code:
.. _codingstyle:

Linux kernel coding style
=========================

This is a short document describing the preferred coding style for the
linux kernel.  Coding style is very personal, and I won't **force** my
views on anybody, but this is what goes for anything that I have to be
able to maintain, and I'd prefer it for most other things too.  Please
at least consider the points made here.

First off, I'd suggest printing out a copy of the GNU coding standards,
and NOT read it.  Burn them, it's a great symbolic gesture.

Anyway, here goes:

1) Indentation
--------------

Tabs are 8 characters, and thus indentations are also 8 characters.
There are heretic movements that try to make indentations 4 (or even 2!)
characters deep, and that is akin to trying to define the value of PI to
be 3.

Rationale: The whole idea behind indentation is to clearly define where
a block of control starts and ends.  Especially when you've been looking
at your screen for 20 straight hours, you'll find it a lot easier to see
how the indentation works if you have large indentations.

Now, some people will claim that having 8-character indentations makes
the code move too far to the right, and makes it hard to read on a
80-character terminal screen.  The answer to that is that if you need
more than 3 levels of indentation, you're screwed anyway, and should fix
your program.

In short, 8-char indents make things easier to read, and have the added
benefit of warning you when you're nesting your functions too deep.
Heed that warning.

The preferred way to ease multiple indentation levels in a switch statement is
to align the ``switch`` and its subordinate ``case`` labels in the same column
instead of ``double-indenting`` the ``case`` labels.  E.g.:

.. code-block:: c

	switch (suffix) {
	case 'G':
	case 'g':
		mem <<= 30;
		break;
	case 'M':
	case 'm':
		mem <<= 20;
		break;
	case 'K':
	case 'k':
		mem <<= 10;
		fallthrough;
	default:
		break;
	}

Don't put multiple statements on a single line unless you have
something to hide:

.. code-block:: c

	if (condition) do_this;
	  do_something_everytime;

Don't use commas to avoid using braces:

.. code-block:: c

	if (condition)
		do_this(), do_that();

Always uses braces for multiple statements:

.. code-block:: c

	if (condition) {
		do_this();
		do_that();
	}

Don't put multiple assignments on a single line either.  Kernel coding style
is super simple.  Avoid tricky expressions.

Outside of comments, documentation and except in Kconfig, spaces are never
used for indentation, and the above example is deliberately broken.

Get a decent editor and don't leave whitespace at the end of lines.

2) Breaking long lines and strings
----------------------------------

Coding style is all about readability and maintainability using commonly
available tools.

The preferred limit on the length of a single line is 80 columns.

Statements longer than 80 columns should be broken into sensible chunks,
unless exceeding 80 columns significantly increases readability and does
not hide information.

Descendants are always substantially shorter than the parent and
are placed substantially to the right.  A very commonly used style
is to align descendants to a function open parenthesis.

These same rules are applied to function headers with a long argument list.

However, never break user-visible strings such as printk messages because
that breaks the ability to grep for them.

3) Placing Braces and Spaces
----------------------------

The other issue that always comes up in C styling is the placement of
braces.  Unlike the indent size, there are few technical reasons to
choose one placement strategy over the other, but the preferred way, as
shown to us by the prophets Kernighan and Ritchie, is to put the opening
brace last on the line, and put the closing brace first, thusly:

.. code-block:: c

	if (x is true) {
		we do y
	}

This applies to all non-function statement blocks (if, switch, for,
while, do).  E.g.:

.. code-block:: c

	switch (action) {
	case KOBJ_ADD:
		return "add";
	case KOBJ_REMOVE:
		return "remove";
	case KOBJ_CHANGE:
		return "change";
	default:
		return NULL;
	}

However, there is one special case, namely functions: they have the
opening brace at the beginning of the next line, thus:

.. code-block:: c

	int function(int x)
	{
		body of function
	}

Heretic people all over the world have claimed that this inconsistency
is ...  well ...  inconsistent, but all right-thinking people know that
(a) K&R are **right** and (b) K&R are right.  Besides, functions are
special anyway (you can't nest them in C).

Note that the closing brace is empty on a line of its own, **except** in
the cases where it is followed by a continuation of the same statement,
ie a ``while`` in a do-statement or an ``else`` in an if-statement, like
this:

.. code-block:: c

	do {
		body of do-loop
	} while (condition);

and

.. code-block:: c

	if (x == y) {
		..
	} else if (x > y) {
		...
	} else {
		....
	}

Rationale: K&R.

Also, note that this brace-placement also minimizes the number of empty
(or almost empty) lines, without any loss of readability.  Thus, as the
supply of new-lines on your screen is not a renewable resource (think
25-line terminal screens here), you have more empty lines to put
comments on.

Do not unnecessarily use braces where a single statement will do.

.. code-block:: c

	if (condition)
		action();

and

.. code-block:: none

	if (condition)
		do_this();
	else
		do_that();

This does not apply if only one branch of a conditional statement is a single
statement; in the latter case use braces in both branches:

.. code-block:: c

	if (condition) {
		do_this();
		do_that();
	} else {
		otherwise();
	}

Also, use braces when a loop contains more than a single simple statement:

.. code-block:: c

	while (condition) {
		if (test)
			do_something();
	}

3.1) Spaces
***********

Linux kernel style for use of spaces depends (mostly) on
function-versus-keyword usage.  Use a space after (most) keywords.  The
notable exceptions are sizeof, typeof, alignof, and __attribute__, which look
somewhat like functions (and are usually used with parentheses in Linux,
although they are not required in the language, as in: ``sizeof info`` after
``struct fileinfo info;`` is declared).

So use a space after these keywords::

	if, switch, case, for, do, while

but not with sizeof, typeof, alignof, or __attribute__.  E.g.,

.. code-block:: c

	s = sizeof(struct file);

Do not add spaces around (inside) parenthesized expressions.  This example is
**bad**:

.. code-block:: c

	s = sizeof( struct file );

When declaring pointer data or a function that returns a pointer type, the
preferred use of ``*`` is adjacent to the data name or function name and not
adjacent to the type name.  Examples:

.. code-block:: c

	char *linux_banner;
	unsigned long long memparse(char *ptr, char **retptr);
	char *match_strdup(substring_t *s);

Use one space around (on each side of) most binary and ternary operators,
such as any of these::

	=  +  -  <  >  *  /  %  |  &  ^  <=  >=  ==  !=  ?  :

but no space after unary operators::

	&  *  +  -  ~  !  sizeof  typeof  alignof  __attribute__  defined

no space before the postfix increment & decrement unary operators::

	++  --

no space after the prefix increment & decrement unary operators::

	++  --

and no space around the ``.`` and ``->`` structure member operators.

Do not leave trailing whitespace at the ends of lines.  Some editors with
``smart`` indentation will insert whitespace at the beginning of new lines as
appropriate, so you can start typing the next line of code right away.
However, some such editors do not remove the whitespace if you end up not
putting a line of code there, such as if you leave a blank line.  As a result,
you end up with lines containing trailing whitespace.

Git will warn you about patches that introduce trailing whitespace, and can
optionally strip the trailing whitespace for you; however, if applying a series
of patches, this may make later patches in the series fail by changing their
context lines.

4) Naming
---------

C is a Spartan language, and your naming conventions should follow suit.
Unlike Modula-2 and Pascal programmers, C programmers do not use cute
names like ThisVariableIsATemporaryCounter. A C programmer would call that
variable ``tmp``, which is much easier to write, and not the least more
difficult to understand.

HOWEVER, while mixed-case names are frowned upon, descriptive names for
global variables are a must.  To call a global function ``foo`` is a
shooting offense.

GLOBAL variables (to be used only if you **really** need them) need to
have descriptive names, as do global functions.  If you have a function
that counts the number of active users, you should call that
``count_active_users()`` or similar, you should **not** call it ``cntusr()``.

Encoding the type of a function into the name (so-called Hungarian
notation) is asinine - the compiler knows the types anyway and can check
those, and it only confuses the programmer.

LOCAL variable names should be short, and to the point.  If you have
some random integer loop counter, it should probably be called ``i``.
Calling it ``loop_counter`` is non-productive, if there is no chance of it
being mis-understood.  Similarly, ``tmp`` can be just about any type of
variable that is used to hold a temporary value.

If you are afraid to mix up your local variable names, you have another
problem, which is called the function-growth-hormone-imbalance syndrome.
See chapter 6 (Functions).

For symbol names and documentation, avoid introducing new usage of
'master / slave' (or 'slave' independent of 'master') and 'blacklist /
whitelist'.

Recommended replacements for 'master / slave' are:
    '{primary,main} / {secondary,replica,subordinate}'
    '{initiator,requester} / {target,responder}'
    '{controller,host} / {device,worker,proxy}'
    'leader / follower'
    'director / performer'

Recommended replacements for 'blacklist/whitelist' are:
    'denylist / allowlist'
    'blocklist / passlist'

Exceptions for introducing new usage is to maintain a userspace ABI/API,
or when updating code for an existing (as of 2020) hardware or protocol
specification that mandates those terms. For new specifications
translate specification usage of the terminology to the kernel coding
standard where possible.

5) Typedefs
-----------

Please don't use things like ``vps_t``.
It's a **mistake** to use typedef for structures and pointers. When you see a

.. code-block:: c

	vps_t a;

in the source, what does it mean?
In contrast, if it says

.. code-block:: c

	struct virtual_container *a;

you can actually tell what ``a`` is.

Lots of people think that typedefs ``help readability``. Not so. They are
useful only for:

 (a) totally opaque objects (where the typedef is actively used to **hide**
     what the object is).

     Example: ``pte_t`` etc. opaque objects that you can only access using
     the proper accessor functions.

     .. note::

       Opaqueness and ``accessor functions`` are not good in themselves.
       The reason we have them for things like pte_t etc. is that there
       really is absolutely **zero** portably accessible information there.

 (b) Clear integer types, where the abstraction **helps** avoid confusion
     whether it is ``int`` or ``long``.

     u8/u16/u32 are perfectly fine typedefs, although they fit into
     category (d) better than here.

     .. note::

       Again - there needs to be a **reason** for this. If something is
       ``unsigned long``, then there's no reason to do

	typedef unsigned long myflags_t;

     but if there is a clear reason for why it under certain circumstances
     might be an ``unsigned int`` and under other configurations might be
     ``unsigned long``, then by all means go ahead and use a typedef.

 (c) when you use sparse to literally create a **new** type for
     type-checking.

 (d) New types which are identical to standard C99 types, in certain
     exceptional circumstances.

     Although it would only take a short amount of time for the eyes and
     brain to become accustomed to the standard types like ``uint32_t``,
     some people object to their use anyway.

     Therefore, the Linux-specific ``u8/u16/u32/u64`` types and their
     signed equivalents which are identical to standard types are
     permitted -- although they are not mandatory in new code of your
     own.

     When editing existing code which already uses one or the other set
     of types, you should conform to the existing choices in that code.

 (e) Types safe for use in userspace.

     In certain structures which are visible to userspace, we cannot
     require C99 types and cannot use the ``u32`` form above. Thus, we
     use __u32 and similar types in all structures which are shared
     with userspace.

Maybe there are other cases too, but the rule should basically be to NEVER
EVER use a typedef unless you can clearly match one of those rules.

In general, a pointer, or a struct that has elements that can reasonably
be directly accessed should **never** be a typedef.

6) Functions
------------

Functions should be short and sweet, and do just one thing.  They should
fit on one or two screenfuls of text (the ISO/ANSI screen size is 80x24,
as we all know), and do one thing and do that well.

The maximum length of a function is inversely proportional to the
complexity and indentation level of that function.  So, if you have a
conceptually simple function that is just one long (but simple)
case-statement, where you have to do lots of small things for a lot of
different cases, it's OK to have a longer function.

However, if you have a complex function, and you suspect that a
less-than-gifted first-year high-school student might not even
understand what the function is all about, you should adhere to the
maximum limits all the more closely.  Use helper functions with
descriptive names (you can ask the compiler to in-line them if you think
it's performance-critical, and it will probably do a better job of it
than you would have done).

Another measure of the function is the number of local variables.  They
shouldn't exceed 5-10, or you're doing something wrong.  Re-think the
function, and split it into smaller pieces.  A human brain can
generally easily keep track of about 7 different things, anything more
and it gets confused.  You know you're brilliant, but maybe you'd like
to understand what you did 2 weeks from now.

In source files, separate functions with one blank line.  If the function is
exported, the **EXPORT** macro for it should follow immediately after the
closing function brace line.  E.g.:

.. code-block:: c

	int system_is_up(void)
	{
		return system_state == SYSTEM_RUNNING;
	}
	EXPORT_SYMBOL(system_is_up);

6.1) Function prototypes
************************

In function prototypes, include parameter names with their data types.
Although this is not required by the C language, it is preferred in Linux
because it is a simple way to add valuable information for the reader.

Do not use the ``extern`` keyword with function declarations as this makes
lines longer and isn't strictly necessary.

When writing function prototypes, please keep the `order of elements regular
<https://lore.kernel.org/mm-commits/CAHk-=wiOCLRny5aifWNhr621kYrJwhfURsa0vFPeUEm8mF0ufg@mail.gmail.com/>`_.
For example, using this function declaration example::

 __init void * __must_check action(enum magic value, size_t size, u8 count,
				   char *fmt, ...) __printf(4, 5) __malloc;

The preferred order of elements for a function prototype is:

- storage class (below, ``static __always_inline``, noting that ``__always_inline``
  is technically an attribute but is treated like ``inline``)
- storage class attributes (here, ``__init`` -- i.e. section declarations, but also
  things like ``__cold``)
- return type (here, ``void *``)
- return type attributes (here, ``__must_check``)
- function name (here, ``action``)
- function parameters (here, ``(enum magic value, size_t size, u8 count, char *fmt, ...)``,
  noting that parameter names should always be included)
- function parameter attributes (here, ``__printf(4, 5)``)
- function behavior attributes (here, ``__malloc``)

Note that for a function **definition** (i.e. the actual function body),
the compiler does not allow function parameter attributes after the
function parameters. In these cases, they should go after the storage
class attributes (e.g. note the changed position of ``__printf(4, 5)``
below, compared to the **declaration** example above)::

 static __always_inline __init __printf(4, 5) void * __must_check action(enum magic value,
		size_t size, u8 count, char *fmt, ...) __malloc
 {
	...
 }

7) Centralized exiting of functions
-----------------------------------

Albeit deprecated by some people, the equivalent of the goto statement is
used frequently by compilers in form of the unconditional jump instruction.

The goto statement comes in handy when a function exits from multiple
locations and some common work such as cleanup has to be done.  If there is no
cleanup needed then just return directly.

Choose label names which say what the goto does or why the goto exists.  An
example of a good name could be ``out_free_buffer:`` if the goto frees ``buffer``.
Avoid using GW-BASIC names like ``err1:`` and ``err2:``, as you would have to
renumber them if you ever add or remove exit paths, and they make correctness
difficult to verify anyway.

The rationale for using gotos is:

- unconditional statements are easier to understand and follow
- nesting is reduced
- errors by not updating individual exit points when making
  modifications are prevented
- saves the compiler work to optimize redundant code away ;)

.. code-block:: c

	int fun(int a)
	{
		int result = 0;
		char *buffer;

		buffer = kmalloc(SIZE, GFP_KERNEL);
		if (!buffer)
			return -ENOMEM;

		if (condition1) {
			while (loop1) {
				...
			}
			result = 1;
			goto out_free_buffer;
		}
		...
	out_free_buffer:
		kfree(buffer);
		return result;
	}

A common type of bug to be aware of is ``one err bugs`` which look like this:

.. code-block:: c

	err:
		kfree(foo->bar);
		kfree(foo);
		return ret;

The bug in this code is that on some exit paths ``foo`` is NULL.  Normally the
fix for this is to split it up into two error labels ``err_free_bar:`` and
``err_free_foo:``:

.. code-block:: c

	 err_free_bar:
		kfree(foo->bar);
	 err_free_foo:
		kfree(foo);
		return ret;

Ideally you should simulate errors to test all exit paths.

8) Commenting
-------------

Comments are good, but there is also a danger of over-commenting.  NEVER
try to explain HOW your code works in a comment: it's much better to
write the code so that the **working** is obvious, and it's a waste of
time to explain badly written code.

Generally, you want your comments to tell WHAT your code does, not HOW.
Also, try to avoid putting comments inside a function body: if the
function is so complex that you need to separately comment parts of it,
you should probably go back to chapter 6 for a while.  You can make
small comments to note or warn about something particularly clever (or
ugly), but try to avoid excess.  Instead, put the comments at the head
of the function, telling people what it does, and possibly WHY it does
it.

When commenting the kernel API functions, please use the kernel-doc format.
See the files at :ref:`Documentation/doc-guide/ <doc_guide>` and
``scripts/kernel-doc`` for details.

The preferred style for long (multi-line) comments is:

.. code-block:: c

	/*
	 * This is the preferred style for multi-line
	 * comments in the Linux kernel source code.
	 * Please use it consistently.
	 *
	 * Description:  A column of asterisks on the left side,
	 * with beginning and ending almost-blank lines.
	 */

For files in net/ and drivers/net/ the preferred style for long (multi-line)
comments is a little different.

.. code-block:: c

	/* The preferred comment style for files in net/ and drivers/net
	 * looks like this.
	 *
	 * It is nearly the same as the generally preferred comment style,
	 * but there is no initial almost-blank line.
	 */

It's also important to comment data, whether they are basic types or derived
types.  To this end, use just one data declaration per line (no commas for
multiple data declarations).  This leaves you room for a small comment on each
item, explaining its use.

9) You've made a mess of it
---------------------------

That's OK, we all do.  You've probably been told by your long-time Unix
user helper that ``GNU emacs`` automatically formats the C sources for
you, and you've noticed that yes, it does do that, but the defaults it
uses are less than desirable (in fact, they are worse than random
typing - an infinite number of monkeys typing into GNU emacs would never
make a good program).

So, you can either get rid of GNU emacs, or change it to use saner
values.  To do the latter, you can stick the following in your .emacs file:

.. code-block:: none

  (defun c-lineup-arglist-tabs-only (ignored)
    "Line up argument lists by tabs, not spaces"
    (let* ((anchor (c-langelem-pos c-syntactic-element))
           (column (c-langelem-2nd-pos c-syntactic-element))
           (offset (- (1+ column) anchor))
           (steps (floor offset c-basic-offset)))
      (* (max steps 1)
         c-basic-offset)))

  (dir-locals-set-class-variables
   'linux-kernel
   '((c-mode . (
          (c-basic-offset . 8)
          (c-label-minimum-indentation . 0)
          (c-offsets-alist . (
                  (arglist-close         . c-lineup-arglist-tabs-only)
                  (arglist-cont-nonempty .
		      (c-lineup-gcc-asm-reg c-lineup-arglist-tabs-only))
                  (arglist-intro         . +)
                  (brace-list-intro      . +)
                  (c                     . c-lineup-C-comments)
                  (case-label            . 0)
                  (comment-intro         . c-lineup-comment)
                  (cpp-define-intro      . +)
                  (cpp-macro             . -1000)
                  (cpp-macro-cont        . +)
                  (defun-block-intro     . +)
                  (else-clause           . 0)
                  (func-decl-cont        . +)
                  (inclass               . +)
                  (inher-cont            . c-lineup-multi-inher)
                  (knr-argdecl-intro     . 0)
                  (label                 . -1000)
                  (statement             . 0)
                  (statement-block-intro . +)
                  (statement-case-intro  . +)
                  (statement-cont        . +)
                  (substatement          . +)
                  ))
          (indent-tabs-mode . t)
          (show-trailing-whitespace . t)
          ))))

  (dir-locals-set-directory-class
   (expand-file-name "~/src/linux-trees")
   'linux-kernel)

This will make emacs go better with the kernel coding style for C
files below ``~/src/linux-trees``.

But even if you fail in getting emacs to do sane formatting, not
everything is lost: use ``indent``.

Now, again, GNU indent has the same brain-dead settings that GNU emacs
has, which is why you need to give it a few command line options.
However, that's not too bad, because even the makers of GNU indent
recognize the authority of K&R (the GNU people aren't evil, they are
just severely misguided in this matter), so you just give indent the
options ``-kr -i8`` (stands for ``K&R, 8 character indents``), or use
``scripts/Lindent``, which indents in the latest style.

``indent`` has a lot of options, and especially when it comes to comment
re-formatting you may want to take a look at the man page.  But
remember: ``indent`` is not a fix for bad programming.

Note that you can also use the ``clang-format`` tool to help you with
these rules, to quickly re-format parts of your code automatically,
and to review full files in order to spot coding style mistakes,
typos and possible improvements. It is also handy for sorting ``#includes``,
for aligning variables/macros, for reflowing text and other similar tasks.
See the file :ref:`Documentation/process/clang-format.rst <clangformat>`
for more details.

10) Kconfig configuration files
-------------------------------

For all of the Kconfig* configuration files throughout the source tree,
the indentation is somewhat different.  Lines under a ``config`` definition
are indented with one tab, while help text is indented an additional two
spaces.  Example::

  config AUDIT
	bool "Auditing support"
	depends on NET
	help
	  Enable auditing infrastructure that can be used with another
	  kernel subsystem, such as SELinux (which requires this for
	  logging of avc messages output).  Does not do system-call
	  auditing without CONFIG_AUDITSYSCALL.

Seriously dangerous features (such as write support for certain
filesystems) should advertise this prominently in their prompt string::

  config ADFS_FS_RW
	bool "ADFS write support (DANGEROUS)"
	depends on ADFS_FS
	...

For full documentation on the configuration files, see the file
Documentation/kbuild/kconfig-language.rst.

11) Data structures
-------------------

Data structures that have visibility outside the single-threaded
environment they are created and destroyed in should always have
reference counts.  In the kernel, garbage collection doesn't exist (and
outside the kernel garbage collection is slow and inefficient), which
means that you absolutely **have** to reference count all your uses.

Reference counting means that you can avoid locking, and allows multiple
users to have access to the data structure in parallel - and not having
to worry about the structure suddenly going away from under them just
because they slept or did something else for a while.

Note that locking is **not** a replacement for reference counting.
Locking is used to keep data structures coherent, while reference
counting is a memory management technique.  Usually both are needed, and
they are not to be confused with each other.

Many data structures can indeed have two levels of reference counting,
when there are users of different ``classes``.  The subclass count counts
the number of subclass users, and decrements the global count just once
when the subclass count goes to zero.

Examples of this kind of ``multi-level-reference-counting`` can be found in
memory management (``struct mm_struct``: mm_users and mm_count), and in
filesystem code (``struct super_block``: s_count and s_active).

Remember: if another thread can find your data structure, and you don't
have a reference count on it, you almost certainly have a bug.

12) Macros, Enums and RTL
-------------------------

Names of macros defining constants and labels in enums are capitalized.

.. code-block:: c

	#define CONSTANT 0x12345

Enums are preferred when defining several related constants.

CAPITALIZED macro names are appreciated but macros resembling functions
may be named in lower case.

Generally, inline functions are preferable to macros resembling functions.

Macros with multiple statements should be enclosed in a do - while block:

.. code-block:: c

	#define macrofun(a, b, c)			\
		do {					\
			if (a == 5)			\
				do_this(b, c);		\
		} while (0)

Things to avoid when using macros:

1) macros that affect control flow:

.. code-block:: c

	#define FOO(x)					\
		do {					\
			if (blah(x) < 0)		\
				return -EBUGGERED;	\
		} while (0)

is a **very** bad idea.  It looks like a function call but exits the ``calling``
function; don't break the internal parsers of those who will read the code.

2) macros that depend on having a local variable with a magic name:

.. code-block:: c

	#define FOO(val) bar(index, val)

might look like a good thing, but it's confusing as hell when one reads the
code and it's prone to breakage from seemingly innocent changes.

3) macros with arguments that are used as l-values: FOO(x) = y; will
bite you if somebody e.g. turns FOO into an inline function.

4) forgetting about precedence: macros defining constants using expressions
must enclose the expression in parentheses. Beware of similar issues with
macros using parameters.

.. code-block:: c

	#define CONSTANT 0x4000
	#define CONSTEXP (CONSTANT | 3)

5) namespace collisions when defining local variables in macros resembling
functions:

.. code-block:: c

	#define FOO(x)				\
	({					\
		typeof(x) ret;			\
		ret = calc_ret(x);		\
		(ret);				\
	})

ret is a common name for a local variable - __foo_ret is less likely
to collide with an existing variable.

The cpp manual deals with macros exhaustively. The gcc internals manual also
covers RTL which is used frequently with assembly language in the kernel.

13) Printing kernel messages
----------------------------

Kernel developers like to be seen as literate. Do mind the spelling
of kernel messages to make a good impression. Do not use incorrect
contractions like ``dont``; use ``do not`` or ``don't`` instead. Make the
messages concise, clear, and unambiguous.

Kernel messages do not have to be terminated with a period.

Printing numbers in parentheses (%d) adds no value and should be avoided.

There are a number of driver model diagnostic macros in <linux/dev_printk.h>
which you should use to make sure messages are matched to the right device
and driver, and are tagged with the right level:  dev_err(), dev_warn(),
dev_info(), and so forth.  For messages that aren't associated with a
particular device, <linux/printk.h> defines pr_notice(), pr_info(),
pr_warn(), pr_err(), etc.

Coming up with good debugging messages can be quite a challenge; and once
you have them, they can be a huge help for remote troubleshooting.  However
debug message printing is handled differently than printing other non-debug
messages.  While the other pr_XXX() functions print unconditionally,
pr_debug() does not; it is compiled out by default, unless either DEBUG is
defined or CONFIG_DYNAMIC_DEBUG is set.  That is true for dev_dbg() also,
and a related convention uses VERBOSE_DEBUG to add dev_vdbg() messages to
the ones already enabled by DEBUG.

Many subsystems have Kconfig debug options to turn on -DDEBUG in the
corresponding Makefile; in other cases specific files #define DEBUG.  And
when a debug message should be unconditionally printed, such as if it is
already inside a debug-related #ifdef section, printk(KERN_DEBUG ...) can be
used.

14) Allocating memory
---------------------

The kernel provides the following general purpose memory allocators:
kmalloc(), kzalloc(), kmalloc_array(), kcalloc(), vmalloc(), and
vzalloc().  Please refer to the API documentation for further information
about them.  :ref:`Documentation/core-api/memory-allocation.rst
<memory_allocation>`

The preferred form for passing a size of a struct is the following:

.. code-block:: c

	p = kmalloc(sizeof(*p), ...);

The alternative form where struct name is spelled out hurts readability and
introduces an opportunity for a bug when the pointer variable type is changed
but the corresponding sizeof that is passed to a memory allocator is not.

Casting the return value which is a void pointer is redundant. The conversion
from void pointer to any other pointer type is guaranteed by the C programming
language.

The preferred form for allocating an array is the following:

.. code-block:: c

	p = kmalloc_array(n, sizeof(...), ...);

The preferred form for allocating a zeroed array is the following:

.. code-block:: c

	p = kcalloc(n, sizeof(...), ...);

Both forms check for overflow on the allocation size n * sizeof(...),
and return NULL if that occurred.

These generic allocation functions all emit a stack dump on failure when used
without __GFP_NOWARN so there is no use in emitting an additional failure
message when NULL is returned.

15) The inline disease
----------------------

There appears to be a common misperception that gcc has a magic "make me
faster" speedup option called ``inline``. While the use of inlines can be
appropriate (for example as a means of replacing macros, see Chapter 12), it
very often is not. Abundant use of the inline keyword leads to a much bigger
kernel, which in turn slows the system as a whole down, due to a bigger
icache footprint for the CPU and simply because there is less memory
available for the pagecache. Just think about it; a pagecache miss causes a
disk seek, which easily takes 5 milliseconds. There are a LOT of cpu cycles
that can go into these 5 milliseconds.

A reasonable rule of thumb is to not put inline at functions that have more
than 3 lines of code in them. An exception to this rule are the cases where
a parameter is known to be a compiletime constant, and as a result of this
constantness you *know* the compiler will be able to optimize most of your
function away at compile time. For a good example of this later case, see
the kmalloc() inline function.

Often people argue that adding inline to functions that are static and used
only once is always a win since there is no space tradeoff. While this is
technically correct, gcc is capable of inlining these automatically without
help, and the maintenance issue of removing the inline when a second user
appears outweighs the potential value of the hint that tells gcc to do
something it would have done anyway.

16) Function return values and names
------------------------------------

Functions can return values of many different kinds, and one of the
most common is a value indicating whether the function succeeded or
failed.  Such a value can be represented as an error-code integer
(-Exxx = failure, 0 = success) or a ``succeeded`` boolean (0 = failure,
non-zero = success).

Mixing up these two sorts of representations is a fertile source of
difficult-to-find bugs.  If the C language included a strong distinction
between integers and booleans then the compiler would find these mistakes
for us... but it doesn't.  To help prevent such bugs, always follow this
convention::

	If the name of a function is an action or an imperative command,
	the function should return an error-code integer.  If the name
	is a predicate, the function should return a "succeeded" boolean.

For example, ``add work`` is a command, and the add_work() function returns 0
for success or -EBUSY for failure.  In the same way, ``PCI device present`` is
a predicate, and the pci_dev_present() function returns 1 if it succeeds in
finding a matching device or 0 if it doesn't.

All EXPORTed functions must respect this convention, and so should all
public functions.  Private (static) functions need not, but it is
recommended that they do.

Functions whose return value is the actual result of a computation, rather
than an indication of whether the computation succeeded, are not subject to
this rule.  Generally they indicate failure by returning some out-of-range
result.  Typical examples would be functions that return pointers; they use
NULL or the ERR_PTR mechanism to report failure.

17) Using bool
--------------

The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
only evaluate to 0 or 1, and implicit or explicit conversion to bool
automatically converts the value to true or false. When using bool types the
!! construction is not needed, which eliminates a class of bugs.

When working with bool values the true and false definitions should be used
instead of 1 and 0.

bool function return types and stack variables are always fine to use whenever
appropriate. Use of bool is encouraged to improve readability and is often a
better option than 'int' for storing boolean values.

Do not use bool if cache line layout or size of the value matters, as its size
and alignment varies based on the compiled architecture. Structures that are
optimized for alignment and size should not use bool.

If a structure has many true/false values, consider consolidating them into a
bitfield with 1 bit members, or using an appropriate fixed width type, such as
u8.

Similarly for function arguments, many true/false values can be consolidated
into a single bitwise 'flags' argument and 'flags' can often be a more
readable alternative if the call-sites have naked true/false constants.

Otherwise limited use of bool in structures and arguments can improve
readability.

18) Don't re-invent the kernel macros
-------------------------------------

The header file include/linux/kernel.h contains a number of macros that
you should use, rather than explicitly coding some variant of them yourself.
For example, if you need to calculate the length of an array, take advantage
of the macro

.. code-block:: c

	#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))

Similarly, if you need to calculate the size of some structure member, use

.. code-block:: c

	#define sizeof_field(t, f) (sizeof(((t*)0)->f))

There are also min() and max() macros that do strict type checking if you
need them.  Feel free to peruse that header file to see what else is already
defined that you shouldn't reproduce in your code.

19) Editor modelines and other cruft
------------------------------------

Some editors can interpret configuration information embedded in source files,
indicated with special markers.  For example, emacs interprets lines marked
like this:

.. code-block:: c

	-*- mode: c -*-

Or like this:

.. code-block:: c

	/*
	Local Variables:
	compile-command: "gcc -DMAGIC_DEBUG_FLAG foo.c"
	End:
	*/

Vim interprets markers that look like this:

.. code-block:: c

	/* vim:set sw=8 noet */

Do not include any of these in source files.  People have their own personal
editor configurations, and your source files should not override them.  This
includes markers for indentation and mode configuration.  People may use their
own custom mode, or may have some other magic method for making indentation
work correctly.

20) Inline assembly
-------------------

In architecture-specific code, you may need to use inline assembly to interface
with CPU or platform functionality.  Don't hesitate to do so when necessary.
However, don't use inline assembly gratuitously when C can do the job.  You can
and should poke hardware from C when possible.

Consider writing simple helper functions that wrap common bits of inline
assembly, rather than repeatedly writing them with slight variations.  Remember
that inline assembly can use C parameters.

Large, non-trivial assembly functions should go in .S files, with corresponding
C prototypes defined in C header files.  The C prototypes for assembly
functions should use ``asmlinkage``.

You may need to mark your asm statement as volatile, to prevent GCC from
removing it if GCC doesn't notice any side effects.  You don't always need to
do so, though, and doing so unnecessarily can limit optimization.

When writing a single inline assembly statement containing multiple
instructions, put each instruction on a separate line in a separate quoted
string, and end each string except the last with ``\n\t`` to properly indent
the next instruction in the assembly output:

.. code-block:: c

	asm ("magic %reg1, #42\n\t"
	     "more_magic %reg2, %reg3"
	     : /* outputs */ : /* inputs */ : /* clobbers */);

21) Conditional Compilation
---------------------------

Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
files; doing so makes code harder to read and logic harder to follow.  Instead,
use such conditionals in a header file defining functions for use in those .c
files, providing no-op stub versions in the #else case, and then call those
functions unconditionally from .c files.  The compiler will avoid generating
any code for the stub calls, producing identical results, but the logic will
remain easy to follow.

Prefer to compile out entire functions, rather than portions of functions or
portions of expressions.  Rather than putting an ifdef in an expression, factor
out part or all of the expression into a separate helper function and apply the
conditional to that function.

If you have a function or variable which may potentially go unused in a
particular configuration, and the compiler would warn about its definition
going unused, mark the definition as __maybe_unused rather than wrapping it in
a preprocessor conditional.  (However, if a function or variable *always* goes
unused, delete it.)

Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
symbol into a C boolean expression, and use it in a normal C conditional:

.. code-block:: c

	if (IS_ENABLED(CONFIG_SOMETHING)) {
		...
	}

The compiler will constant-fold the conditional away, and include or exclude
the block of code just as with an #ifdef, so this will not add any runtime
overhead.  However, this approach still allows the C compiler to see the code
inside the block, and check it for correctness (syntax, types, symbol
references, etc).  Thus, you still have to use an #ifdef if the code inside the
block references symbols that will not exist if the condition is not met.

At the end of any non-trivial #if or #ifdef block (more than a few lines),
place a comment after the #endif on the same line, noting the conditional
expression used.  For instance:

.. code-block:: c

	#ifdef CONFIG_SOMETHING
	...
	#endif /* CONFIG_SOMETHING */

22) Do not crash the kernel
---------------------------

In general, the decision to crash the kernel belongs to the user, rather
than to the kernel developer.

Avoid panic()
*************

panic() should be used with care and primarily only during system boot.
panic() is, for example, acceptable when running out of memory during boot and
not being able to continue.

Use WARN() rather than BUG()
****************************

Do not add new code that uses any of the BUG() variants, such as BUG(),
BUG_ON(), or VM_BUG_ON(). Instead, use a WARN*() variant, preferably
WARN_ON_ONCE(), and possibly with recovery code. Recovery code is not
required if there is no reasonable way to at least partially recover.

"I'm too lazy to do error handling" is not an excuse for using BUG(). Major
internal corruptions with no way of continuing may still use BUG(), but need
good justification.

Use WARN_ON_ONCE() rather than WARN() or WARN_ON()
**************************************************

WARN_ON_ONCE() is generally preferred over WARN() or WARN_ON(), because it
is common for a given warning condition, if it occurs at all, to occur
multiple times. This can fill up and wrap the kernel log, and can even slow
the system enough that the excessive logging turns into its own, additional
problem.

Do not WARN lightly
*******************

WARN*() is intended for unexpected, this-should-never-happen situations.
WARN*() macros are not to be used for anything that is expected to happen
during normal operation. These are not pre- or post-condition asserts, for
example. Again: WARN*() must not be used for a condition that is expected
to trigger easily, for example, by user space actions. pr_warn_once() is a
possible alternative, if you need to notify the user of a problem.

Do not worry about panic_on_warn users
**************************************

A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an
available kernel option, and that many users set this option. This is why
there is a "Do not WARN lightly" writeup, above. However, the existence of
panic_on_warn users is not a valid reason to avoid the judicious use
WARN*(). That is because, whoever enables panic_on_warn has explicitly
asked the kernel to crash if a WARN*() fires, and such users must be
prepared to deal with the consequences of a system that is somewhat more
likely to crash.

Use BUILD_BUG_ON() for compile-time assertions
**********************************************

The use of BUILD_BUG_ON() is acceptable and encouraged, because it is a
compile-time assertion that has no effect at runtime.

Appendix I) References
----------------------

The C Programming Language, Second Edition
by Brian W. Kernighan and Dennis M. Ritchie.
Prentice Hall, Inc., 1988.
ISBN 0-13-110362-8 (paperback), 0-13-110370-9 (hardback).

The Practice of Programming
by Brian W. Kernighan and Rob Pike.
Addison-Wesley, Inc., 1999.
ISBN 0-201-61586-X.

GNU manuals - where in compliance with K&R and this text - for cpp, gcc,
gcc internals and indent, all available from https://www.gnu.org/manual/

WG14 is the international standardization working group for the programming
language C, URL: http://www.open-std.org/JTC1/SC22/WG14/

Kernel :ref:`process/coding-style.rst <codingstyle>`, by greg@kroah.com at OLS 2002:
http://www.kroah.com/linux/talks/ols_2002_kernel_codingstyle_talk/html/


code:
.. SPDX-License-Identifier: GPL-2.0

.. _deprecated:

=====================================================================
Deprecated Interfaces, Language Features, Attributes, and Conventions
=====================================================================

In a perfect world, it would be possible to convert all instances of
some deprecated API into the new API and entirely remove the old API in
a single development cycle. However, due to the size of the kernel, the
maintainership hierarchy, and timing, it's not always feasible to do these
kinds of conversions at once. This means that new instances may sneak into
the kernel while old ones are being removed, only making the amount of
work to remove the API grow. In order to educate developers about what
has been deprecated and why, this list has been created as a place to
point when uses of deprecated things are proposed for inclusion in the
kernel.

__deprecated
------------
While this attribute does visually mark an interface as deprecated,
it `does not produce warnings during builds any more
<https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
because one of the standing goals of the kernel is to build without
warnings and no one was actually doing anything to remove these deprecated
interfaces. While using `__deprecated` is nice to note an old API in
a header file, it isn't the full solution. Such interfaces must either
be fully removed from the kernel, or added to this file to discourage
others from using them in the future.

BUG() and BUG_ON()
------------------
Use WARN() and WARN_ON() instead, and handle the "impossible"
error condition as gracefully as possible. While the BUG()-family
of APIs were originally designed to act as an "impossible situation"
assert and to kill a kernel thread "safely", they turn out to just be
too risky. (e.g. "In what order do locks need to be released? Have
various states been restored?") Very commonly, using BUG() will
destabilize a system or entirely break it, which makes it impossible
to debug or even get viable crash reports. Linus has `very strong
<https://lore.kernel.org/lkml/CA+55aFy6jNLsywVYdGp83AMrXBo_P-pkjkphPGrO=82SPKCpLQ@mail.gmail.com/>`_
feelings `about this
<https://lore.kernel.org/lkml/CAHk-=whDHsbK3HTOpTF=ue_o04onRwTEaK_ZoJp_fjbqq4+=Jw@mail.gmail.com/>`_.

Note that the WARN()-family should only be used for "expected to
be unreachable" situations. If you want to warn about "reachable
but undesirable" situations, please use the pr_warn()-family of
functions. System owners may have set the *panic_on_warn* sysctl,
to make sure their systems do not continue running in the face of
"unreachable" conditions. (For example, see commits like `this one
<https://git.kernel.org/linus/d4689846881d160a4d12a514e991a740bcb5d65a>`_.)

open-coded arithmetic in allocator arguments
--------------------------------------------
Dynamic size calculations (especially multiplication) should not be
performed in memory allocator (or similar) function arguments due to the
risk of them overflowing. This could lead to values wrapping around and a
smaller allocation being made than the caller was expecting. Using those
allocations could lead to linear overflows of heap memory and other
misbehaviors. (One exception to this is literal values where the compiler
can warn if they might overflow. However, the preferred way in these
cases is to refactor the code as suggested below to avoid the open-coded
arithmetic.)

For example, do not use ``count * size`` as an argument, as in::

	foo = kmalloc(count * size, GFP_KERNEL);

Instead, the 2-factor form of the allocator should be used::

	foo = kmalloc_array(count, size, GFP_KERNEL);

Specifically, kmalloc() can be replaced with kmalloc_array(), and
kzalloc() can be replaced with kcalloc().

If no 2-factor form is available, the saturate-on-overflow helpers should
be used::

	bar = vmalloc(array_size(count, size));

Another common case to avoid is calculating the size of a structure with
a trailing array of others structures, as in::

	header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
			 GFP_KERNEL);

Instead, use the helper::

	header = kzalloc(struct_size(header, item, count), GFP_KERNEL);

.. note:: If you are using struct_size() on a structure containing a zero-length
        or a one-element array as a trailing array member, please refactor such
        array usage and switch to a `flexible array member
        <#zero-length-and-one-element-arrays>`_ instead.

For other calculations, please compose the use of the size_mul(),
size_add(), and size_sub() helpers. For example, in the case of::

	foo = krealloc(current_size + chunk_size * (count - 3), GFP_KERNEL);

Instead, use the helpers::

	foo = krealloc(size_add(current_size,
				size_mul(chunk_size,
					 size_sub(count, 3))), GFP_KERNEL);

For more details, also see array3_size() and flex_array_size(),
as well as the related check_mul_overflow(), check_add_overflow(),
check_sub_overflow(), and check_shl_overflow() family of functions.

simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
----------------------------------------------------------------------
The simple_strtol(), simple_strtoll(),
simple_strtoul(), and simple_strtoull() functions
explicitly ignore overflows, which may lead to unexpected results
in callers. The respective kstrtol(), kstrtoll(),
kstrtoul(), and kstrtoull() functions tend to be the
correct replacements, though note that those require the string to be
NUL or newline terminated.

strcpy()
--------
strcpy() performs no bounds checking on the destination buffer. This
could result in linear overflows beyond the end of the buffer, leading to
all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various
compiler flags help reduce the risk of using this function, there is
no good reason to add new uses of this function. The safe replacement
is strscpy(), though care must be given to any cases where the return
value of strcpy() was used, since strscpy() does not return a pointer to
the destination, but rather a count of non-NUL bytes copied (or negative
errno when it truncates).

strncpy() on NUL-terminated strings
-----------------------------------
Use of strncpy() does not guarantee that the destination buffer will
be NUL terminated. This can lead to various linear read overflows and
other misbehavior due to the missing termination. It also NUL-pads
the destination buffer if the source contents are shorter than the
destination buffer size, which may be a needless performance penalty
for callers using only NUL-terminated strings.

When the destination is required to be NUL-terminated, the replacement is
strscpy(), though care must be given to any cases where the return value
of strncpy() was used, since strscpy() does not return a pointer to the
destination, but rather a count of non-NUL bytes copied (or negative
errno when it truncates). Any cases still needing NUL-padding should
instead use strscpy_pad().

If a caller is using non-NUL-terminated strings, strtomem() should be
used, and the destinations should be marked with the `__nonstring
<https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
attribute to avoid future compiler warnings. For cases still needing
NUL-padding, strtomem_pad() can be used.

strlcpy()
---------
strlcpy() reads the entire source buffer first (since the return value
is meant to match that of strlen()). This read may exceed the destination
size limit. This is both inefficient and can lead to linear read overflows
if a source string is not NUL-terminated. The safe replacement is strscpy(),
though care must be given to any cases where the return value of strlcpy()
is used, since strscpy() will return negative errno values when it truncates.

%p format specifier
-------------------
Traditionally, using "%p" in format strings would lead to regular address
exposure flaws in dmesg, proc, sysfs, etc. Instead of leaving these to
be exploitable, all "%p" uses in the kernel are being printed as a hashed
value, rendering them unusable for addressing. New uses of "%p" should not
be added to the kernel. For text addresses, using "%pS" is likely better,
as it produces the more useful symbol name instead. For nearly everything
else, just do not add "%p" at all.

Paraphrasing Linus's current `guidance <https://lore.kernel.org/lkml/CA+55aFwQEd_d40g4mUCSsVRZzrFPUJt74vc6PPpb675hYNXcKw@mail.gmail.com/>`_:

- If the hashed "%p" value is pointless, ask yourself whether the pointer
  itself is important. Maybe it should be removed entirely?
- If you really think the true pointer value is important, why is some
  system state or user privilege level considered "special"? If you think
  you can justify it (in comments and commit log) well enough to stand
  up to Linus's scrutiny, maybe you can use "%px", along with making sure
  you have sensible permissions.

If you are debugging something where "%p" hashing is causing problems,
you can temporarily boot with the debug flag "`no_hash_pointers
<https://git.kernel.org/linus/5ead723a20e0447bc7db33dc3070b420e5f80aa6>`_".

Variable Length Arrays (VLAs)
-----------------------------
Using stack VLAs produces much worse machine code than statically
sized stack arrays. While these non-trivial `performance issues
<https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
eliminate VLAs, they are also a security risk. Dynamic growth of a stack
array may exceed the remaining memory in the stack segment. This could
lead to a crash, possible overwriting sensitive contents at the end of the
stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)

Implicit switch case fall-through
---------------------------------
The C language allows switch cases to fall through to the next case
when a "break" statement is missing at the end of a case. This, however,
introduces ambiguity in the code, as it's not always clear if the missing
break is intentional or a bug. For example, it's not obvious just from
looking at the code if `STATE_ONE` is intentionally designed to fall
through into `STATE_TWO`::

	switch (value) {
	case STATE_ONE:
		do_something();
	case STATE_TWO:
		do_other();
		break;
	default:
		WARN("unknown state");
	}

As there have been a long list of flaws `due to missing "break" statements
<https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
implicit fall-through. In order to identify intentional fall-through
cases, we have adopted a pseudo-keyword macro "fallthrough" which
expands to gcc's extension `__attribute__((__fallthrough__))
<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_.
(When the C17/C18  `[[fallthrough]]` syntax is more commonly supported by
C compilers, static analyzers, and IDEs, we can switch to using that syntax
for the macro pseudo-keyword.)

All switch/case blocks must end in one of:

* break;
* fallthrough;
* continue;
* goto <label>;
* return [expression];

Zero-length and one-element arrays
----------------------------------
There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use `"flexible array members" <https://en.wikipedia.org/wiki/Flexible_array_member>`_
for these cases. The older style of one-element or zero-length arrays should
no longer be used.

In older C code, dynamically sized trailing elements were done by specifying
a one-element array at the end of a structure::

        struct something {
                size_t count;
                struct foo items[1];
        };

This led to fragile size calculations via sizeof() (which would need to
remove the size of the single trailing element to get a correct size of
the "header"). A `GNU C extension <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_
was introduced to allow for zero-length arrays, to avoid these kinds of
size problems::

        struct something {
                size_t count;
                struct foo items[0];
        };

But this led to other problems, and didn't solve some problems shared by
both styles, like not being able to detect when such an array is accidentally
being used _not_ at the end of a structure (which could happen directly, or
when such a struct was in unions, structs of structs, etc).

C99 introduced "flexible array members", which lacks a numeric size for
the array declaration entirely::

        struct something {
                size_t count;
                struct foo items[];
        };

This is the way the kernel expects dynamically sized trailing elements
to be declared. It allows the compiler to generate errors when the
flexible array does not occur last in the structure, which helps to prevent
some kind of `undefined behavior
<https://git.kernel.org/linus/76497732932f15e7323dc805e8ea8dc11bb587cf>`_
bugs from being inadvertently introduced to the codebase. It also allows
the compiler to correctly analyze array sizes (via sizeof(),
`CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOUNDS`). For instance,
there is no mechanism that warns us that the following application of the
sizeof() operator to a zero-length array always results in zero::

        struct something {
                size_t count;
                struct foo items[0];
        };

        struct something *instance;

        instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
        instance->count = count;

        size = sizeof(instance->items) * instance->count;
        memcpy(instance->items, source, size);

At the last line of code above, ``size`` turns out to be ``zero``, when one might
have thought it represents the total size in bytes of the dynamic memory recently
allocated for the trailing array ``items``. Here are a couple examples of this
issue: `link 1
<https://git.kernel.org/linus/f2cd32a443da694ac4e28fbf4ac6f9d5cc63a539>`_,
`link 2
<https://git.kernel.org/linus/ab91c2a89f86be2898cee208d492816ec238b2cf>`_.
Instead, `flexible array members have incomplete type, and so the sizeof()
operator may not be applied <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
so any misuse of such operators will be immediately noticed at build time.

With respect to one-element arrays, one has to be acutely aware that `such arrays
occupy at least as much space as a single object of the type
<https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
hence they contribute to the size of the enclosing structure. This is prone
to error every time people want to calculate the total size of dynamic memory
to allocate for a structure containing an array of this kind as a member::

        struct something {
                size_t count;
                struct foo items[1];
        };

        struct something *instance;

        instance = kmalloc(struct_size(instance, items, count - 1), GFP_KERNEL);
        instance->count = count;

        size = sizeof(instance->items) * instance->count;
        memcpy(instance->items, source, size);

In the example above, we had to remember to calculate ``count - 1`` when using
the struct_size() helper, otherwise we would have --unintentionally-- allocated
memory for one too many ``items`` objects. The cleanest and least error-prone way
to implement this is through the use of a `flexible array member`, together with
struct_size() and flex_array_size() helpers::

        struct something {
                size_t count;
                struct foo items[];
        };

        struct something *instance;

        instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
        instance->count = count;

        memcpy(instance->items, source, flex_array_size(instance, items, instance->count));

repo_name: angery/linux


code:
.. _development_process_main:

A guide to the Kernel Development Process
=========================================

Contents:

.. toctree::
   :numbered:
   :maxdepth: 2

   1.Intro
   2.Process
   3.Early-stage
   4.Coding
   5.Posting
   6.Followthrough
   7.AdvancedTopics
   8.Conclusion

The purpose of this document is to help developers (and their managers)
work with the development community with a minimum of frustration.  It is
an attempt to document how this community works in a way which is
accessible to those who are not intimately familiar with Linux kernel
development (or, indeed, free software development in general).  While
there is some technical material here, this is very much a process-oriented
discussion which does not require a deep knowledge of kernel programming to
understand.


code:
.. _email_clients:

Email clients info for Linux
============================

Git
---

These days most developers use ``git send-email`` instead of regular
email clients.  The man page for this is quite good.  On the receiving
end, maintainers use ``git am`` to apply the patches.

If you are new to ``git`` then send your first patch to yourself.  Save it
as raw text including all the headers.  Run ``git am raw_email.txt`` and
then review the changelog with ``git log``.  When that works then send
the patch to the appropriate mailing list(s).

General Preferences
-------------------

Patches for the Linux kernel are submitted via email, preferably as
inline text in the body of the email.  Some maintainers accept
attachments, but then the attachments should have content-type
``text/plain``.  However, attachments are generally frowned upon because
it makes quoting portions of the patch more difficult in the patch
review process.

It's also strongly recommended that you use plain text in your email body,
for patches and other emails alike. https://useplaintext.email may be useful
for information on how to configure your preferred email client, as well as
listing recommended email clients should you not already have a preference.

Email clients that are used for Linux kernel patches should send the
patch text untouched.  For example, they should not modify or delete tabs
or spaces, even at the beginning or end of lines.

Don't send patches with ``format=flowed``.  This can cause unexpected
and unwanted line breaks.

Don't let your email client do automatic word wrapping for you.
This can also corrupt your patch.

Email clients should not modify the character set encoding of the text.
Emailed patches should be in ASCII or UTF-8 encoding only.
If you configure your email client to send emails with UTF-8 encoding,
you avoid some possible charset problems.

Email clients should generate and maintain "References:" or "In-Reply-To:"
headers so that mail threading is not broken.

Copy-and-paste (or cut-and-paste) usually does not work for patches
because tabs are converted to spaces.  Using xclipboard, xclip, and/or
xcutsel may work, but it's best to test this for yourself or just avoid
copy-and-paste.

Don't use PGP/GPG signatures in mail that contains patches.
This breaks many scripts that read and apply the patches.
(This should be fixable.)

It's a good idea to send a patch to yourself, save the received message,
and successfully apply it with 'patch' before sending patches to Linux
mailing lists.

Some email client (MUA) hints
-----------------------------

Here are some specific MUA configuration hints for editing and sending
patches for the Linux kernel.  These are not meant to be complete
software package configuration summaries.

Legend:

- TUI = text-based user interface
- GUI = graphical user interface

Alpine (TUI)
************

Config options:

In the :menuselection:`Sending Preferences` section:

- :menuselection:`Do Not Send Flowed Text` must be ``enabled``
- :menuselection:`Strip Whitespace Before Sending` must be ``disabled``

When composing the message, the cursor should be placed where the patch
should appear, and then pressing :kbd:`CTRL-R` let you specify the patch file
to insert into the message.

Claws Mail (GUI)
****************

Works. Some people use this successfully for patches.

To insert a patch use :menuselection:`Message-->Insert File` (:kbd:`CTRL-I`)
or an external editor.

If the inserted patch has to be edited in the Claws composition window
"Auto wrapping" in
:menuselection:`Configuration-->Preferences-->Compose-->Wrapping` should be
disabled.

Evolution (GUI)
***************

Some people use this successfully for patches.

When composing mail select: Preformat
  from :menuselection:`Format-->Paragraph Style-->Preformatted` (:kbd:`CTRL-7`)
  or the toolbar

Then use:
:menuselection:`Insert-->Text File...` (:kbd:`ALT-N x`)
to insert the patch.

You can also ``diff -Nru old.c new.c | xclip``, select
:menuselection:`Preformat`, then paste with the middle button.

Kmail (GUI)
***********

Some people use Kmail successfully for patches.

The default setting of not composing in HTML is appropriate; do not
enable it.

When composing an email, under options, uncheck "word wrap". The only
disadvantage is any text you type in the email will not be word-wrapped
so you will have to manually word wrap text before the patch. The easiest
way around this is to compose your email with word wrap enabled, then save
it as a draft. Once you pull it up again from your drafts it is now hard
word-wrapped and you can uncheck "word wrap" without losing the existing
wrapping.

At the bottom of your email, put the commonly-used patch delimiter before
inserting your patch:  three hyphens (``---``).

Then from the :menuselection:`Message` menu item, select
:menuselection:`insert file` and choose your patch.
As an added bonus you can customise the message creation toolbar menu
and put the :menuselection:`insert file` icon there.

Make the composer window wide enough so that no lines wrap. As of
KMail 1.13.5 (KDE 4.5.4), KMail will apply word wrapping when sending
the email if the lines wrap in the composer window. Having word wrapping
disabled in the Options menu isn't enough. Thus, if your patch has very
long lines, you must make the composer window very wide before sending
the email. See: https://bugs.kde.org/show_bug.cgi?id=174034

You can safely GPG sign attachments, but inlined text is preferred for
patches so do not GPG sign them.  Signing patches that have been inserted
as inlined text will make them tricky to extract from their 7-bit encoding.

If you absolutely must send patches as attachments instead of inlining
them as text, right click on the attachment and select :menuselection:`properties`,
and highlight :menuselection:`Suggest automatic display` to make the attachment
inlined to make it more viewable.

When saving patches that are sent as inlined text, select the email that
contains the patch from the message list pane, right click and select
:menuselection:`save as`.  You can use the whole email unmodified as a patch
if it was properly composed.  Emails are saved as read-write for user only so
you will have to chmod them to make them group and world readable if you copy
them elsewhere.

Lotus Notes (GUI)
*****************

Run away from it.

IBM Verse (Web GUI)
*******************

See Lotus Notes.

Mutt (TUI)
**********

Plenty of Linux developers use ``mutt``, so it must work pretty well.

Mutt doesn't come with an editor, so whatever editor you use should be
used in a way that there are no automatic linebreaks.  Most editors have
an :menuselection:`insert file` option that inserts the contents of a file
unaltered.

To use ``vim`` with mutt::

  set editor="vi"

If using xclip, type the command::

  :set paste

before middle button or shift-insert or use::

  :r filename

if you want to include the patch inline.
(a)ttach works fine without ``set paste``.

You can also generate patches with ``git format-patch`` and then use Mutt
to send them::

    $ mutt -H 0001-some-bug-fix.patch

Config options:

It should work with default settings.
However, it's a good idea to set the ``send_charset`` to::

  set send_charset="us-ascii:utf-8"

Mutt is highly customizable. Here is a minimum configuration to start
using Mutt to send patches through Gmail::

  # .muttrc
  # ================  IMAP ====================
  set imap_user = 'yourusername@gmail.com'
  set imap_pass = 'yourpassword'
  set spoolfile = imaps://imap.gmail.com/INBOX
  set folder = imaps://imap.gmail.com/
  set record="imaps://imap.gmail.com/[Gmail]/Sent Mail"
  set postponed="imaps://imap.gmail.com/[Gmail]/Drafts"
  set mbox="imaps://imap.gmail.com/[Gmail]/All Mail"

  # ================  SMTP  ====================
  set smtp_url = "smtp://username@smtp.gmail.com:587/"
  set smtp_pass = $imap_pass
  set ssl_force_tls = yes # Require encrypted connection

  # ================  Composition  ====================
  set editor = `echo \$EDITOR`
  set edit_headers = yes  # See the headers when editing
  set charset = UTF-8     # value of $LANG; also fallback for send_charset
  # Sender, email address, and sign-off line must match
  unset use_domain        # because joe@localhost is just embarrassing
  set realname = "YOUR NAME"
  set from = "username@gmail.com"
  set use_from = yes

The Mutt docs have lots more information:

    https://gitlab.com/muttmua/mutt/-/wikis/UseCases/Gmail

    http://www.mutt.org/doc/manual/

Pine (TUI)
**********

Pine has had some whitespace truncation issues in the past, but these
should all be fixed now.

Use alpine (pine's successor) if you can.

Config options:

- ``quell-flowed-text`` is needed for recent versions
- the ``no-strip-whitespace-before-send`` option is needed

Sylpheed (GUI)
**************

- Works well for inlining text (or using attachments).
- Allows use of an external editor.
- Is slow on large folders.
- Won't do TLS SMTP auth over a non-SSL connection.
- Has a helpful ruler bar in the compose window.
- Adding addresses to address book doesn't understand the display name
  properly.

Thunderbird (GUI)
*****************

Thunderbird is an Outlook clone that likes to mangle text, but there are ways
to coerce it into behaving.

After doing the modifications, this includes installing the extensions,
you need to restart Thunderbird.

- Allow use of an external editor:

  The easiest thing to do with Thunderbird and patches is to use extensions
  which open your favorite external editor.

  Here are some example extensions which are capable of doing this.

  - "External Editor Revived"

    https://github.com/Frederick888/external-editor-revived

    https://addons.thunderbird.net/en-GB/thunderbird/addon/external-editor-revived/

    It requires installing a "native messaging host".
    Please read the wiki which can be found here:
    https://github.com/Frederick888/external-editor-revived/wiki

  - "External Editor"

    https://github.com/exteditor/exteditor

    To do this, download and install the extension, then open the
    :menuselection:`compose` window, add a button for it using
    :menuselection:`View-->Toolbars-->Customize...`
    then just click on the new button when you wish to use the external editor.

    Please note that "External Editor" requires that your editor must not
    fork, or in other words, the editor must not return before closing.
    You may have to pass additional flags or change the settings of your
    editor. Most notably if you are using gvim then you must pass the -f
    option to gvim by putting ``/usr/bin/gvim --nofork"`` (if the binary is in
    ``/usr/bin``) to the text editor field in :menuselection:`external editor`
    settings. If you are using some other editor then please read its manual
    to find out how to do this.

To beat some sense out of the internal editor, do this:

- Edit your Thunderbird config settings so that it won't use ``format=flowed``!
  Go to your main window and find the button for your main dropdown menu.
  :menuselection:`Main Menu-->Preferences-->General-->Config Editor...`
  to bring up the thunderbird's registry editor.

  - Set ``mailnews.send_plaintext_flowed`` to ``false``

  - Set ``mailnews.wraplength`` from ``72`` to ``0``

- Don't write HTML messages! Go to the main window
  :menuselection:`Main Menu-->Account Settings-->youracc@server.something-->Composition & Addressing`!
  There you can disable the option "Compose messages in HTML format".

- Open messages only as plain text! Go to the main window
  :menuselection:`Main Menu-->View-->Message Body As-->Plain Text`!

TkRat (GUI)
***********

Works.  Use "Insert file..." or external editor.

Gmail (Web GUI)
***************

Does not work for sending patches.

Gmail web client converts tabs to spaces automatically.

At the same time it wraps lines every 78 chars with CRLF style line breaks
although tab2space problem can be solved with external editor.

Another problem is that Gmail will base64-encode any message that has a
non-ASCII character. That includes things like European names.


code:
.. _embargoed_hardware_issues:

Embargoed hardware issues
=========================

Scope
-----

Hardware issues which result in security problems are a different category
of security bugs than pure software bugs which only affect the Linux
kernel.

Hardware issues like Meltdown, Spectre, L1TF etc. must be treated
differently because they usually affect all Operating Systems ("OS") and
therefore need coordination across different OS vendors, distributions,
hardware vendors and other parties. For some of the issues, software
mitigations can depend on microcode or firmware updates, which need further
coordination.

.. _Contact:

Contact
-------

The Linux kernel hardware security team is separate from the regular Linux
kernel security team.

The team only handles the coordination of embargoed hardware security
issues.  Reports of pure software security bugs in the Linux kernel are not
handled by this team and the reporter will be guided to contact the regular
Linux kernel security team (:ref:`Documentation/admin-guide/
<securitybugs>`) instead.

The team can be contacted by email at <hardware-security@kernel.org>. This
is a private list of security officers who will help you to coordinate an
issue according to our documented process.

The list is encrypted and email to the list can be sent by either PGP or
S/MIME encrypted and must be signed with the reporter's PGP key or S/MIME
certificate. The list's PGP key and S/MIME certificate are available from
the following URLs:

  - PGP: https://www.kernel.org/static/files/hardware-security.asc
  - S/MIME: https://www.kernel.org/static/files/hardware-security.crt

While hardware security issues are often handled by the affected hardware
vendor, we welcome contact from researchers or individuals who have
identified a potential hardware flaw.

Hardware security officers
^^^^^^^^^^^^^^^^^^^^^^^^^^

The current team of hardware security officers:

  - Linus Torvalds (Linux Foundation Fellow)
  - Greg Kroah-Hartman (Linux Foundation Fellow)
  - Thomas Gleixner (Linux Foundation Fellow)

Operation of mailing-lists
^^^^^^^^^^^^^^^^^^^^^^^^^^

The encrypted mailing-lists which are used in our process are hosted on
Linux Foundation's IT infrastructure. By providing this service, members
of Linux Foundation's IT operations personnel technically have the
ability to access the embargoed information, but are obliged to
confidentiality by their employment contract. Linux Foundation IT
personnel are also responsible for operating and managing the rest of
kernel.org infrastructure.

The Linux Foundation's current director of IT Project infrastructure is
Konstantin Ryabitsev.

Non-disclosure agreements
-------------------------

The Linux kernel hardware security team is not a formal body and therefore
unable to enter into any non-disclosure agreements.  The kernel community
is aware of the sensitive nature of such issues and offers a Memorandum of
Understanding instead.

Memorandum of Understanding
---------------------------

The Linux kernel community has a deep understanding of the requirement to
keep hardware security issues under embargo for coordination between
different OS vendors, distributors, hardware vendors and other parties.

The Linux kernel community has successfully handled hardware security
issues in the past and has the necessary mechanisms in place to allow
community compliant development under embargo restrictions.

The Linux kernel community has a dedicated hardware security team for
initial contact, which oversees the process of handling such issues under
embargo rules.

The hardware security team identifies the developers (domain experts) who
will form the initial response team for a particular issue. The initial
response team can bring in further developers (domain experts) to address
the issue in the best technical way.

All involved developers pledge to adhere to the embargo rules and to keep
the received information confidential. Violation of the pledge will lead to
immediate exclusion from the current issue and removal from all related
mailing-lists. In addition, the hardware security team will also exclude
the offender from future issues. The impact of this consequence is a highly
effective deterrent in our community. In case a violation happens the
hardware security team will inform the involved parties immediately. If you
or anyone becomes aware of a potential violation, please report it
immediately to the Hardware security officers.

Process
^^^^^^^

Due to the globally distributed nature of Linux kernel development,
face-to-face meetings are almost impossible to address hardware security
issues.  Phone conferences are hard to coordinate due to time zones and
other factors and should be only used when absolutely necessary. Encrypted
email has been proven to be the most effective and secure communication
method for these types of issues.

Start of Disclosure
"""""""""""""""""""

Disclosure starts by contacting the Linux kernel hardware security team by
email. This initial contact should contain a description of the problem and
a list of any known affected hardware. If your organization builds or
distributes the affected hardware, we encourage you to also consider what
other hardware could be affected.

The hardware security team will provide an incident-specific encrypted
mailing-list which will be used for initial discussion with the reporter,
further disclosure and coordination.

The hardware security team will provide the disclosing party a list of
developers (domain experts) who should be informed initially about the
issue after confirming with the developers  that they will adhere to this
Memorandum of Understanding and the documented process. These developers
form the initial response team and will be responsible for handling the
issue after initial contact. The hardware security team is supporting the
response team, but is not necessarily involved in the mitigation
development process.

While individual developers might be covered by a non-disclosure agreement
via their employer, they cannot enter individual non-disclosure agreements
in their role as Linux kernel developers. They will, however, agree to
adhere to this documented process and the Memorandum of Understanding.

The disclosing party should provide a list of contacts for all other
entities who have already been, or should be, informed about the issue.
This serves several purposes:

 - The list of disclosed entities allows communication across the
   industry, e.g. other OS vendors, HW vendors, etc.

 - The disclosed entities can be contacted to name experts who should
   participate in the mitigation development.

 - If an expert which is required to handle an issue is employed by an
   listed entity or member of an listed entity, then the response teams can
   request the disclosure of that expert from that entity. This ensures
   that the expert is also part of the entity's response team.

Disclosure
""""""""""

The disclosing party provides detailed information to the initial response
team via the specific encrypted mailing-list.

From our experience the technical documentation of these issues is usually
a sufficient starting point and further technical clarification is best
done via email.

Mitigation development
""""""""""""""""""""""

The initial response team sets up an encrypted mailing-list or repurposes
an existing one if appropriate.

Using a mailing-list is close to the normal Linux development process and
has been successfully used in developing mitigations for various hardware
security issues in the past.

The mailing-list operates in the same way as normal Linux development.
Patches are posted, discussed and reviewed and if agreed on applied to a
non-public git repository which is only accessible to the participating
developers via a secure connection. The repository contains the main
development branch against the mainline kernel and backport branches for
stable kernel versions as necessary.

The initial response team will identify further experts from the Linux
kernel developer community as needed. Bringing in experts can happen at any
time of the development process and needs to be handled in a timely manner.

If an expert is employed by or member of an entity on the disclosure list
provided by the disclosing party, then participation will be requested from
the relevant entity.

If not, then the disclosing party will be informed about the experts
participation. The experts are covered by the Memorandum of Understanding
and the disclosing party is requested to acknowledge the participation. In
case that the disclosing party has a compelling reason to object, then this
objection has to be raised within five work days and resolved with the
incident team immediately. If the disclosing party does not react within
five work days this is taken as silent acknowledgement.

After acknowledgement or resolution of an objection the expert is disclosed
by the incident team and brought into the development process.

Coordinated release
"""""""""""""""""""

The involved parties will negotiate the date and time where the embargo
ends. At that point the prepared mitigations are integrated into the
relevant kernel trees and published.

While we understand that hardware security issues need coordinated embargo
time, the embargo time should be constrained to the minimum time which is
required for all involved parties to develop, test and prepare the
mitigations. Extending embargo time artificially to meet conference talk
dates or other non-technical reasons is creating more work and burden for
the involved developers and response teams as the patches need to be kept
up to date in order to follow the ongoing upstream kernel development,
which might create conflicting changes.

CVE assignment
""""""""""""""

Neither the hardware security team nor the initial response team assign
CVEs, nor are CVEs required for the development process. If CVEs are
provided by the disclosing party they can be used for documentation
purposes.

Process ambassadors
-------------------

For assistance with this process we have established ambassadors in various
organizations, who can answer questions about or provide guidance on the
reporting process and further handling. Ambassadors are not involved in the
disclosure of a particular issue, unless requested by a response team or by
an involved disclosed party. The current ambassadors list:

  ============= ========================================================
  AMD		Tom Lendacky <thomas.lendacky@amd.com>
  Ampere	Darren Hart <darren@os.amperecomputing.com>
  ARM		Catalin Marinas <catalin.marinas@arm.com>
  IBM Power	Anton Blanchard <anton@linux.ibm.com>
  IBM Z		Christian Borntraeger <borntraeger@de.ibm.com>
  Intel		Tony Luck <tony.luck@intel.com>
  Qualcomm	Trilok Soni <tsoni@codeaurora.org>

  Microsoft	James Morris <jamorris@linux.microsoft.com>
  VMware
  Xen		Andrew Cooper <andrew.cooper3@citrix.com>

  Canonical	John Johansen <john.johansen@canonical.com>
  Debian	Ben Hutchings <ben@decadent.org.uk>
  Oracle	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
  Red Hat	Josh Poimboeuf <jpoimboe@redhat.com>
  SUSE		Jiri Kosina <jkosina@suse.cz>

  Amazon
  Google	Kees Cook <keescook@chromium.org>

  GCC
  LLVM		Nick Desaulniers <ndesaulniers@google.com>
  ============= ========================================================

If you want your organization to be added to the ambassadors list, please
contact the hardware security team. The nominated ambassador has to
understand and support our process fully and is ideally well connected in
the Linux kernel community.

Encrypted mailing-lists
-----------------------

We use encrypted mailing-lists for communication. The operating principle
of these lists is that email sent to the list is encrypted either with the
list's PGP key or with the list's S/MIME certificate. The mailing-list
software decrypts the email and re-encrypts it individually for each
subscriber with the subscriber's PGP key or S/MIME certificate. Details
about the mailing-list software and the setup which is used to ensure the
security of the lists and protection of the data can be found here:
https://korg.wiki.kernel.org/userdoc/remail.

List keys
^^^^^^^^^

For initial contact see :ref:`Contact`. For incident specific mailing-lists
the key and S/MIME certificate are conveyed to the subscribers by email
sent from the specific list.

Subscription to incident specific lists
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Subscription is handled by the response teams. Disclosed parties who want
to participate in the communication send a list of potential subscribers to
the response team so the response team can validate subscription requests.

Each subscriber needs to send a subscription request to the response team
by email. The email must be signed with the subscriber's PGP key or S/MIME
certificate. If a PGP key is used, it must be available from a public key
server and is ideally connected to the Linux kernel's PGP web of trust. See
also: https://www.kernel.org/signature.html.

The response team verifies that the subscriber request is valid and adds
the subscriber to the list. After subscription the subscriber will receive
email from the mailing-list which is signed either with the list's PGP key
or the list's S/MIME certificate. The subscriber's email client can extract
the PGP key or the S/MIME certificate from the signature so the subscriber
can send encrypted email to the list.