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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
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Within the kernel proper, th
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