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 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 in_interrupt()
(include/linux/preempt.h
) is false.
Warning
Beware that if you have preemption or softirqs disabled (see below),
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
in_softirq()
macro (include/linux/preempt.h
).
Warning
Beware that this will return a false positive if a bottom half lock 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 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¶
printk()
¶
Defined in include/linux/printk.h
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);
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.
copy_to_user()
/ copy_from_user()
/ get_user()
/ put_user()
¶
Defined in include/linux/uaccess.h
/ asm/uaccess.h
[SLEEPS]
put_user()
and 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.
copy_to_user()
and copy_from_user()
are
more general: they copy an arbitrary amount of data to and from
userspace.
Warning
Unlike put_user()
and 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.
kmalloc()
/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
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 __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
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 alloc_bootmem()
routine.
Before inventing your own cache of often-used objects consider using a
slab cache in include/linux/slab.h
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.
mdelay()
/udelay()
¶
Defined in include/asm/delay.h
/ include/linux/delay.h
The udelay()
and ndelay()
functions can be
used for small pauses. Do not use large values with them as you risk
overflow - the helper function mdelay()
is useful here, or
consider msleep()
.
cpu_to_be32()
/be32_to_cpu()
/cpu_to_le32()
/le32_to_cpu()
¶
Defined in include/asm/byteorder.h
The 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:
be32_to_cpu()
, etc.
There are two major variations of these functions: the pointer
variation, such as 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 cpu_to_be32s()
, which
convert value referred to by the pointer, and return void.
local_irq_save()
/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 local_irq_disable()
and
local_irq_enable()
.
local_bh_disable()
/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.
smp_processor_id()
¶
Defined in include/linux/smp.h
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 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 EXPORT_SYMBOL()
or EXPORT_SYMBOL_GPL()
- this
will break.
__initcall()
/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
module_init()
and module_exit()
macros it
is easy to write code without #ifdefs which can operate both as a module
or built into the kernel.
The 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
module_init()
macro becomes equivalent to
__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.
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’).
try_module_get()
/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
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
module_put()
when you’re finished.
Most registerable structures have an owner field, such as in the
struct 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 wait_queue_head_t
, and then processes
which want to wait for that condition declare a wait_queue_entry_t
referring to themselves, and place that in the queue.
Declaring¶
You declare a wait_queue_head_t
using the
DECLARE_WAIT_QUEUE_HEAD()
macro, or using the
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: 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
wait_event()
version ignores signals.
Waking Up Queued Tasks¶
Call 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 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 atomic_t
variables.
atomic_read()
and atomic_set()
get and set
the counter, atomic_add()
, atomic_sub()
,
atomic_inc()
, atomic_dec()
, and
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. set_bit()
,
clear_bit()
and change_bit()
set, clear,
and flip the given bit. test_and_set_bit()
,
test_and_clear_bit()
and
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.
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.
EXPORT_SYMBOL_GPL()
¶
Defined in include/linux/export.h
Similar to EXPORT_SYMBOL()
except that the symbols
exported by EXPORT_SYMBOL_GPL()
can only be seen by
modules with a 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.
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 Symbol Namespaces
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 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, 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 ERR_PTR()
(include/linux/err.h
) to encode a
negative error number into a pointer, and IS_ERR()
and
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 theCREDITS
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 inDocumentation/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 inDocumentation/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 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.