1.. SPDX-License-Identifier: GPL-2.0
2
3.. _deprecated:
4
5=====================================================================
6Deprecated Interfaces, Language Features, Attributes, and Conventions
7=====================================================================
8
9In a perfect world, it would be possible to convert all instances of
10some deprecated API into the new API and entirely remove the old API in
11a single development cycle. However, due to the size of the kernel, the
12maintainership hierarchy, and timing, it's not always feasible to do these
13kinds of conversions at once. This means that new instances may sneak into
14the kernel while old ones are being removed, only making the amount of
15work to remove the API grow. In order to educate developers about what
16has been deprecated and why, this list has been created as a place to
17point when uses of deprecated things are proposed for inclusion in the
18kernel.
19
20__deprecated
21------------
22While this attribute does visually mark an interface as deprecated,
23it `does not produce warnings during builds any more
24<https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
25because one of the standing goals of the kernel is to build without
26warnings and no one was actually doing anything to remove these deprecated
27interfaces. While using `__deprecated` is nice to note an old API in
28a header file, it isn't the full solution. Such interfaces must either
29be fully removed from the kernel, or added to this file to discourage
30others from using them in the future.
31
32open-coded arithmetic in allocator arguments
33--------------------------------------------
34Dynamic size calculations (especially multiplication) should not be
35performed in memory allocator (or similar) function arguments due to the
36risk of them overflowing. This could lead to values wrapping around and a
37smaller allocation being made than the caller was expecting. Using those
38allocations could lead to linear overflows of heap memory and other
39misbehaviors. (One exception to this is literal values where the compiler
40can warn if they might overflow. Though using literals for arguments as
41suggested below is also harmless.)
42
43For example, do not use ``count * size`` as an argument, as in::
44
45	foo = kmalloc(count * size, GFP_KERNEL);
46
47Instead, the 2-factor form of the allocator should be used::
48
49	foo = kmalloc_array(count, size, GFP_KERNEL);
50
51If no 2-factor form is available, the saturate-on-overflow helpers should
52be used::
53
54	bar = vmalloc(array_size(count, size));
55
56Another common case to avoid is calculating the size of a structure with
57a trailing array of others structures, as in::
58
59	header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
60			 GFP_KERNEL);
61
62Instead, use the helper::
63
64	header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
65
66See :c:func:`array_size`, :c:func:`array3_size`, and :c:func:`struct_size`,
67for more details as well as the related :c:func:`check_add_overflow` and
68:c:func:`check_mul_overflow` family of functions.
69
70simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
71----------------------------------------------------------------------
72The :c:func:`simple_strtol`, :c:func:`simple_strtoll`,
73:c:func:`simple_strtoul`, and :c:func:`simple_strtoull` functions
74explicitly ignore overflows, which may lead to unexpected results
75in callers. The respective :c:func:`kstrtol`, :c:func:`kstrtoll`,
76:c:func:`kstrtoul`, and :c:func:`kstrtoull` functions tend to be the
77correct replacements, though note that those require the string to be
78NUL or newline terminated.
79
80strcpy()
81--------
82:c:func:`strcpy` performs no bounds checking on the destination
83buffer. This could result in linear overflows beyond the
84end of the buffer, leading to all kinds of misbehaviors. While
85`CONFIG_FORTIFY_SOURCE=y` and various compiler flags help reduce the
86risk of using this function, there is no good reason to add new uses of
87this function. The safe replacement is :c:func:`strscpy`.
88
89strncpy() on NUL-terminated strings
90-----------------------------------
91Use of :c:func:`strncpy` does not guarantee that the destination buffer
92will be NUL terminated. This can lead to various linear read overflows
93and other misbehavior due to the missing termination. It also NUL-pads the
94destination buffer if the source contents are shorter than the destination
95buffer size, which may be a needless performance penalty for callers using
96only NUL-terminated strings. The safe replacement is :c:func:`strscpy`.
97(Users of :c:func:`strscpy` still needing NUL-padding will need an
98explicit :c:func:`memset` added.)
99
100If a caller is using non-NUL-terminated strings, :c:func:`strncpy()` can
101still be used, but destinations should be marked with the `__nonstring
102<https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
103attribute to avoid future compiler warnings.
104
105strlcpy()
106---------
107:c:func:`strlcpy` reads the entire source buffer first, possibly exceeding
108the given limit of bytes to copy. This is inefficient and can lead to
109linear read overflows if a source string is not NUL-terminated. The
110safe replacement is :c:func:`strscpy`.
111
112Variable Length Arrays (VLAs)
113-----------------------------
114Using stack VLAs produces much worse machine code than statically
115sized stack arrays. While these non-trivial `performance issues
116<https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
117eliminate VLAs, they are also a security risk. Dynamic growth of a stack
118array may exceed the remaining memory in the stack segment. This could
119lead to a crash, possible overwriting sensitive contents at the end of the
120stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
121memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
122
123Implicit switch case fall-through
124---------------------------------
125The C language allows switch cases to "fall through" when
126a "break" statement is missing at the end of a case. This,
127however, introduces ambiguity in the code, as it's not always
128clear if the missing break is intentional or a bug. As there
129have been a long list of flaws `due to missing "break" statements
130<https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
131"implicit fall-through". In order to identify an intentional fall-through
132case, we have adopted the marking used by static analyzers: a comment
133saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
134is more widely handled by C compilers, static analyzers, and IDEs, we can
135switch to using that instead.
136