Documentation / kernel-hacking / hacking.rst

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.. _kernel_hacking_hack:

Unreliable Guide To Hacking The Linux Kernel

:Author: Rusty Russell


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

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

.. 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

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

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.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).

    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.


Common Routines


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``


: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

:c:func:`copy_to_user()` and :c:func:`copy_from_user()` are
more general: they copy an arbitrary amount of data to and from

.. 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.


Defined in ``include/linux/slab.h``


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:

    May sleep and swap to free memory. Only allowed in user context, but
    is the most reliable way to allocate memory.

    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.

    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()`

Before inventing your own cache of often-used objects consider using a
slab cache in ``include/linux/slab.h``


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.


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()`.


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.


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

.. _local_bh_disable:


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.


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


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.


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.


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').


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``


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.


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


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.


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.


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.


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.


Defined in ``include/linux/export.h``

This is the variant of `EXPORT_SYMBOL()` that allows specifying a symbol
namespace. Symbol Namespaces are documented in


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

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

-  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.


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.


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

   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

Kernel Cantrips

Some favorites from browsing the source. Feel free to add to this list.


    #define ndelay(n) (__builtin_constant_p(n) ? \
            ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \


     * 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))


    #define copy_to_user(to,from,n)                         \
            (__builtin_constant_p(n) ?                      \
             __constant_copy_to_user((to),(from),(n)) :     \


     * 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.
            .asciz "compatibility"

    /* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
            .align 4
            .asciz "compatible"


            /* 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 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.