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 ============================================
 Unreliable Guide To Hacking The Linux Kernel
 ============================================
 
 :Author: Rusty Russell
 
 Introduction
 ============
 
 Welcome, gentle reader, to Rusty's Remarkably Unreliable Guide to Linux
 Kernel Hacking. This document describes the common routines and general
 requirements for kernel code: its goal is to serve as a primer for Linux
 kernel development for experienced C programmers. I avoid implementation
 details: that's what the code is for, and I ignore whole tracts of
 useful routines.
 
 Before you read this, please understand that I never wanted to write
 this document, being grossly under-qualified, but I always wanted to
 read it, and this was the only way. I hope it will grow into a
 compendium of best practice, common starting points and random
 information.
 
 The Players
 ===========
 
 At any time each of the CPUs in a system can be:
 
 -  not associated with any process, serving a hardware interrupt;
 
 -  not associated with any process, serving a softirq or tasklet;
 
 -  running in kernel space, associated with a process (user context);
 
 -  running a process in user space.
 
 There is an ordering between these. The bottom two can preempt each
 other, but above that is a strict hierarchy: each can only be preempted
 by the ones above it. For example, while a softirq is running on a CPU,
 no other softirq will preempt it, but a hardware interrupt can. However,
 any other CPUs in the system execute independently.
 
 We'll see a number of ways that the user context can block interrupts,
 to become truly non-preemptable.
 
 User Context
 ------------
 
 User context is when you are coming in from a system call or other trap:
 like userspace, you can be preempted by more important tasks and by
 interrupts. You can sleep, by calling :c:func:`schedule()`.
 
 .. note::
 
     You are always in user context on module load and unload, and on
     operations on the block device layer.
 
 In user context, the ``current`` pointer (indicating the task we are
 currently executing) is valid, and :c:func:`in_interrupt()`
 (``include/linux/preempt.h``) is false.
 
 .. warning::
 
     Beware that if you have preemption or softirqs disabled (see below),
     :c:func:`in_interrupt()` will return a false positive.
 
 Hardware Interrupts (Hard IRQs)
 -------------------------------
 
 Timer ticks, network cards and keyboard are examples of real hardware
 which produce interrupts at any time. The kernel runs interrupt
 handlers, which services the hardware. The kernel guarantees that this
 handler is never re-entered: if the same interrupt arrives, it is queued
 (or dropped). Because it disables interrupts, this handler has to be
 fast: frequently it simply acknowledges the interrupt, marks a 'software
 interrupt' for execution and exits.
 
 You can tell you are in a hardware interrupt, because in_hardirq() returns
 true.
 
 .. warning::
 
     Beware that this will return a false positive if interrupts are
     disabled (see below).
 
 Software Interrupt Context: Softirqs and Tasklets
 -------------------------------------------------
 
 Whenever a system call is about to return to userspace, or a hardware
 interrupt handler exits, any 'software interrupts' which are marked
 pending (usually by hardware interrupts) are run (``kernel/softirq.c``).
 
 Much of the real interrupt handling work is done here. Early in the
 transition to SMP, there were only 'bottom halves' (BHs), which didn't
 take advantage of multiple CPUs. Shortly after we switched from wind-up
 computers made of match-sticks and snot, we abandoned this limitation
 and switched to 'softirqs'.
 
 ``include/linux/interrupt.h`` lists the different softirqs. A very
 important softirq is the timer softirq (``include/linux/timer.h``): you
 can register to have it call functions for you in a given length of
 time.
 
 Softirqs are often a pain to deal with, since the same softirq will run
 simultaneously on more than one CPU. For this reason, tasklets
 (``include/linux/interrupt.h``) are more often used: they are
 dynamically-registrable (meaning you can have as many as you want), and
 they also guarantee that any tasklet will only run on one CPU at any
 time, although different tasklets can run simultaneously.
 
 .. warning::
 
     The name 'tasklet' is misleading: they have nothing to do with
     'tasks'.
 
 You can tell you are in a softirq (or tasklet) using the
 :c:func:`in_softirq()` macro (``include/linux/preempt.h``).
 
 .. warning::
 
     Beware that this will return a false positive if a
     :ref:`botton half lock <local_bh_disable>` is held.
 
 Some Basic Rules
 ================
 
 No memory protection
     If you corrupt memory, whether in user context or interrupt context,
     the whole machine will crash. Are you sure you can't do what you
     want in userspace?
 
 No floating point or MMX
     The FPU context is not saved; even in user context the FPU state
     probably won't correspond with the current process: you would mess
     with some user process' FPU state. If you really want to do this,
     you would have to explicitly save/restore the full FPU state (and
     avoid context switches). It is generally a bad idea; use fixed point
     arithmetic first.
 
 A rigid stack limit
     Depending on configuration options the kernel stack is about 3K to
     6K for most 32-bit architectures: it's about 14K on most 64-bit
     archs, and often shared with interrupts so you can't use it all.
     Avoid deep recursion and huge local arrays on the stack (allocate
     them dynamically instead).
 
 The Linux kernel is portable
     Let's keep it that way. Your code should be 64-bit clean, and
     endian-independent. You should also minimize CPU specific stuff,
     e.g. inline assembly should be cleanly encapsulated and minimized to
     ease porting. Generally it should be restricted to the
     architecture-dependent part of the kernel tree.
 
 ioctls: Not writing a new system call
 =====================================
 
 A system call generally looks like this::
 
     asmlinkage long sys_mycall(int arg)
     {
             return 0;
     }
 
 
 First, in most cases you don't want to create a new system call. You
 create a character device and implement an appropriate ioctl for it.
 This is much more flexible than system calls, doesn't have to be entered
 in every architecture's ``include/asm/unistd.h`` and
 ``arch/kernel/entry.S`` file, and is much more likely to be accepted by
 Linus.
 
 If all your routine does is read or write some parameter, consider
 implementing a :c:func:`sysfs()` interface instead.
 
 Inside the ioctl you're in user context to a process. When a error
 occurs you return a negated errno (see
 ``include/uapi/asm-generic/errno-base.h``,
 ``include/uapi/asm-generic/errno.h`` and ``include/linux/errno.h``),
 otherwise you return 0.
 
 After you slept you should check if a signal occurred: the Unix/Linux
 way of handling signals is to temporarily exit the system call with the
 ``-ERESTARTSYS`` error. The system call entry code will switch back to
 user context, process the signal handler and then your system call will
 be restarted (unless the user disabled that). So you should be prepared
 to process the restart, e.g. if you're in the middle of manipulating
 some data structure.
 
 ::
 
     if (signal_pending(current))
             return -ERESTARTSYS;
 
 
 If you're doing longer computations: first think userspace. If you
 **really** want to do it in kernel you should regularly check if you need
 to give up the CPU (remember there is cooperative multitasking per CPU).
 Idiom::
 
     cond_resched(); /* Will sleep */
 
 
 A short note on interface design: the UNIX system call motto is "Provide
 mechanism not policy".
 
 Recipes for Deadlock
 ====================
 
 You cannot call any routines which may sleep, unless:
 
 -  You are in user context.
 
 -  You do not own any spinlocks.
 
 -  You have interrupts enabled (actually, Andi Kleen says that the
    scheduling code will enable them for you, but that's probably not
    what you wanted).
 
 Note that some functions may sleep implicitly: common ones are the user
 space access functions (\*_user) and memory allocation functions
 without ``GFP_ATOMIC``.
 
 You should always compile your kernel ``CONFIG_DEBUG_ATOMIC_SLEEP`` on,
 and it will warn you if you break these rules. If you **do** break the
 rules, you will eventually lock up your box.
 
 Really.
 
 Common Routines
 ===============
 
 :c:func:`printk()`
 ------------------
 
 Defined in ``include/linux/printk.h``
 
 :c:func:`printk()` feeds kernel messages to the console, dmesg, and
 the syslog daemon. It is useful for debugging and reporting errors, and
 can be used inside interrupt context, but use with caution: a machine
 which has its console flooded with printk messages is unusable. It uses
 a format string mostly compatible with ANSI C printf, and C string
 concatenation to give it a first "priority" argument::
 
     printk(KERN_INFO "i = %u\n", i);
 
 
 See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are
 interpreted by syslog as the level. Special case: for printing an IP
 address use::
 
     __be32 ipaddress;
     printk(KERN_INFO "my ip: %pI4\n", &ipaddress);
 
 
 :c:func:`printk()` internally uses a 1K buffer and does not catch
 overruns. Make sure that will be enough.
 
 .. note::
 
     You will know when you are a real kernel hacker when you start
     typoing printf as printk in your user programs :)
 
 .. note::
 
     Another sidenote: the original Unix Version 6 sources had a comment
     on top of its printf function: "Printf should not be used for
     chit-chat". You should follow that advice.
 
 :c:func:`copy_to_user()` / :c:func:`copy_from_user()` / :c:func:`get_user()` / :c:func:`put_user()`
 ---------------------------------------------------------------------------------------------------
 
 Defined in ``include/linux/uaccess.h`` / ``asm/uaccess.h``
 
 **[SLEEPS]**
 
 :c:func:`put_user()` and :c:func:`get_user()` are used to get
 and put single values (such as an int, char, or long) from and to
 userspace. A pointer into userspace should never be simply dereferenced:
 data should be copied using these routines. Both return ``-EFAULT`` or
 0.
 
 :c:func:`copy_to_user()` and :c:func:`copy_from_user()` are
 more general: they copy an arbitrary amount of data to and from
 userspace.
 
 .. warning::
 
     Unlike :c:func:`put_user()` and :c:func:`get_user()`, they
     return the amount of uncopied data (ie. 0 still means success).
 
 [Yes, this objectionable interface makes me cringe. The flamewar comes
 up every year or so. --RR.]
 
 The functions may sleep implicitly. This should never be called outside
 user context (it makes no sense), with interrupts disabled, or a
 spinlock held.
 
 :c:func:`kmalloc()`/:c:func:`kfree()`
 -------------------------------------
 
 Defined in ``include/linux/slab.h``
 
 **[MAY SLEEP: SEE BELOW]**
 
 These routines are used to dynamically request pointer-aligned chunks of
 memory, like malloc and free do in userspace, but
 :c:func:`kmalloc()` takes an extra flag word. Important values:
 
 ``GFP_KERNEL``
     May sleep and swap to free memory. Only allowed in user context, but
     is the most reliable way to allocate memory.
 
 ``GFP_ATOMIC``
     Don't sleep. Less reliable than ``GFP_KERNEL``, but may be called
     from interrupt context. You should **really** have a good
     out-of-memory error-handling strategy.
 
 ``GFP_DMA``
     Allocate ISA DMA lower than 16MB. If you don't know what that is you
     don't need it. Very unreliable.
 
 If you see a sleeping function called from invalid context warning
 message, then maybe you called a sleeping allocation function from
 interrupt context without ``GFP_ATOMIC``. You should really fix that.
 Run, don't walk.
 
 If you are allocating at least ``PAGE_SIZE`` (``asm/page.h`` or
 ``asm/page_types.h``) bytes, consider using :c:func:`__get_free_pages()`
 (``include/linux/gfp.h``). It takes an order argument (0 for page sized,
 1 for double page, 2 for four pages etc.) and the same memory priority
 flag word as above.
 
 If you are allocating more than a page worth of bytes you can use
 :c:func:`vmalloc()`. It'll allocate virtual memory in the kernel
 map. This block is not contiguous in physical memory, but the MMU makes
 it look like it is for you (so it'll only look contiguous to the CPUs,
 not to external device drivers). If you really need large physically
 contiguous memory for some weird device, you have a problem: it is
 poorly supported in Linux because after some time memory fragmentation
 in a running kernel makes it hard. The best way is to allocate the block
 early in the boot process via the :c:func:`alloc_bootmem()`
 routine.
 
 Before inventing your own cache of often-used objects consider using a
 slab cache in ``include/linux/slab.h``
 
 :c:macro:`current`
 ------------------
 
 Defined in ``include/asm/current.h``
 
 This global variable (really a macro) contains a pointer to the current
 task structure, so is only valid in user context. For example, when a
 process makes a system call, this will point to the task structure of
 the calling process. It is **not NULL** in interrupt context.
 
 :c:func:`mdelay()`/:c:func:`udelay()`
 -------------------------------------
 
 Defined in ``include/asm/delay.h`` / ``include/linux/delay.h``
 
 The :c:func:`udelay()` and :c:func:`ndelay()` functions can be
 used for small pauses. Do not use large values with them as you risk
 overflow - the helper function :c:func:`mdelay()` is useful here, or
 consider :c:func:`msleep()`.
 
 :c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()`
 -----------------------------------------------------------------------------------------------
 
 Defined in ``include/asm/byteorder.h``
 
 The :c:func:`cpu_to_be32()` family (where the "32" can be replaced
 by 64 or 16, and the "be" can be replaced by "le") are the general way
 to do endian conversions in the kernel: they return the converted value.
 All variations supply the reverse as well:
 :c:func:`be32_to_cpu()`, etc.
 
 There are two major variations of these functions: the pointer
 variation, such as :c:func:`cpu_to_be32p()`, which take a pointer
 to the given type, and return the converted value. The other variation
 is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which
 convert value referred to by the pointer, and return void.
 
 :c:func:`local_irq_save()`/:c:func:`local_irq_restore()`
 --------------------------------------------------------
 
 Defined in ``include/linux/irqflags.h``
 
 These routines disable hard interrupts on the local CPU, and restore
 them. They are reentrant; saving the previous state in their one
 ``unsigned long flags`` argument. If you know that interrupts are
 enabled, you can simply use :c:func:`local_irq_disable()` and
 :c:func:`local_irq_enable()`.
 
 .. _local_bh_disable:
 
 :c:func:`local_bh_disable()`/:c:func:`local_bh_enable()`
 --------------------------------------------------------
 
 Defined in ``include/linux/bottom_half.h``
 
 
 These routines disable soft interrupts on the local CPU, and restore
 them. They are reentrant; if soft interrupts were disabled before, they
 will still be disabled after this pair of functions has been called.
 They prevent softirqs and tasklets from running on the current CPU.
 
 :c:func:`smp_processor_id()`
 ----------------------------
 
 Defined in ``include/linux/smp.h``
 
 :c:func:`get_cpu()` disables preemption (so you won't suddenly get
 moved to another CPU) and returns the current processor number, between
 0 and ``NR_CPUS``. Note that the CPU numbers are not necessarily
 continuous. You return it again with :c:func:`put_cpu()` when you
 are done.
 
 If you know you cannot be preempted by another task (ie. you are in
 interrupt context, or have preemption disabled) you can use
 smp_processor_id().
 
 ``__init``/``__exit``/``__initdata``
 ------------------------------------
 
 Defined in  ``include/linux/init.h``
 
 After boot, the kernel frees up a special section; functions marked with
 ``__init`` and data structures marked with ``__initdata`` are dropped
 after boot is complete: similarly modules discard this memory after
 initialization. ``__exit`` is used to declare a function which is only
 required on exit: the function will be dropped if this file is not
 compiled as a module. See the header file for use. Note that it makes no
 sense for a function marked with ``__init`` to be exported to modules
 with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this
 will break.
 
 :c:func:`__initcall()`/:c:func:`module_init()`
 ----------------------------------------------
 
 Defined in  ``include/linux/init.h`` / ``include/linux/module.h``
 
 Many parts of the kernel are well served as a module
 (dynamically-loadable parts of the kernel). Using the
 :c:func:`module_init()` and :c:func:`module_exit()` macros it
 is easy to write code without #ifdefs which can operate both as a module
 or built into the kernel.
 
 The :c:func:`module_init()` macro defines which function is to be
 called at module insertion time (if the file is compiled as a module),
 or at boot time: if the file is not compiled as a module the
 :c:func:`module_init()` macro becomes equivalent to
 :c:func:`__initcall()`, which through linker magic ensures that
 the function is called on boot.
 
 The function can return a negative error number to cause module loading
 to fail (unfortunately, this has no effect if the module is compiled
 into the kernel). This function is called in user context with
 interrupts enabled, so it can sleep.
 
 :c:func:`module_exit()`
 -----------------------
 
 
 Defined in  ``include/linux/module.h``
 
 This macro defines the function to be called at module removal time (or
 never, in the case of the file compiled into the kernel). It will only
 be called if the module usage count has reached zero. This function can
 also sleep, but cannot fail: everything must be cleaned up by the time
 it returns.
 
 Note that this macro is optional: if it is not present, your module will
 not be removable (except for 'rmmod -f').
 
 :c:func:`try_module_get()`/:c:func:`module_put()`
 -------------------------------------------------
 
 Defined in ``include/linux/module.h``
 
 These manipulate the module usage count, to protect against removal (a
 module also can't be removed if another module uses one of its exported
 symbols: see below). Before calling into module code, you should call
 :c:func:`try_module_get()` on that module: if it fails, then the
 module is being removed and you should act as if it wasn't there.
 Otherwise, you can safely enter the module, and call
 :c:func:`module_put()` when you're finished.
 
 Most registerable structures have an owner field, such as in the
 :c:type:`struct file_operations <file_operations>` structure.
 Set this field to the macro ``THIS_MODULE``.
 
 Wait Queues ``include/linux/wait.h``
 ====================================
 
 **[SLEEPS]**
 
 A wait queue is used to wait for someone to wake you up when a certain
 condition is true. They must be used carefully to ensure there is no
 race condition. You declare a :c:type:`wait_queue_head_t`, and then processes
 which want to wait for that condition declare a :c:type:`wait_queue_entry_t`
 referring to themselves, and place that in the queue.
 
 Declaring
 ---------
 
 You declare a ``wait_queue_head_t`` using the
 :c:func:`DECLARE_WAIT_QUEUE_HEAD()` macro, or using the
 :c:func:`init_waitqueue_head()` routine in your initialization
 code.
 
 Queuing
 -------
 
 Placing yourself in the waitqueue is fairly complex, because you must
 put yourself in the queue before checking the condition. There is a
 macro to do this: :c:func:`wait_event_interruptible()`
 (``include/linux/wait.h``) The first argument is the wait queue head, and
 the second is an expression which is evaluated; the macro returns 0 when
 this expression is true, or ``-ERESTARTSYS`` if a signal is received. The
 :c:func:`wait_event()` version ignores signals.
 
 Waking Up Queued Tasks
 ----------------------
 
 Call :c:func:`wake_up()` (``include/linux/wait.h``), which will wake
 up every process in the queue. The exception is if one has
 ``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will
 not be woken. There are other variants of this basic function available
 in the same header.
 
 Atomic Operations
 =================
 
 Certain operations are guaranteed atomic on all platforms. The first
 class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``);
 this contains a signed integer (at least 32 bits long), and you must use
 these functions to manipulate or read :c:type:`atomic_t` variables.
 :c:func:`atomic_read()` and :c:func:`atomic_set()` get and set
 the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`,
 :c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and
 :c:func:`atomic_dec_and_test()` (returns true if it was
 decremented to zero).
 
 Yes. It returns true (i.e. != 0) if the atomic variable is zero.
 
 Note that these functions are slower than normal arithmetic, and so
 should not be used unnecessarily.
 
 The second class of atomic operations is atomic bit operations on an
 ``unsigned long``, defined in ``include/linux/bitops.h``. These
 operations generally take a pointer to the bit pattern, and a bit
 number: 0 is the least significant bit. :c:func:`set_bit()`,
 :c:func:`clear_bit()` and :c:func:`change_bit()` set, clear,
 and flip the given bit. :c:func:`test_and_set_bit()`,
 :c:func:`test_and_clear_bit()` and
 :c:func:`test_and_change_bit()` do the same thing, except return
 true if the bit was previously set; these are particularly useful for
 atomically setting flags.
 
 It is possible to call these operations with bit indices greater than
 ``BITS_PER_LONG``. The resulting behavior is strange on big-endian
 platforms though so it is a good idea not to do this.
 
 Symbols
 =======
 
 Within the kernel proper, the normal linking rules apply (ie. unless a
 symbol is declared to be file scope with the ``static`` keyword, it can
 be used anywhere in the kernel). However, for modules, a special
 exported symbol table is kept which limits the entry points to the
 kernel proper. Modules can also export symbols.
 
 :c:func:`EXPORT_SYMBOL()`
 -------------------------
 
 Defined in ``include/linux/export.h``
 
 This is the classic method of exporting a symbol: dynamically loaded
 modules will be able to use the symbol as normal.
 
 :c:func:`EXPORT_SYMBOL_GPL()`
 -----------------------------
 
 Defined in ``include/linux/export.h``
 
 Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols
 exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by
 modules with a :c:func:`MODULE_LICENSE()` that specifies a GPL
 compatible license. It implies that the function is considered an
 internal implementation issue, and not really an interface. Some
 maintainers and developers may however require EXPORT_SYMBOL_GPL()
 when adding any new APIs or functionality.
 
 :c:func:`EXPORT_SYMBOL_NS()`
 ----------------------------
 
 Defined in ``include/linux/export.h``
 
 This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
 Documentation/core-api/symbol-namespaces.rst
 
 :c:func:`EXPORT_SYMBOL_NS_GPL()`
 --------------------------------
 
 Defined in ``include/linux/export.h``
 
 This is the variant of `EXPORT_SYMBOL_GPL()` that allows specifying a symbol
 namespace. Symbol Namespaces are documented in
 Documentation/core-api/symbol-namespaces.rst
 
 Routines and Conventions
 ========================
 
 Double-linked lists ``include/linux/list.h``
 --------------------------------------------
 
 There used to be three sets of linked-list routines in the kernel
 headers, but this one is the winner. If you don't have some particular
 pressing need for a single list, it's a good choice.
 
 In particular, :c:func:`list_for_each_entry()` is useful.
 
 Return Conventions
 ------------------
 
 For code called in user context, it's very common to defy C convention,
 and return 0 for success, and a negative error number (eg. ``-EFAULT``) for
 failure. This can be unintuitive at first, but it's fairly widespread in
 the kernel.
 
 Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a
 negative error number into a pointer, and :c:func:`IS_ERR()` and
 :c:func:`PTR_ERR()` to get it back out again: avoids a separate
 pointer parameter for the error number. Icky, but in a good way.
 
 Breaking Compilation
 --------------------
 
 Linus and the other developers sometimes change function or structure
 names in development kernels; this is not done just to keep everyone on
 their toes: it reflects a fundamental change (eg. can no longer be
 called with interrupts on, or does extra checks, or doesn't do checks
 which were caught before). Usually this is accompanied by a fairly
 complete note to the appropriate kernel development mailing list; search
 the archives. Simply doing a global replace on the file usually makes
 things **worse**.
 
 Initializing structure members
 ------------------------------
 
 The preferred method of initializing structures is to use designated
 initialisers, as defined by ISO C99, eg::
 
     static struct block_device_operations opt_fops = {
             .open               = opt_open,
             .release            = opt_release,
             .ioctl              = opt_ioctl,
             .check_media_change = opt_media_change,
     };
 
 
 This makes it easy to grep for, and makes it clear which structure
 fields are set. You should do this because it looks cool.
 
 GNU Extensions
 --------------
 
 GNU Extensions are explicitly allowed in the Linux kernel. Note that
 some of the more complex ones are not very well supported, due to lack
 of general use, but the following are considered standard (see the GCC
 info page section "C Extensions" for more details - Yes, really the info
 page, the man page is only a short summary of the stuff in info).
 
 -  Inline functions
 
 -  Statement expressions (ie. the ({ and }) constructs).
 
 -  Declaring attributes of a function / variable / type
    (__attribute__)
 
 -  typeof
 
 -  Zero length arrays
 
 -  Macro varargs
 
 -  Arithmetic on void pointers
 
 -  Non-Constant initializers
 
 -  Assembler Instructions (not outside arch/ and include/asm/)
 
 -  Function names as strings (__func__).
 
 -  __builtin_constant_p()
 
 Be wary when using long long in the kernel, the code gcc generates for
 it is horrible and worse: division and multiplication does not work on
 i386 because the GCC runtime functions for it are missing from the
 kernel environment.
 
 C++
 ---
 
 Using C++ in the kernel is usually a bad idea, because the kernel does
 not provide the necessary runtime environment and the include files are
 not tested for it. It is still possible, but not recommended. If you
 really want to do this, forget about exceptions at least.
 
 #if
 ---
 
 It is generally considered cleaner to use macros in header files (or at
 the top of .c files) to abstract away functions rather than using \`#if'
 pre-processor statements throughout the source code.
 
 Putting Your Stuff in the Kernel
 ================================
 
 In order to get your stuff into shape for official inclusion, or even to
 make a neat patch, there's administrative work to be done:
 
 -  Figure out who are the owners of the code you've been modifying. Look
    at the top of the source files, inside the ``MAINTAINERS`` file, and
    last of all in the ``CREDITS`` file. You should coordinate with these
    people to make sure you're not duplicating effort, or trying something
    that's already been rejected.
 
    Make sure you put your name and email address at the top of any files
    you create or modify significantly. This is the first place people
    will look when they find a bug, or when **they** want to make a change.
 
 -  Usually you want a configuration option for your kernel hack. Edit
    ``Kconfig`` in the appropriate directory. The Config language is
    simple to use by cut and paste, and there's complete documentation in
    ``Documentation/kbuild/kconfig-language.rst``.
 
    In your description of the option, make sure you address both the
    expert user and the user who knows nothing about your feature.
    Mention incompatibilities and issues here. **Definitely** end your
    description with “if in doubt, say N” (or, occasionally, \`Y'); this
    is for people who have no idea what you are talking about.
 
 -  Edit the ``Makefile``: the CONFIG variables are exported here so you
    can usually just add a "obj-$(CONFIG_xxx) += xxx.o" line. The syntax
    is documented in ``Documentation/kbuild/makefiles.rst``.
 
 -  Put yourself in ``CREDITS`` if you consider what you've done
    noteworthy, usually beyond a single file (your name should be at the
    top of the source files anyway). ``MAINTAINERS`` means you want to be
    consulted when changes are made to a subsystem, and hear about bugs;
    it implies a more-than-passing commitment to some part of the code.
 
 -  Finally, don't forget to read
    ``Documentation/process/submitting-patches.rst``
 
 Kernel Cantrips
 ===============
 
 Some favorites from browsing the source. Feel free to add to this list.
 
 ``arch/x86/include/asm/delay.h``::
 
     #define ndelay(n) (__builtin_constant_p(n) ? \
             ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \
             __ndelay(n))
 
 
 ``include/linux/fs.h``::
 
     /*
      * Kernel pointers have redundant information, so we can use a
      * scheme where we can return either an error code or a dentry
      * pointer with the same return value.
      *
      * This should be a per-architecture thing, to allow different
      * error and pointer decisions.
      */
      #define ERR_PTR(err)    ((void *)((long)(err)))
      #define PTR_ERR(ptr)    ((long)(ptr))
      #define IS_ERR(ptr)     ((unsigned long)(ptr) > (unsigned long)(-1000))
 
 ``arch/x86/include/asm/uaccess_32.h:``::
 
     #define copy_to_user(to,from,n)                         \
             (__builtin_constant_p(n) ?                      \
              __constant_copy_to_user((to),(from),(n)) :     \
              __generic_copy_to_user((to),(from),(n)))
 
 
 ``arch/sparc/kernel/head.S:``::
 
     /*
      * Sun people can't spell worth damn. "compatability" indeed.
      * At least we *know* we can't spell, and use a spell-checker.
      */
 
     /* Uh, actually Linus it is I who cannot spell. Too much murky
      * Sparc assembly will do this to ya.
      */
     C_LABEL(cputypvar):
             .asciz "compatibility"
 
     /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
             .align 4
     C_LABEL(cputypvar_sun4m):
             .asciz "compatible"
 
 
 ``arch/sparc/lib/checksum.S:``::
 
             /* Sun, you just can't beat me, you just can't.  Stop trying,
              * give up.  I'm serious, I am going to kick the living shit
              * out of you, game over, lights out.
              */
 
 
 Thanks
 ======
 
 Thanks to Andi Kleen for the idea, answering my questions, fixing my
 mistakes, filling content, etc. Philipp Rumpf for more spelling and
 clarity fixes, and some excellent non-obvious points. Werner Almesberger
 for giving me a great summary of :c:func:`disable_irq()`, and Jes
 Sorensen and Andrea Arcangeli added caveats. Michael Elizabeth Chastain
 for checking and adding to the Configure section. Telsa Gwynne for
 teaching me DocBook.

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