1.. _kernel_hacking_hack: 2 3============================================ 4Unreliable Guide To Hacking The Linux Kernel 5============================================ 6 7:Author: Rusty Russell 8 9Introduction 10============ 11 12Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux 13Kernel Hacking. This document describes the common routines and general 14requirements for kernel code: its goal is to serve as a primer for Linux 15kernel development for experienced C programmers. I avoid implementation 16details: that's what the code is for, and I ignore whole tracts of 17useful routines. 18 19Before you read this, please understand that I never wanted to write 20this document, being grossly under-qualified, but I always wanted to 21read it, and this was the only way. I hope it will grow into a 22compendium of best practice, common starting points and random 23information. 24 25The Players 26=========== 27 28At any time each of the CPUs in a system can be: 29 30- not associated with any process, serving a hardware interrupt; 31 32- not associated with any process, serving a softirq or tasklet; 33 34- running in kernel space, associated with a process (user context); 35 36- running a process in user space. 37 38There is an ordering between these. The bottom two can preempt each 39other, but above that is a strict hierarchy: each can only be preempted 40by the ones above it. For example, while a softirq is running on a CPU, 41no other softirq will preempt it, but a hardware interrupt can. However, 42any other CPUs in the system execute independently. 43 44We'll see a number of ways that the user context can block interrupts, 45to become truly non-preemptable. 46 47User Context 48------------ 49 50User context is when you are coming in from a system call or other trap: 51like userspace, you can be preempted by more important tasks and by 52interrupts. You can sleep, by calling :c:func:`schedule()`. 53 54.. note:: 55 56 You are always in user context on module load and unload, and on 57 operations on the block device layer. 58 59In user context, the ``current`` pointer (indicating the task we are 60currently executing) is valid, and :c:func:`in_interrupt()` 61(``include/linux/preempt.h``) is false. 62 63.. warning:: 64 65 Beware that if you have preemption or softirqs disabled (see below), 66 :c:func:`in_interrupt()` will return a false positive. 67 68Hardware Interrupts (Hard IRQs) 69------------------------------- 70 71Timer ticks, network cards and keyboard are examples of real hardware 72which produce interrupts at any time. The kernel runs interrupt 73handlers, which services the hardware. The kernel guarantees that this 74handler is never re-entered: if the same interrupt arrives, it is queued 75(or dropped). Because it disables interrupts, this handler has to be 76fast: frequently it simply acknowledges the interrupt, marks a 'software 77interrupt' for execution and exits. 78 79You can tell you are in a hardware interrupt, because in_hardirq() returns 80true. 81 82.. warning:: 83 84 Beware that this will return a false positive if interrupts are 85 disabled (see below). 86 87Software Interrupt Context: Softirqs and Tasklets 88------------------------------------------------- 89 90Whenever a system call is about to return to userspace, or a hardware 91interrupt handler exits, any 'software interrupts' which are marked 92pending (usually by hardware interrupts) are run (``kernel/softirq.c``). 93 94Much of the real interrupt handling work is done here. Early in the 95transition to SMP, there were only 'bottom halves' (BHs), which didn't 96take advantage of multiple CPUs. Shortly after we switched from wind-up 97computers made of match-sticks and snot, we abandoned this limitation 98and switched to 'softirqs'. 99 100``include/linux/interrupt.h`` lists the different softirqs. A very 101important softirq is the timer softirq (``include/linux/timer.h``): you 102can register to have it call functions for you in a given length of 103time. 104 105Softirqs are often a pain to deal with, since the same softirq will run 106simultaneously on more than one CPU. For this reason, tasklets 107(``include/linux/interrupt.h``) are more often used: they are 108dynamically-registrable (meaning you can have as many as you want), and 109they also guarantee that any tasklet will only run on one CPU at any 110time, although different tasklets can run simultaneously. 111 112.. warning:: 113 114 The name 'tasklet' is misleading: they have nothing to do with 115 'tasks'. 116 117You can tell you are in a softirq (or tasklet) using the 118:c:func:`in_softirq()` macro (``include/linux/preempt.h``). 119 120.. warning:: 121 122 Beware that this will return a false positive if a 123 :ref:`bottom half lock <local_bh_disable>` is held. 124 125Some Basic Rules 126================ 127 128No memory protection 129 If you corrupt memory, whether in user context or interrupt context, 130 the whole machine will crash. Are you sure you can't do what you 131 want in userspace? 132 133No floating point or MMX 134 The FPU context is not saved; even in user context the FPU state 135 probably won't correspond with the current process: you would mess 136 with some user process' FPU state. If you really want to do this, 137 you would have to explicitly save/restore the full FPU state (and 138 avoid context switches). It is generally a bad idea; use fixed point 139 arithmetic first. 140 141A rigid stack limit 142 Depending on configuration options the kernel stack is about 3K to 143 6K for most 32-bit architectures: it's about 14K on most 64-bit 144 archs, and often shared with interrupts so you can't use it all. 145 Avoid deep recursion and huge local arrays on the stack (allocate 146 them dynamically instead). 147 148The Linux kernel is portable 149 Let's keep it that way. Your code should be 64-bit clean, and 150 endian-independent. You should also minimize CPU specific stuff, 151 e.g. inline assembly should be cleanly encapsulated and minimized to 152 ease porting. Generally it should be restricted to the 153 architecture-dependent part of the kernel tree. 154 155ioctls: Not writing a new system call 156===================================== 157 158A system call generally looks like this:: 159 160 asmlinkage long sys_mycall(int arg) 161 { 162 return 0; 163 } 164 165 166First, in most cases you don't want to create a new system call. You 167create a character device and implement an appropriate ioctl for it. 168This is much more flexible than system calls, doesn't have to be entered 169in every architecture's ``include/asm/unistd.h`` and 170``arch/kernel/entry.S`` file, and is much more likely to be accepted by 171Linus. 172 173If all your routine does is read or write some parameter, consider 174implementing a :c:func:`sysfs()` interface instead. 175 176Inside the ioctl you're in user context to a process. When a error 177occurs you return a negated errno (see 178``include/uapi/asm-generic/errno-base.h``, 179``include/uapi/asm-generic/errno.h`` and ``include/linux/errno.h``), 180otherwise you return 0. 181 182After you slept you should check if a signal occurred: the Unix/Linux 183way of handling signals is to temporarily exit the system call with the 184``-ERESTARTSYS`` error. The system call entry code will switch back to 185user context, process the signal handler and then your system call will 186be restarted (unless the user disabled that). So you should be prepared 187to process the restart, e.g. if you're in the middle of manipulating 188some data structure. 189 190:: 191 192 if (signal_pending(current)) 193 return -ERESTARTSYS; 194 195 196If you're doing longer computations: first think userspace. If you 197**really** want to do it in kernel you should regularly check if you need 198to give up the CPU (remember there is cooperative multitasking per CPU). 199Idiom:: 200 201 cond_resched(); /* Will sleep */ 202 203 204A short note on interface design: the UNIX system call motto is "Provide 205mechanism not policy". 206 207Recipes for Deadlock 208==================== 209 210You cannot call any routines which may sleep, unless: 211 212- You are in user context. 213 214- You do not own any spinlocks. 215 216- You have interrupts enabled (actually, Andi Kleen says that the 217 scheduling code will enable them for you, but that's probably not 218 what you wanted). 219 220Note that some functions may sleep implicitly: common ones are the user 221space access functions (\*_user) and memory allocation functions 222without ``GFP_ATOMIC``. 223 224You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on, 225and it will warn you if you break these rules. If you **do** break the 226rules, you will eventually lock up your box. 227 228Really. 229 230Common Routines 231=============== 232 233:c:func:`printk()` 234------------------ 235 236Defined in ``include/linux/printk.h`` 237 238:c:func:`printk()` feeds kernel messages to the console, dmesg, and 239the syslog daemon. It is useful for debugging and reporting errors, and 240can be used inside interrupt context, but use with caution: a machine 241which has its console flooded with printk messages is unusable. It uses 242a format string mostly compatible with ANSI C printf, and C string 243concatenation to give it a first "priority" argument:: 244 245 printk(KERN_INFO "i = %u\n", i); 246 247 248See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are 249interpreted by syslog as the level. Special case: for printing an IP 250address use:: 251 252 __be32 ipaddress; 253 printk(KERN_INFO "my ip: %pI4\n", &ipaddress); 254 255 256:c:func:`printk()` internally uses a 1K buffer and does not catch 257overruns. Make sure that will be enough. 258 259.. note:: 260 261 You will know when you are a real kernel hacker when you start 262 typoing printf as printk in your user programs :) 263 264.. note:: 265 266 Another sidenote: the original Unix Version 6 sources had a comment 267 on top of its printf function: "Printf should not be used for 268 chit-chat". You should follow that advice. 269 270:c:func:`copy_to_user()` / :c:func:`copy_from_user()` / :c:func:`get_user()` / :c:func:`put_user()` 271--------------------------------------------------------------------------------------------------- 272 273Defined in ``include/linux/uaccess.h`` / ``asm/uaccess.h`` 274 275**[SLEEPS]** 276 277:c:func:`put_user()` and :c:func:`get_user()` are used to get 278and put single values (such as an int, char, or long) from and to 279userspace. A pointer into userspace should never be simply dereferenced: 280data should be copied using these routines. Both return ``-EFAULT`` or 2810. 282 283:c:func:`copy_to_user()` and :c:func:`copy_from_user()` are 284more general: they copy an arbitrary amount of data to and from 285userspace. 286 287.. warning:: 288 289 Unlike :c:func:`put_user()` and :c:func:`get_user()`, they 290 return the amount of uncopied data (ie. 0 still means success). 291 292[Yes, this objectionable interface makes me cringe. The flamewar comes 293up every year or so. --RR.] 294 295The functions may sleep implicitly. This should never be called outside 296user context (it makes no sense), with interrupts disabled, or a 297spinlock held. 298 299:c:func:`kmalloc()`/:c:func:`kfree()` 300------------------------------------- 301 302Defined in ``include/linux/slab.h`` 303 304**[MAY SLEEP: SEE BELOW]** 305 306These routines are used to dynamically request pointer-aligned chunks of 307memory, like malloc and free do in userspace, but 308:c:func:`kmalloc()` takes an extra flag word. Important values: 309 310``GFP_KERNEL`` 311 May sleep and swap to free memory. Only allowed in user context, but 312 is the most reliable way to allocate memory. 313 314``GFP_ATOMIC`` 315 Don't sleep. Less reliable than ``GFP_KERNEL``, but may be called 316 from interrupt context. You should **really** have a good 317 out-of-memory error-handling strategy. 318 319``GFP_DMA`` 320 Allocate ISA DMA lower than 16MB. If you don't know what that is you 321 don't need it. Very unreliable. 322 323If you see a sleeping function called from invalid context warning 324message, then maybe you called a sleeping allocation function from 325interrupt context without ``GFP_ATOMIC``. You should really fix that. 326Run, don't walk. 327 328If you are allocating at least ``PAGE_SIZE`` (``asm/page.h`` or 329``asm/page_types.h``) bytes, consider using :c:func:`__get_free_pages()` 330(``include/linux/gfp.h``). It takes an order argument (0 for page sized, 3311 for double page, 2 for four pages etc.) and the same memory priority 332flag word as above. 333 334If you are allocating more than a page worth of bytes you can use 335:c:func:`vmalloc()`. It'll allocate virtual memory in the kernel 336map. This block is not contiguous in physical memory, but the MMU makes 337it look like it is for you (so it'll only look contiguous to the CPUs, 338not to external device drivers). If you really need large physically 339contiguous memory for some weird device, you have a problem: it is 340poorly supported in Linux because after some time memory fragmentation 341in a running kernel makes it hard. The best way is to allocate the block 342early in the boot process via the :c:func:`alloc_bootmem()` 343routine. 344 345Before inventing your own cache of often-used objects consider using a 346slab cache in ``include/linux/slab.h`` 347 348:c:macro:`current` 349------------------ 350 351Defined in ``include/asm/current.h`` 352 353This global variable (really a macro) contains a pointer to the current 354task structure, so is only valid in user context. For example, when a 355process makes a system call, this will point to the task structure of 356the calling process. It is **not NULL** in interrupt context. 357 358:c:func:`mdelay()`/:c:func:`udelay()` 359------------------------------------- 360 361Defined in ``include/asm/delay.h`` / ``include/linux/delay.h`` 362 363The :c:func:`udelay()` and :c:func:`ndelay()` functions can be 364used for small pauses. Do not use large values with them as you risk 365overflow - the helper function :c:func:`mdelay()` is useful here, or 366consider :c:func:`msleep()`. 367 368:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()` 369----------------------------------------------------------------------------------------------- 370 371Defined in ``include/asm/byteorder.h`` 372 373The :c:func:`cpu_to_be32()` family (where the "32" can be replaced 374by 64 or 16, and the "be" can be replaced by "le") are the general way 375to do endian conversions in the kernel: they return the converted value. 376All variations supply the reverse as well: 377:c:func:`be32_to_cpu()`, etc. 378 379There are two major variations of these functions: the pointer 380variation, such as :c:func:`cpu_to_be32p()`, which take a pointer 381to the given type, and return the converted value. The other variation 382is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which 383convert value referred to by the pointer, and return void. 384 385:c:func:`local_irq_save()`/:c:func:`local_irq_restore()` 386-------------------------------------------------------- 387 388Defined in ``include/linux/irqflags.h`` 389 390These routines disable hard interrupts on the local CPU, and restore 391them. They are reentrant; saving the previous state in their one 392``unsigned long flags`` argument. If you know that interrupts are 393enabled, you can simply use :c:func:`local_irq_disable()` and 394:c:func:`local_irq_enable()`. 395 396.. _local_bh_disable: 397 398:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()` 399-------------------------------------------------------- 400 401Defined in ``include/linux/bottom_half.h`` 402 403 404These routines disable soft interrupts on the local CPU, and restore 405them. They are reentrant; if soft interrupts were disabled before, they 406will still be disabled after this pair of functions has been called. 407They prevent softirqs and tasklets from running on the current CPU. 408 409:c:func:`smp_processor_id()` 410---------------------------- 411 412Defined in ``include/linux/smp.h`` 413 414:c:func:`get_cpu()` disables preemption (so you won't suddenly get 415moved to another CPU) and returns the current processor number, between 4160 and ``NR_CPUS``. Note that the CPU numbers are not necessarily 417continuous. You return it again with :c:func:`put_cpu()` when you 418are done. 419 420If you know you cannot be preempted by another task (ie. you are in 421interrupt context, or have preemption disabled) you can use 422smp_processor_id(). 423 424``__init``/``__exit``/``__initdata`` 425------------------------------------ 426 427Defined in ``include/linux/init.h`` 428 429After boot, the kernel frees up a special section; functions marked with 430``__init`` and data structures marked with ``__initdata`` are dropped 431after boot is complete: similarly modules discard this memory after 432initialization. ``__exit`` is used to declare a function which is only 433required on exit: the function will be dropped if this file is not 434compiled as a module. See the header file for use. Note that it makes no 435sense for a function marked with ``__init`` to be exported to modules 436with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this 437will break. 438 439:c:func:`__initcall()`/:c:func:`module_init()` 440---------------------------------------------- 441 442Defined in ``include/linux/init.h`` / ``include/linux/module.h`` 443 444Many parts of the kernel are well served as a module 445(dynamically-loadable parts of the kernel). Using the 446:c:func:`module_init()` and :c:func:`module_exit()` macros it 447is easy to write code without #ifdefs which can operate both as a module 448or built into the kernel. 449 450The :c:func:`module_init()` macro defines which function is to be 451called at module insertion time (if the file is compiled as a module), 452or at boot time: if the file is not compiled as a module the 453:c:func:`module_init()` macro becomes equivalent to 454:c:func:`__initcall()`, which through linker magic ensures that 455the function is called on boot. 456 457The function can return a negative error number to cause module loading 458to fail (unfortunately, this has no effect if the module is compiled 459into the kernel). This function is called in user context with 460interrupts enabled, so it can sleep. 461 462:c:func:`module_exit()` 463----------------------- 464 465 466Defined in ``include/linux/module.h`` 467 468This macro defines the function to be called at module removal time (or 469never, in the case of the file compiled into the kernel). It will only 470be called if the module usage count has reached zero. This function can 471also sleep, but cannot fail: everything must be cleaned up by the time 472it returns. 473 474Note that this macro is optional: if it is not present, your module will 475not be removable (except for 'rmmod -f'). 476 477:c:func:`try_module_get()`/:c:func:`module_put()` 478------------------------------------------------- 479 480Defined in ``include/linux/module.h`` 481 482These manipulate the module usage count, to protect against removal (a 483module also can't be removed if another module uses one of its exported 484symbols: see below). Before calling into module code, you should call 485:c:func:`try_module_get()` on that module: if it fails, then the 486module is being removed and you should act as if it wasn't there. 487Otherwise, you can safely enter the module, and call 488:c:func:`module_put()` when you're finished. 489 490Most registerable structures have an owner field, such as in the 491:c:type:`struct file_operations <file_operations>` structure. 492Set this field to the macro ``THIS_MODULE``. 493 494Wait Queues ``include/linux/wait.h`` 495==================================== 496 497**[SLEEPS]** 498 499A wait queue is used to wait for someone to wake you up when a certain 500condition is true. They must be used carefully to ensure there is no 501race condition. You declare a :c:type:`wait_queue_head_t`, and then processes 502which want to wait for that condition declare a :c:type:`wait_queue_entry_t` 503referring to themselves, and place that in the queue. 504 505Declaring 506--------- 507 508You declare a ``wait_queue_head_t`` using the 509:c:func:`DECLARE_WAIT_QUEUE_HEAD()` macro, or using the 510:c:func:`init_waitqueue_head()` routine in your initialization 511code. 512 513Queuing 514------- 515 516Placing yourself in the waitqueue is fairly complex, because you must 517put yourself in the queue before checking the condition. There is a 518macro to do this: :c:func:`wait_event_interruptible()` 519(``include/linux/wait.h``) The first argument is the wait queue head, and 520the second is an expression which is evaluated; the macro returns 0 when 521this expression is true, or ``-ERESTARTSYS`` if a signal is received. The 522:c:func:`wait_event()` version ignores signals. 523 524Waking Up Queued Tasks 525---------------------- 526 527Call :c:func:`wake_up()` (``include/linux/wait.h``), which will wake 528up every process in the queue. The exception is if one has 529``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will 530not be woken. There are other variants of this basic function available 531in the same header. 532 533Atomic Operations 534================= 535 536Certain operations are guaranteed atomic on all platforms. The first 537class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``); 538this contains a signed integer (at least 32 bits long), and you must use 539these functions to manipulate or read :c:type:`atomic_t` variables. 540:c:func:`atomic_read()` and :c:func:`atomic_set()` get and set 541the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`, 542:c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and 543:c:func:`atomic_dec_and_test()` (returns true if it was 544decremented to zero). 545 546Yes. It returns true (i.e. != 0) if the atomic variable is zero. 547 548Note that these functions are slower than normal arithmetic, and so 549should not be used unnecessarily. 550 551The second class of atomic operations is atomic bit operations on an 552``unsigned long``, defined in ``include/linux/bitops.h``. These 553operations generally take a pointer to the bit pattern, and a bit 554number: 0 is the least significant bit. :c:func:`set_bit()`, 555:c:func:`clear_bit()` and :c:func:`change_bit()` set, clear, 556and flip the given bit. :c:func:`test_and_set_bit()`, 557:c:func:`test_and_clear_bit()` and 558:c:func:`test_and_change_bit()` do the same thing, except return 559true if the bit was previously set; these are particularly useful for 560atomically setting flags. 561 562It is possible to call these operations with bit indices greater than 563``BITS_PER_LONG``. The resulting behavior is strange on big-endian 564platforms though so it is a good idea not to do this. 565 566Symbols 567======= 568 569Within the kernel proper, the normal linking rules apply (ie. unless a 570symbol is declared to be file scope with the ``static`` keyword, it can 571be used anywhere in the kernel). However, for modules, a special 572exported symbol table is kept which limits the entry points to the 573kernel proper. Modules can also export symbols. 574 575:c:func:`EXPORT_SYMBOL()` 576------------------------- 577 578Defined in ``include/linux/export.h`` 579 580This is the classic method of exporting a symbol: dynamically loaded 581modules will be able to use the symbol as normal. 582 583:c:func:`EXPORT_SYMBOL_GPL()` 584----------------------------- 585 586Defined in ``include/linux/export.h`` 587 588Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols 589exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by 590modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL 591compatible license. It implies that the function is considered an 592internal implementation issue, and not really an interface. Some 593maintainers and developers may however require EXPORT_SYMBOL_GPL() 594when adding any new APIs or functionality. 595 596:c:func:`EXPORT_SYMBOL_NS()` 597---------------------------- 598 599Defined in ``include/linux/export.h`` 600 601This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol 602namespace. Symbol Namespaces are documented in 603Documentation/core-api/symbol-namespaces.rst 604 605:c:func:`EXPORT_SYMBOL_NS_GPL()` 606-------------------------------- 607 608Defined in ``include/linux/export.h`` 609 610This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol 611namespace. Symbol Namespaces are documented in 612Documentation/core-api/symbol-namespaces.rst 613 614Routines and Conventions 615======================== 616 617Double-linked lists ``include/linux/list.h`` 618-------------------------------------------- 619 620There used to be three sets of linked-list routines in the kernel 621headers, but this one is the winner. If you don't have some particular 622pressing need for a single list, it's a good choice. 623 624In particular, :c:func:`list_for_each_entry()` is useful. 625 626Return Conventions 627------------------ 628 629For code called in user context, it's very common to defy C convention, 630and return 0 for success, and a negative error number (eg. ``-EFAULT``) for 631failure. This can be unintuitive at first, but it's fairly widespread in 632the kernel. 633 634Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a 635negative error number into a pointer, and :c:func:`IS_ERR()` and 636:c:func:`PTR_ERR()` to get it back out again: avoids a separate 637pointer parameter for the error number. Icky, but in a good way. 638 639Breaking Compilation 640-------------------- 641 642Linus and the other developers sometimes change function or structure 643names in development kernels; this is not done just to keep everyone on 644their toes: it reflects a fundamental change (eg. can no longer be 645called with interrupts on, or does extra checks, or doesn't do checks 646which were caught before). Usually this is accompanied by a fairly 647complete note to the appropriate kernel development mailing list; search 648the archives. Simply doing a global replace on the file usually makes 649things **worse**. 650 651Initializing structure members 652------------------------------ 653 654The preferred method of initializing structures is to use designated 655initialisers, as defined by ISO C99, eg:: 656 657 static struct block_device_operations opt_fops = { 658 .open = opt_open, 659 .release = opt_release, 660 .ioctl = opt_ioctl, 661 .check_media_change = opt_media_change, 662 }; 663 664 665This makes it easy to grep for, and makes it clear which structure 666fields are set. You should do this because it looks cool. 667 668GNU Extensions 669-------------- 670 671GNU Extensions are explicitly allowed in the Linux kernel. Note that 672some of the more complex ones are not very well supported, due to lack 673of general use, but the following are considered standard (see the GCC 674info page section "C Extensions" for more details - Yes, really the info 675page, the man page is only a short summary of the stuff in info). 676 677- Inline functions 678 679- Statement expressions (ie. the ({ and }) constructs). 680 681- Declaring attributes of a function / variable / type 682 (__attribute__) 683 684- typeof 685 686- Zero length arrays 687 688- Macro varargs 689 690- Arithmetic on void pointers 691 692- Non-Constant initializers 693 694- Assembler Instructions (not outside arch/ and include/asm/) 695 696- Function names as strings (__func__). 697 698- __builtin_constant_p() 699 700Be wary when using long long in the kernel, the code gcc generates for 701it is horrible and worse: division and multiplication does not work on 702i386 because the GCC runtime functions for it are missing from the 703kernel environment. 704 705C++ 706--- 707 708Using C++ in the kernel is usually a bad idea, because the kernel does 709not provide the necessary runtime environment and the include files are 710not tested for it. It is still possible, but not recommended. If you 711really want to do this, forget about exceptions at least. 712 713#if 714--- 715 716It is generally considered cleaner to use macros in header files (or at 717the top of .c files) to abstract away functions rather than using \`#if' 718pre-processor statements throughout the source code. 719 720Putting Your Stuff in the Kernel 721================================ 722 723In order to get your stuff into shape for official inclusion, or even to 724make a neat patch, there's administrative work to be done: 725 726- Figure out who are the owners of the code you've been modifying. Look 727 at the top of the source files, inside the ``MAINTAINERS`` file, and 728 last of all in the ``CREDITS`` file. You should coordinate with these 729 people to make sure you're not duplicating effort, or trying something 730 that's already been rejected. 731 732 Make sure you put your name and email address at the top of any files 733 you create or modify significantly. This is the first place people 734 will look when they find a bug, or when **they** want to make a change. 735 736- Usually you want a configuration option for your kernel hack. Edit 737 ``Kconfig`` in the appropriate directory. The Config language is 738 simple to use by cut and paste, and there's complete documentation in 739 ``Documentation/kbuild/kconfig-language.rst``. 740 741 In your description of the option, make sure you address both the 742 expert user and the user who knows nothing about your feature. 743 Mention incompatibilities and issues here. **Definitely** end your 744 description with “if in doubt, say N” (or, occasionally, \`Y'); this 745 is for people who have no idea what you are talking about. 746 747- Edit the ``Makefile``: the CONFIG variables are exported here so you 748 can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax 749 is documented in ``Documentation/kbuild/makefiles.rst``. 750 751- Put yourself in ``CREDITS`` if you consider what you've done 752 noteworthy, usually beyond a single file (your name should be at the 753 top of the source files anyway). ``MAINTAINERS`` means you want to be 754 consulted when changes are made to a subsystem, and hear about bugs; 755 it implies a more-than-passing commitment to some part of the code. 756 757- Finally, don't forget to read 758 ``Documentation/process/submitting-patches.rst`` 759 760Kernel Cantrips 761=============== 762 763Some favorites from browsing the source. Feel free to add to this list. 764 765``arch/x86/include/asm/delay.h``:: 766 767 #define ndelay(n) (__builtin_constant_p(n) ? \ 768 ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ 769 __ndelay(n)) 770 771 772``include/linux/fs.h``:: 773 774 /* 775 * Kernel pointers have redundant information, so we can use a 776 * scheme where we can return either an error code or a dentry 777 * pointer with the same return value. 778 * 779 * This should be a per-architecture thing, to allow different 780 * error and pointer decisions. 781 */ 782 #define ERR_PTR(err) ((void *)((long)(err))) 783 #define PTR_ERR(ptr) ((long)(ptr)) 784 #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) 785 786``arch/x86/include/asm/uaccess_32.h:``:: 787 788 #define copy_to_user(to,from,n) \ 789 (__builtin_constant_p(n) ? \ 790 __constant_copy_to_user((to),(from),(n)) : \ 791 __generic_copy_to_user((to),(from),(n))) 792 793 794``arch/sparc/kernel/head.S:``:: 795 796 /* 797 * Sun people can't spell worth damn. "compatability" indeed. 798 * At least we *know* we can't spell, and use a spell-checker. 799 */ 800 801 /* Uh, actually Linus it is I who cannot spell. Too much murky 802 * Sparc assembly will do this to ya. 803 */ 804 C_LABEL(cputypvar): 805 .asciz "compatibility" 806 807 /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */ 808 .align 4 809 C_LABEL(cputypvar_sun4m): 810 .asciz "compatible" 811 812 813``arch/sparc/lib/checksum.S:``:: 814 815 /* Sun, you just can't beat me, you just can't. Stop trying, 816 * give up. I'm serious, I am going to kick the living shit 817 * out of you, game over, lights out. 818 */ 819 820 821Thanks 822====== 823 824Thanks to Andi Kleen for the idea, answering my questions, fixing my 825mistakes, filling content, etc. Philipp Rumpf for more spelling and 826clarity fixes, and some excellent non-obvious points. Werner Almesberger 827for giving me a great summary of :c:func:`disable_irq()`, and Jes 828Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain 829for checking and adding to the Configure section. Telsa Gwynne for 830teaching me DocBook. 831