Documentation/process/deprecated.rst - linux - Git at Google

 .. 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.

 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. Though using literals for arguments as
 suggested below is also harmless.)

 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);

 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);

 See :c:func:`array_size`, :c:func:`array3_size`, and :c:func:`struct_size`,
 for more details as well as the related :c:func:`check_add_overflow` and
 :c:func:`check_mul_overflow` family of functions.

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

 strcpy()
 --------
 :c:func:`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 :c:func:`strscpy`.

 strncpy() on NUL-terminated strings
 -----------------------------------
 Use of :c:func:`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. The safe replacement is :c:func:`strscpy`.
 (Users of :c:func:`strscpy` still needing NUL-padding will need an
 explicit :c:func:`memset` added.)

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

 strlcpy()
 ---------
 :c:func:`strlcpy` reads the entire source buffer first, possibly exceeding
 the given limit of bytes to copy. This is inefficient and can lead to
 linear read overflows if a source string is not NUL-terminated. The
 safe replacement is :c:func:`strscpy`.

 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" 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. 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 an intentional fall-through
 case, we have adopted the marking used by static analyzers: a comment
 saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
 is more widely handled by C compilers, static analyzers, and IDEs, we can
 switch to using that instead.
	.. 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.

	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. Though using literals for arguments as
	suggested below is also harmless.)

	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);

	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);

	See :c:func:`array_size`, :c:func:`array3_size`, and :c:func:`struct_size`,
	for more details as well as the related :c:func:`check_add_overflow` and
	:c:func:`check_mul_overflow` family of functions.

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

	strcpy()
	--------
	:c:func:`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 :c:func:`strscpy`.

	strncpy() on NUL-terminated strings
	-----------------------------------
	Use of :c:func:`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. The safe replacement is :c:func:`strscpy`.
	(Users of :c:func:`strscpy` still needing NUL-padding will need an
	explicit :c:func:`memset` added.)

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

	strlcpy()
	---------
	:c:func:`strlcpy` reads the entire source buffer first, possibly exceeding
	the given limit of bytes to copy. This is inefficient and can lead to
	linear read overflows if a source string is not NUL-terminated. The
	safe replacement is :c:func:`strscpy`.

	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" 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. 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 an intentional fall-through
	case, we have adopted the marking used by static analyzers: a comment
	saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
	is more widely handled by C compilers, static analyzers, and IDEs, we can
	switch to using that instead.