1 .. _kernel_hacking_hack:
2
3 ============================================
4 Unreliable Guide To Hacking The Linux Kernel
5 ============================================
6
7 :Author: Rusty Russell
8
9 Introduction
10 ============
11
12 Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
13 Kernel Hacking. This document describes the common routines and general
14 requirements for kernel code: its goal is to serve as a primer for Linux
15 kernel development for experienced C programmers. I avoid implementation
16 details: that's what the code is for, and I ignore whole tracts of
17 useful routines.
18
19 Before you read this, please understand that I never wanted to write
20 this document, being grossly under-qualified, but I always wanted to
21 read it, and this was the only way. I hope it will grow into a
22 compendium of best practice, common starting points and random
23 information.
24
25 The Players
26 ===========
27
28 At 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
38 There is an ordering between these. The bottom two can preempt each
39 other, but above that is a strict hierarchy: each can only be preempted
40 by the ones above it. For example, while a softirq is running on a CPU,
41 no other softirq will preempt it, but a hardware interrupt can. However,
42 any other CPUs in the system execute independently.
43
44 We'll see a number of ways that the user context can block interrupts,
45 to become truly non-preemptable.
46
47 User Context
48 ------------
49
50 User context is when you are coming in from a system call or other trap:
51 like userspace, you can be preempted by more important tasks and by
52 interrupts. 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
59 In user context, the ``current`` pointer (indicating the task we are
60 currently 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
68 Hardware Interrupts (Hard IRQs)
69 -------------------------------
70
71 Timer ticks, network cards and keyboard are examples of real hardware
72 which produce interrupts at any time. The kernel runs interrupt
73 handlers, which services the hardware. The kernel guarantees that this
74 handler is never re-entered: if the same interrupt arrives, it is queued
75 (or dropped). Because it disables interrupts, this handler has to be
76 fast: frequently it simply acknowledges the interrupt, marks a 'software
77 interrupt' for execution and exits.
78
79 You can tell you are in a hardware interrupt, because in_hardirq() returns
80 true.
81
82 .. warning::
83
84 Beware that this will return a false positive if interrupts are
85 disabled (see below).
86
87 Software Interrupt Context: Softirqs and Tasklets
88 -------------------------------------------------
89
90 Whenever a system call is about to return to userspace, or a hardware
91 interrupt handler exits, any 'software interrupts' which are marked
92 pending (usually by hardware interrupts) are run (``kernel/softirq.c``).
93
94 Much of the real interrupt handling work is done here. Early in the
95 transition to SMP, there were only 'bottom halves' (BHs), which didn't
96 take advantage of multiple CPUs. Shortly after we switched from wind-up
97 computers made of match-sticks and snot, we abandoned this limitation
98 and switched to 'softirqs'.
99
100 ``include/linux/interrupt.h`` lists the different softirqs. A very
101 important softirq is the timer softirq (``include/linux/timer.h``): you
102 can register to have it call functions for you in a given length of
103 time.
104
105 Softirqs are often a pain to deal with, since the same softirq will run
106 simultaneously on more than one CPU. For this reason, tasklets
107 (``include/linux/interrupt.h``) are more often used: they are
108 dynamically-registrable (meaning you can have as many as you want), and
109 they also guarantee that any tasklet will only run on one CPU at any
110 time, 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
117 You 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
125 Some Basic Rules
126 ================
127
128 No 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
133 No 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
141 A 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
148 The 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
155 ioctls: Not writing a new system call
156 =====================================
157
158 A system call generally looks like this::
159
160 asmlinkage long sys_mycall(int arg)
161 {
162 return 0;
163 }
164
165
166 First, in most cases you don't want to create a new system call. You
167 create a character device and implement an appropriate ioctl for it.
168 This is much more flexible than system calls, doesn't have to be entered
169 in every architecture's ``include/asm/unistd.h`` and
170 ``arch/kernel/entry.S`` file, and is much more likely to be accepted by
171 Linus.
172
173 If all your routine does is read or write some parameter, consider
174 implementing a :c:func:`sysfs()` interface instead.
175
176 Inside the ioctl you're in user context to a process. When a error
177 occurs 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``),
180 otherwise you return 0.
181
182 After you slept you should check if a signal occurred: the Unix/Linux
183 way of handling signals is to temporarily exit the system call with the
184 ``-ERESTARTSYS`` error. The system call entry code will switch back to
185 user context, process the signal handler and then your system call will
186 be restarted (unless the user disabled that). So you should be prepared
187 to process the restart, e.g. if you're in the middle of manipulating
188 some data structure.
189
190 ::
191
192 if (signal_pending(current))
193 return -ERESTARTSYS;
194
195
196 If 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
198 to give up the CPU (remember there is cooperative multitasking per CPU).
199 Idiom::
200
201 cond_resched(); /* Will sleep */
202
203
204 A short note on interface design: the UNIX system call motto is "Provide
205 mechanism not policy".
206
207 Recipes for Deadlock
208 ====================
209
210 You 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
220 Note that some functions may sleep implicitly: common ones are the user
221 space access functions (\*_user) and memory allocation functions
222 without ``GFP_ATOMIC``.
223
224 You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on,
225 and it will warn you if you break these rules. If you **do** break the
226 rules, you will eventually lock up your box.
227
228 Really.
229
230 Common Routines
231 ===============
232
233 :c:func:`printk()`
234 ------------------
235
236 Defined in ``include/linux/printk.h``
237
238 :c:func:`printk()` feeds kernel messages to the console, dmesg, and
239 the syslog daemon. It is useful for debugging and reporting errors, and
240 can be used inside interrupt context, but use with caution: a machine
241 which has its console flooded with printk messages is unusable. It uses
242 a format string mostly compatible with ANSI C printf, and C string
243 concatenation to give it a first "priority" argument::
244
245 printk(KERN_INFO "i = %u\n", i);
246
247
248 See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are
249 interpreted by syslog as the level. Special case: for printing an IP
250 address 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
257 overruns. 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
273 Defined 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
278 and put single values (such as an int, char, or long) from and to
279 userspace. A pointer into userspace should never be simply dereferenced:
280 data should be copied using these routines. Both return ``-EFAULT`` or
281 0.
282
283 :c:func:`copy_to_user()` and :c:func:`copy_from_user()` are
284 more general: they copy an arbitrary amount of data to and from
285 userspace.
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
293 up every year or so. --RR.]
294
295 The functions may sleep implicitly. This should never be called outside
296 user context (it makes no sense), with interrupts disabled, or a
297 spinlock held.
298
299 :c:func:`kmalloc()`/:c:func:`kfree()`
300 -------------------------------------
301
302 Defined in ``include/linux/slab.h``
303
304 **[MAY SLEEP: SEE BELOW]**
305
306 These routines are used to dynamically request pointer-aligned chunks of
307 memory, 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
323 If you see a sleeping function called from invalid context warning
324 message, then maybe you called a sleeping allocation function from
325 interrupt context without ``GFP_ATOMIC``. You should really fix that.
326 Run, don't walk.
327
328 If 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,
331 1 for double page, 2 for four pages etc.) and the same memory priority
332 flag word as above.
333
334 If 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
336 map. This block is not contiguous in physical memory, but the MMU makes
337 it look like it is for you (so it'll only look contiguous to the CPUs,
338 not to external device drivers). If you really need large physically
339 contiguous memory for some weird device, you have a problem: it is
340 poorly supported in Linux because after some time memory fragmentation
341 in a running kernel makes it hard. The best way is to allocate the block
342 early in the boot process via the :c:func:`alloc_bootmem()`
343 routine.
344
345 Before inventing your own cache of often-used objects consider using a
346 slab cache in ``include/linux/slab.h``
347
348 :c:macro:`current`
349 ------------------
350
351 Defined in ``include/asm/current.h``
352
353 This global variable (really a macro) contains a pointer to the current
354 task structure, so is only valid in user context. For example, when a
355 process makes a system call, this will point to the task structure of
356 the calling process. It is **not NULL** in interrupt context.
357
358 :c:func:`mdelay()`/:c:func:`udelay()`
359 -------------------------------------
360
361 Defined in ``include/asm/delay.h`` / ``include/linux/delay.h``
362
363 The :c:func:`udelay()` and :c:func:`ndelay()` functions can be
364 used for small pauses. Do not use large values with them as you risk
365 overflow - the helper function :c:func:`mdelay()` is useful here, or
366 consider :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
371 Defined in ``include/asm/byteorder.h``
372
373 The :c:func:`cpu_to_be32()` family (where the "32" can be replaced
374 by 64 or 16, and the "be" can be replaced by "le") are the general way
375 to do endian conversions in the kernel: they return the converted value.
376 All variations supply the reverse as well:
377 :c:func:`be32_to_cpu()`, etc.
378
379 There are two major variations of these functions: the pointer
380 variation, such as :c:func:`cpu_to_be32p()`, which take a pointer
381 to the given type, and return the converted value. The other variation
382 is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which
383 convert value referred to by the pointer, and return void.
384
385 :c:func:`local_irq_save()`/:c:func:`local_irq_restore()`
386 --------------------------------------------------------
387
388 Defined in ``include/linux/irqflags.h``
389
390 These routines disable hard interrupts on the local CPU, and restore
391 them. They are reentrant; saving the previous state in their one
392 ``unsigned long flags`` argument. If you know that interrupts are
393 enabled, 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
401 Defined in ``include/linux/bottom_half.h``
402
403
404 These routines disable soft interrupts on the local CPU, and restore
405 them. They are reentrant; if soft interrupts were disabled before, they
406 will still be disabled after this pair of functions has been called.
407 They prevent softirqs and tasklets from running on the current CPU.
408
409 :c:func:`smp_processor_id()`
410 ----------------------------
411
412 Defined in ``include/linux/smp.h``
413
414 :c:func:`get_cpu()` disables preemption (so you won't suddenly get
415 moved to another CPU) and returns the current processor number, between
416 0 and ``NR_CPUS``. Note that the CPU numbers are not necessarily
417 continuous. You return it again with :c:func:`put_cpu()` when you
418 are done.
419
420 If you know you cannot be preempted by another task (ie. you are in
421 interrupt context, or have preemption disabled) you can use
422 smp_processor_id().
423
424 ``__init``/``__exit``/``__initdata``
425 ------------------------------------
426
427 Defined in ``include/linux/init.h``
428
429 After boot, the kernel frees up a special section; functions marked with
430 ``__init`` and data structures marked with ``__initdata`` are dropped
431 after boot is complete: similarly modules discard this memory after
432 initialization. ``__exit`` is used to declare a function which is only
433 required on exit: the function will be dropped if this file is not
434 compiled as a module. See the header file for use. Note that it makes no
435 sense for a function marked with ``__init`` to be exported to modules
436 with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this
437 will break.
438
439 :c:func:`__initcall()`/:c:func:`module_init()`
440 ----------------------------------------------
441
442 Defined in ``include/linux/init.h`` / ``include/linux/module.h``
443
444 Many 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
447 is easy to write code without #ifdefs which can operate both as a module
448 or built into the kernel.
449
450 The :c:func:`module_init()` macro defines which function is to be
451 called at module insertion time (if the file is compiled as a module),
452 or 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
455 the function is called on boot.
456
457 The function can return a negative error number to cause module loading
458 to fail (unfortunately, this has no effect if the module is compiled
459 into the kernel). This function is called in user context with
460 interrupts enabled, so it can sleep.
461
462 :c:func:`module_exit()`
463 -----------------------
464
465
466 Defined in ``include/linux/module.h``
467
468 This macro defines the function to be called at module removal time (or
469 never, in the case of the file compiled into the kernel). It will only
470 be called if the module usage count has reached zero. This function can
471 also sleep, but cannot fail: everything must be cleaned up by the time
472 it returns.
473
474 Note that this macro is optional: if it is not present, your module will
475 not be removable (except for 'rmmod -f').
476
477 :c:func:`try_module_get()`/:c:func:`module_put()`
478 -------------------------------------------------
479
480 Defined in ``include/linux/module.h``
481
482 These manipulate the module usage count, to protect against removal (a
483 module also can't be removed if another module uses one of its exported
484 symbols: see below). Before calling into module code, you should call
485 :c:func:`try_module_get()` on that module: if it fails, then the
486 module is being removed and you should act as if it wasn't there.
487 Otherwise, you can safely enter the module, and call
488 :c:func:`module_put()` when you're finished.
489
490 Most registerable structures have an owner field, such as in the
491 :c:type:`struct file_operations <file_operations>` structure.
492 Set this field to the macro ``THIS_MODULE``.
493
494 Wait Queues ``include/linux/wait.h``
495 ====================================
496
497 **[SLEEPS]**
498
499 A wait queue is used to wait for someone to wake you up when a certain
500 condition is true. They must be used carefully to ensure there is no
501 race condition. You declare a :c:type:`wait_queue_head_t`, and then processes
502 which want to wait for that condition declare a :c:type:`wait_queue_entry_t`
503 referring to themselves, and place that in the queue.
504
505 Declaring
506 ---------
507
508 You 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
511 code.
512
513 Queuing
514 -------
515
516 Placing yourself in the waitqueue is fairly complex, because you must
517 put yourself in the queue before checking the condition. There is a
518 macro to do this: :c:func:`wait_event_interruptible()`
519 (``include/linux/wait.h``) The first argument is the wait queue head, and
520 the second is an expression which is evaluated; the macro returns 0 when
521 this expression is true, or ``-ERESTARTSYS`` if a signal is received. The
522 :c:func:`wait_event()` version ignores signals.
523
524 Waking Up Queued Tasks
525 ----------------------
526
527 Call :c:func:`wake_up()` (``include/linux/wait.h``), which will wake
528 up every process in the queue. The exception is if one has
529 ``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will
530 not be woken. There are other variants of this basic function available
531 in the same header.
532
533 Atomic Operations
534 =================
535
536 Certain operations are guaranteed atomic on all platforms. The first
537 class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``);
538 this contains a signed integer (at least 32 bits long), and you must use
539 these functions to manipulate or read :c:type:`atomic_t` variables.
540 :c:func:`atomic_read()` and :c:func:`atomic_set()` get and set
541 the 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
544 decremented to zero).
545
546 Yes. It returns true (i.e. != 0) if the atomic variable is zero.
547
548 Note that these functions are slower than normal arithmetic, and so
549 should not be used unnecessarily.
550
551 The second class of atomic operations is atomic bit operations on an
552 ``unsigned long``, defined in ``include/linux/bitops.h``. These
553 operations generally take a pointer to the bit pattern, and a bit
554 number: 0 is the least significant bit. :c:func:`set_bit()`,
555 :c:func:`clear_bit()` and :c:func:`change_bit()` set, clear,
556 and 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
559 true if the bit was previously set; these are particularly useful for
560 atomically setting flags.
561
562 It is possible to call these operations with bit indices greater than
563 ``BITS_PER_LONG``. The resulting behavior is strange on big-endian
564 platforms though so it is a good idea not to do this.
565
566 Symbols
567 =======
568
569 Within the kernel proper, the normal linking rules apply (ie. unless a
570 symbol is declared to be file scope with the ``static`` keyword, it can
571 be used anywhere in the kernel). However, for modules, a special
572 exported symbol table is kept which limits the entry points to the
573 kernel proper. Modules can also export symbols.
574
575 :c:func:`EXPORT_SYMBOL()`
576 -------------------------
577
578 Defined in ``include/linux/export.h``
579
580 This is the classic method of exporting a symbol: dynamically loaded
581 modules will be able to use the symbol as normal.
582
583 :c:func:`EXPORT_SYMBOL_GPL()`
584 -----------------------------
585
586 Defined in ``include/linux/export.h``
587
588 Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols
589 exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by
590 modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL
591 compatible license. It implies that the function is considered an
592 internal implementation issue, and not really an interface. Some
593 maintainers and developers may however require EXPORT_SYMBOL_GPL()
594 when adding any new APIs or functionality.
595
596 :c:func:`EXPORT_SYMBOL_NS()`
597 ----------------------------
598
599 Defined in ``include/linux/export.h``
600
601 This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol
602 namespace. Symbol Namespaces are documented in
603 Documentation/core-api/symbol-namespaces.rst
604
605 :c:func:`EXPORT_SYMBOL_NS_GPL()`
606 --------------------------------
607
608 Defined in ``include/linux/export.h``
609
610 This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol
611 namespace. Symbol Namespaces are documented in
612 Documentation/core-api/symbol-namespaces.rst
613
614 Routines and Conventions
615 ========================
616
617 Double-linked lists ``include/linux/list.h``
618 --------------------------------------------
619
620 There used to be three sets of linked-list routines in the kernel
621 headers, but this one is the winner. If you don't have some particular
622 pressing need for a single list, it's a good choice.
623
624 In particular, :c:func:`list_for_each_entry()` is useful.
625
626 Return Conventions
627 ------------------
628
629 For code called in user context, it's very common to defy C convention,
630 and return 0 for success, and a negative error number (eg. ``-EFAULT``) for
631 failure. This can be unintuitive at first, but it's fairly widespread in
632 the kernel.
633
634 Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a
635 negative 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
637 pointer parameter for the error number. Icky, but in a good way.
638
639 Breaking Compilation
640 --------------------
641
642 Linus and the other developers sometimes change function or structure
643 names in development kernels; this is not done just to keep everyone on
644 their toes: it reflects a fundamental change (eg. can no longer be
645 called with interrupts on, or does extra checks, or doesn't do checks
646 which were caught before). Usually this is accompanied by a fairly
647 complete note to the appropriate kernel development mailing list; search
648 the archives. Simply doing a global replace on the file usually makes
649 things **worse**.
650
651 Initializing structure members
652 ------------------------------
653
654 The preferred method of initializing structures is to use designated
655 initialisers, 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
665 This makes it easy to grep for, and makes it clear which structure
666 fields are set. You should do this because it looks cool.
667
668 GNU Extensions
669 --------------
670
671 GNU Extensions are explicitly allowed in the Linux kernel. Note that
672 some of the more complex ones are not very well supported, due to lack
673 of general use, but the following are considered standard (see the GCC
674 info page section "C Extensions" for more details - Yes, really the info
675 page, 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
700 Be wary when using long long in the kernel, the code gcc generates for
701 it is horrible and worse: division and multiplication does not work on
702 i386 because the GCC runtime functions for it are missing from the
703 kernel environment.
704
705 C++
706 ---
707
708 Using C++ in the kernel is usually a bad idea, because the kernel does
709 not provide the necessary runtime environment and the include files are
710 not tested for it. It is still possible, but not recommended. If you
711 really want to do this, forget about exceptions at least.
712
713 #if
714 ---
715
716 It is generally considered cleaner to use macros in header files (or at
717 the top of .c files) to abstract away functions rather than using \`#if'
718 pre-processor statements throughout the source code.
719
720 Putting Your Stuff in the Kernel
721 ================================
722
723 In order to get your stuff into shape for official inclusion, or even to
724 make 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
760 Kernel Cantrips
761 ===============
762
763 Some 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
821 Thanks
822 ======
823
824 Thanks to Andi Kleen for the idea, answering my questions, fixing my
825 mistakes, filling content, etc. Philipp Rumpf for more spelling and
826 clarity fixes, and some excellent non-obvious points. Werner Almesberger
827 for giving me a great summary of :c:func:`disable_irq()`, and Jes
828 Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain
829 for checking and adding to the Configure section. Telsa Gwynne for
830 teaching me DocBook.
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