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+.. _kernel_hacking_hack:
+
+============================================
+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
+:c:func:`in_irq()` 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', and probably more to do with some bad vodka Alexey
+    Kuznetsov had at the time.
+
+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 moronic 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:func:`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
+:doc:`../core-api/symbol-namespaces`
+
+: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
+:doc:`../core-api/symbol-namespaces`
+
+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 linux-kernel mailing list; search the archive.
+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 whose pond you've been pissing in. Look at the top of the
+   source files, inside the ``MAINTAINERS`` file, and last of all in the
+   ``CREDITS`` file. You should coordinate with this person 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 mangle 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've done something 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`` and possibly
+   ``Documentation/process/submitting-drivers.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.