235Defined in ``include/linux/printk.h`` 236 237:c:func:`printk()` feeds kernel messages to the console, dmesg, and 238the syslog daemon. It is useful for debugging and reporting errors, and 239can be used inside interrupt context, but use with caution: a machine 240which has its console flooded with printk messages is unusable. It uses 241a format string mostly compatible with ANSI C printf, and C string 242concatenation to give it a first "priority" argument:: 243 244 printk(KERN_INFO "i = %u\n", i); 245 246 247See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are 248interpreted by syslog as the level. Special case: for printing an IP 249address use:: 250 251 __be32 ipaddress; 252 printk(KERN_INFO "my ip: %pI4\n", &ipaddress); 253 254 255:c:func:`printk()` internally uses a 1K buffer and does not catch 256overruns. Make sure that will be enough. 257 258.. note:: 259 260 You will know when you are a real kernel hacker when you start 261 typoing printf as printk in your user programs :) 262 263.. note:: 264 265 Another sidenote: the original Unix Version 6 sources had a comment 266 on top of its printf function: "Printf should not be used for 267 chit-chat". You should follow that advice. 268 269:c:func:`copy_to_user()` / :c:func:`copy_from_user()` / :c:func:`get_user()` / :c:func:`put_user()` 270--------------------------------------------------------------------------------------------------- 271 272Defined in ``include/linux/uaccess.h`` / ``asm/uaccess.h`` 273 274**[SLEEPS]** 275 276:c:func:`put_user()` and :c:func:`get_user()` are used to get 277and put single values (such as an int, char, or long) from and to 278userspace. A pointer into userspace should never be simply dereferenced: 279data should be copied using these routines. Both return ``-EFAULT`` or 2800. 281 282:c:func:`copy_to_user()` and :c:func:`copy_from_user()` are 283more general: they copy an arbitrary amount of data to and from 284userspace. 285 286.. warning:: 287 288 Unlike :c:func:`put_user()` and :c:func:`get_user()`, they 289 return the amount of uncopied data (ie. 0 still means success). 290 291[Yes, this moronic interface makes me cringe. The flamewar comes up 292every year or so. --RR.] 293 294The functions may sleep implicitly. This should never be called outside 295user context (it makes no sense), with interrupts disabled, or a 296spinlock held. 297 298:c:func:`kmalloc()`/:c:func:`kfree()` 299------------------------------------- 300 301Defined in ``include/linux/slab.h`` 302 303**[MAY SLEEP: SEE BELOW]** 304 305These routines are used to dynamically request pointer-aligned chunks of 306memory, like malloc and free do in userspace, but 307:c:func:`kmalloc()` takes an extra flag word. Important values: 308 309``GFP_KERNEL`` 310 May sleep and swap to free memory. Only allowed in user context, but 311 is the most reliable way to allocate memory. 312 313``GFP_ATOMIC`` 314 Don't sleep. Less reliable than ``GFP_KERNEL``, but may be called 315 from interrupt context. You should **really** have a good 316 out-of-memory error-handling strategy. 317 318``GFP_DMA`` 319 Allocate ISA DMA lower than 16MB. If you don't know what that is you 320 don't need it. Very unreliable. 321 322If you see a sleeping function called from invalid context warning 323message, then maybe you called a sleeping allocation function from 324interrupt context without ``GFP_ATOMIC``. You should really fix that. 325Run, don't walk. 326 327If you are allocating at least ``PAGE_SIZE`` (``asm/page.h`` or 328``asm/page_types.h``) bytes, consider using :c:func:`__get_free_pages()` 329(``include/linux/gfp.h``). It takes an order argument (0 for page sized, 3301 for double page, 2 for four pages etc.) and the same memory priority 331flag word as above. 332 333If you are allocating more than a page worth of bytes you can use 334:c:func:`vmalloc()`. It'll allocate virtual memory in the kernel 335map. This block is not contiguous in physical memory, but the MMU makes 336it look like it is for you (so it'll only look contiguous to the CPUs, 337not to external device drivers). If you really need large physically 338contiguous memory for some weird device, you have a problem: it is 339poorly supported in Linux because after some time memory fragmentation 340in a running kernel makes it hard. The best way is to allocate the block 341early in the boot process via the :c:func:`alloc_bootmem()` 342routine. 343 344Before inventing your own cache of often-used objects consider using a 345slab cache in ``include/linux/slab.h`` 346 347:c:func:`current()` 348------------------- 349 350Defined in ``include/asm/current.h`` 351 352This global variable (really a macro) contains a pointer to the current 353task structure, so is only valid in user context. For example, when a 354process makes a system call, this will point to the task structure of 355the calling process. It is **not NULL** in interrupt context. 356 357:c:func:`mdelay()`/:c:func:`udelay()` 358------------------------------------- 359 360Defined in ``include/asm/delay.h`` / ``include/linux/delay.h`` 361 362The :c:func:`udelay()` and :c:func:`ndelay()` functions can be 363used for small pauses. Do not use large values with them as you risk 364overflow - the helper function :c:func:`mdelay()` is useful here, or 365consider :c:func:`msleep()`. 366 367:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()` 368----------------------------------------------------------------------------------------------- 369 370Defined in ``include/asm/byteorder.h`` 371 372The :c:func:`cpu_to_be32()` family (where the "32" can be replaced 373by 64 or 16, and the "be" can be replaced by "le") are the general way 374to do endian conversions in the kernel: they return the converted value. 375All variations supply the reverse as well: 376:c:func:`be32_to_cpu()`, etc. 377 378There are two major variations of these functions: the pointer 379variation, such as :c:func:`cpu_to_be32p()`, which take a pointer 380to the given type, and return the converted value. The other variation 381is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which 382convert value referred to by the pointer, and return void. 383 384:c:func:`local_irq_save()`/:c:func:`local_irq_restore()` 385-------------------------------------------------------- 386 387Defined in ``include/linux/irqflags.h`` 388 389These routines disable hard interrupts on the local CPU, and restore 390them. They are reentrant; saving the previous state in their one 391``unsigned long flags`` argument. If you know that interrupts are 392enabled, you can simply use :c:func:`local_irq_disable()` and 393:c:func:`local_irq_enable()`. 394 395.. _local_bh_disable: 396 397:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()` 398-------------------------------------------------------- 399 400Defined in ``include/linux/bottom_half.h`` 401 402 403These routines disable soft interrupts on the local CPU, and restore 404them. They are reentrant; if soft interrupts were disabled before, they 405will still be disabled after this pair of functions has been called. 406They prevent softirqs and tasklets from running on the current CPU. 407 408:c:func:`smp_processor_id()` 409---------------------------- 410 411Defined in ``include/linux/smp.h`` 412 413:c:func:`get_cpu()` disables preemption (so you won't suddenly get 414moved to another CPU) and returns the current processor number, between 4150 and ``NR_CPUS``. Note that the CPU numbers are not necessarily 416continuous. You return it again with :c:func:`put_cpu()` when you 417are done. 418 419If you know you cannot be preempted by another task (ie. you are in 420interrupt context, or have preemption disabled) you can use 421smp_processor_id(). 422 423``__init``/``__exit``/``__initdata`` 424------------------------------------ 425 426Defined in ``include/linux/init.h`` 427 428After boot, the kernel frees up a special section; functions marked with 429``__init`` and data structures marked with ``__initdata`` are dropped 430after boot is complete: similarly modules discard this memory after 431initialization. ``__exit`` is used to declare a function which is only 432required on exit: the function will be dropped if this file is not 433compiled as a module. See the header file for use. Note that it makes no 434sense for a function marked with ``__init`` to be exported to modules 435with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this 436will break. 437 438:c:func:`__initcall()`/:c:func:`module_init()` 439---------------------------------------------- 440 441Defined in ``include/linux/init.h`` / ``include/linux/module.h`` 442 443Many parts of the kernel are well served as a module 444(dynamically-loadable parts of the kernel). Using the 445:c:func:`module_init()` and :c:func:`module_exit()` macros it 446is easy to write code without #ifdefs which can operate both as a module 447or built into the kernel. 448 449The :c:func:`module_init()` macro defines which function is to be 450called at module insertion time (if the file is compiled as a module), 451or at boot time: if the file is not compiled as a module the 452:c:func:`module_init()` macro becomes equivalent to 453:c:func:`__initcall()`, which through linker magic ensures that 454the function is called on boot. 455 456The function can return a negative error number to cause module loading 457to fail (unfortunately, this has no effect if the module is compiled 458into the kernel). This function is called in user context with 459interrupts enabled, so it can sleep. 460 461:c:func:`module_exit()` 462----------------------- 463 464 465Defined in ``include/linux/module.h`` 466 467This macro defines the function to be called at module removal time (or 468never, in the case of the file compiled into the kernel). It will only 469be called if the module usage count has reached zero. This function can 470also sleep, but cannot fail: everything must be cleaned up by the time 471it returns. 472 473Note that this macro is optional: if it is not present, your module will 474not be removable (except for 'rmmod -f'). 475 476:c:func:`try_module_get()`/:c:func:`module_put()` 477------------------------------------------------- 478 479Defined in ``include/linux/module.h`` 480 481These manipulate the module usage count, to protect against removal (a 482module also can't be removed if another module uses one of its exported 483symbols: see below). Before calling into module code, you should call 484:c:func:`try_module_get()` on that module: if it fails, then the 485module is being removed and you should act as if it wasn't there. 486Otherwise, you can safely enter the module, and call 487:c:func:`module_put()` when you're finished. 488 489Most registerable structures have an owner field, such as in the 490:c:type:`struct file_operations <file_operations>` structure. 491Set this field to the macro ``THIS_MODULE``. 492 493Wait Queues ``include/linux/wait.h`` 494==================================== 495 496**[SLEEPS]** 497 498A wait queue is used to wait for someone to wake you up when a certain 499condition is true. They must be used carefully to ensure there is no 500race condition. You declare a :c:type:`wait_queue_head_t`, and then processes 501which want to wait for that condition declare a :c:type:`wait_queue_entry_t` 502referring to themselves, and place that in the queue. 503 504Declaring 505--------- 506 507You declare a ``wait_queue_head_t`` using the 508:c:func:`DECLARE_WAIT_QUEUE_HEAD()` macro, or using the 509:c:func:`init_waitqueue_head()` routine in your initialization 510code. 511 512Queuing 513------- 514 515Placing yourself in the waitqueue is fairly complex, because you must 516put yourself in the queue before checking the condition. There is a 517macro to do this: :c:func:`wait_event_interruptible()` 518(``include/linux/wait.h``) The first argument is the wait queue head, and 519the second is an expression which is evaluated; the macro returns 0 when 520this expression is true, or ``-ERESTARTSYS`` if a signal is received. The 521:c:func:`wait_event()` version ignores signals. 522 523Waking Up Queued Tasks 524---------------------- 525 526Call :c:func:`wake_up()` (``include/linux/wait.h``), which will wake 527up every process in the queue. The exception is if one has 528``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will 529not be woken. There are other variants of this basic function available 530in the same header. 531 532Atomic Operations 533================= 534 535Certain operations are guaranteed atomic on all platforms. The first 536class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``); 537this contains a signed integer (at least 32 bits long), and you must use 538these functions to manipulate or read :c:type:`atomic_t` variables. 539:c:func:`atomic_read()` and :c:func:`atomic_set()` get and set 540the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`, 541:c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and 542:c:func:`atomic_dec_and_test()` (returns true if it was 543decremented to zero). 544 545Yes. It returns true (i.e. != 0) if the atomic variable is zero. 546 547Note that these functions are slower than normal arithmetic, and so 548should not be used unnecessarily. 549 550The second class of atomic operations is atomic bit operations on an 551``unsigned long``, defined in ``include/linux/bitops.h``. These 552operations generally take a pointer to the bit pattern, and a bit 553number: 0 is the least significant bit. :c:func:`set_bit()`, 554:c:func:`clear_bit()` and :c:func:`change_bit()` set, clear, 555and flip the given bit. :c:func:`test_and_set_bit()`, 556:c:func:`test_and_clear_bit()` and 557:c:func:`test_and_change_bit()` do the same thing, except return 558true if the bit was previously set; these are particularly useful for 559atomically setting flags. 560 561It is possible to call these operations with bit indices greater than 562``BITS_PER_LONG``. The resulting behavior is strange on big-endian 563platforms though so it is a good idea not to do this. 564 565Symbols 566======= 567 568Within the kernel proper, the normal linking rules apply (ie. unless a 569symbol is declared to be file scope with the ``static`` keyword, it can 570be used anywhere in the kernel). However, for modules, a special 571exported symbol table is kept which limits the entry points to the 572kernel proper. Modules can also export symbols. 573 574:c:func:`EXPORT_SYMBOL()` 575------------------------- 576 577Defined in ``include/linux/export.h`` 578 579This is the classic method of exporting a symbol: dynamically loaded 580modules will be able to use the symbol as normal. 581 582:c:func:`EXPORT_SYMBOL_GPL()` 583----------------------------- 584 585Defined in ``include/linux/export.h`` 586 587Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols 588exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by 589modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL 590compatible license. It implies that the function is considered an 591internal implementation issue, and not really an interface. Some 592maintainers and developers may however require EXPORT_SYMBOL_GPL() 593when adding any new APIs or functionality. 594 595Routines and Conventions 596======================== 597 598Double-linked lists ``include/linux/list.h`` 599-------------------------------------------- 600 601There used to be three sets of linked-list routines in the kernel 602headers, but this one is the winner. If you don't have some particular 603pressing need for a single list, it's a good choice. 604 605In particular, :c:func:`list_for_each_entry()` is useful. 606 607Return Conventions 608------------------ 609 610For code called in user context, it's very common to defy C convention, 611and return 0 for success, and a negative error number (eg. ``-EFAULT``) for 612failure. This can be unintuitive at first, but it's fairly widespread in 613the kernel. 614 615Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a 616negative error number into a pointer, and :c:func:`IS_ERR()` and 617:c:func:`PTR_ERR()` to get it back out again: avoids a separate 618pointer parameter for the error number. Icky, but in a good way. 619 620Breaking Compilation 621-------------------- 622 623Linus and the other developers sometimes change function or structure 624names in development kernels; this is not done just to keep everyone on 625their toes: it reflects a fundamental change (eg. can no longer be 626called with interrupts on, or does extra checks, or doesn't do checks 627which were caught before). Usually this is accompanied by a fairly 628complete note to the linux-kernel mailing list; search the archive. 629Simply doing a global replace on the file usually makes things **worse**. 630 631Initializing structure members 632------------------------------ 633 634The preferred method of initializing structures is to use designated 635initialisers, as defined by ISO C99, eg:: 636 637 static struct block_device_operations opt_fops = { 638 .open = opt_open, 639 .release = opt_release, 640 .ioctl = opt_ioctl, 641 .check_media_change = opt_media_change, 642 }; 643 644 645This makes it easy to grep for, and makes it clear which structure 646fields are set. You should do this because it looks cool. 647 648GNU Extensions 649-------------- 650 651GNU Extensions are explicitly allowed in the Linux kernel. Note that 652some of the more complex ones are not very well supported, due to lack 653of general use, but the following are considered standard (see the GCC 654info page section "C Extensions" for more details - Yes, really the info 655page, the man page is only a short summary of the stuff in info). 656 657- Inline functions 658 659- Statement expressions (ie. the ({ and }) constructs). 660 661- Declaring attributes of a function / variable / type 662 (__attribute__) 663 664- typeof 665 666- Zero length arrays 667 668- Macro varargs 669 670- Arithmetic on void pointers 671 672- Non-Constant initializers 673 674- Assembler Instructions (not outside arch/ and include/asm/) 675 676- Function names as strings (__func__). 677 678- __builtin_constant_p() 679 680Be wary when using long long in the kernel, the code gcc generates for 681it is horrible and worse: division and multiplication does not work on 682i386 because the GCC runtime functions for it are missing from the 683kernel environment. 684 685C++ 686--- 687 688Using C++ in the kernel is usually a bad idea, because the kernel does 689not provide the necessary runtime environment and the include files are 690not tested for it. It is still possible, but not recommended. If you 691really want to do this, forget about exceptions at least. 692 693#if 694--- 695 696It is generally considered cleaner to use macros in header files (or at 697the top of .c files) to abstract away functions rather than using \`#if' 698pre-processor statements throughout the source code. 699 700Putting Your Stuff in the Kernel 701================================ 702 703In order to get your stuff into shape for official inclusion, or even to 704make a neat patch, there's administrative work to be done: 705 706- Figure out whose pond you've been pissing in. Look at the top of the 707 source files, inside the ``MAINTAINERS`` file, and last of all in the 708 ``CREDITS`` file. You should coordinate with this person to make sure 709 you're not duplicating effort, or trying something that's already 710 been rejected. 711 712 Make sure you put your name and EMail address at the top of any files 713 you create or mangle significantly. This is the first place people 714 will look when they find a bug, or when **they** want to make a change. 715 716- Usually you want a configuration option for your kernel hack. Edit 717 ``Kconfig`` in the appropriate directory. The Config language is 718 simple to use by cut and paste, and there's complete documentation in 719 ``Documentation/kbuild/kconfig-language.txt``. 720 721 In your description of the option, make sure you address both the 722 expert user and the user who knows nothing about your feature. 723 Mention incompatibilities and issues here. **Definitely** end your 724 description with “if in doubt, say N” (or, occasionally, \`Y'); this 725 is for people who have no idea what you are talking about. 726 727- Edit the ``Makefile``: the CONFIG variables are exported here so you 728 can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax 729 is documented in ``Documentation/kbuild/makefiles.txt``. 730 731- Put yourself in ``CREDITS`` if you've done something noteworthy, 732 usually beyond a single file (your name should be at the top of the 733 source files anyway). ``MAINTAINERS`` means you want to be consulted 734 when changes are made to a subsystem, and hear about bugs; it implies 735 a more-than-passing commitment to some part of the code. 736 737- Finally, don't forget to read 738 ``Documentation/process/submitting-patches.rst`` and possibly 739 ``Documentation/process/submitting-drivers.rst``. 740 741Kernel Cantrips 742=============== 743 744Some favorites from browsing the source. Feel free to add to this list. 745 746``arch/x86/include/asm/delay.h``:: 747 748 #define ndelay(n) (__builtin_constant_p(n) ? \ 749 ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ 750 __ndelay(n)) 751 752 753``include/linux/fs.h``:: 754 755 /* 756 * Kernel pointers have redundant information, so we can use a 757 * scheme where we can return either an error code or a dentry 758 * pointer with the same return value. 759 * 760 * This should be a per-architecture thing, to allow different 761 * error and pointer decisions. 762 */ 763 #define ERR_PTR(err) ((void *)((long)(err))) 764 #define PTR_ERR(ptr) ((long)(ptr)) 765 #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) 766 767``arch/x86/include/asm/uaccess_32.h:``:: 768 769 #define copy_to_user(to,from,n) \ 770 (__builtin_constant_p(n) ? \ 771 __constant_copy_to_user((to),(from),(n)) : \ 772 __generic_copy_to_user((to),(from),(n))) 773 774 775``arch/sparc/kernel/head.S:``:: 776 777 /* 778 * Sun people can't spell worth damn. "compatability" indeed. 779 * At least we *know* we can't spell, and use a spell-checker. 780 */ 781 782 /* Uh, actually Linus it is I who cannot spell. Too much murky 783 * Sparc assembly will do this to ya. 784 */ 785 C_LABEL(cputypvar): 786 .asciz "compatibility" 787 788 /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */ 789 .align 4 790 C_LABEL(cputypvar_sun4m): 791 .asciz "compatible" 792 793 794``arch/sparc/lib/checksum.S:``:: 795 796 /* Sun, you just can't beat me, you just can't. Stop trying, 797 * give up. I'm serious, I am going to kick the living shit 798 * out of you, game over, lights out. 799 */ 800 801 802Thanks 803====== 804 805Thanks to Andi Kleen for the idea, answering my questions, fixing my 806mistakes, filling content, etc. Philipp Rumpf for more spelling and 807clarity fixes, and some excellent non-obvious points. Werner Almesberger 808for giving me a great summary of :c:func:`disable_irq()`, and Jes 809Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain 810for checking and adding to the Configure section. Telsa Gwynne for 811teaching me DocBook. 812