1 .. SPDX-License-Identifier: GPL-2.0
2
3 .. _deprecated:
4
5 =====================================================================
6 Deprecated Interfaces, Language Features, Attributes, and Conventions
7 =====================================================================
8
9 In a perfect world, it would be possible to convert all instances of
10 some deprecated API into the new API and entirely remove the old API in
11 a single development cycle. However, due to the size of the kernel, the
12 maintainership hierarchy, and timing, it's not always feasible to do these
13 kinds of conversions at once. This means that new instances may sneak into
14 the kernel while old ones are being removed, only making the amount of
15 work to remove the API grow. In order to educate developers about what
16 has been deprecated and why, this list has been created as a place to
17 point when uses of deprecated things are proposed for inclusion in the
18 kernel.
19
20 __deprecated
21 ------------
22 While this attribute does visually mark an interface as deprecated,
23 it `does not produce warnings during builds any more
24 <https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
25 because one of the standing goals of the kernel is to build without
26 warnings and no one was actually doing anything to remove these deprecated
27 interfaces. While using `__deprecated` is nice to note an old API in
28 a header file, it isn't the full solution. Such interfaces must either
29 be fully removed from the kernel, or added to this file to discourage
30 others from using them in the future.
31
32 BUG() and BUG_ON()
33 ------------------
34 Use WARN() and WARN_ON() instead, and handle the "impossible"
35 error condition as gracefully as possible. While the BUG()-family
36 of APIs were originally designed to act as an "impossible situation"
37 assert and to kill a kernel thread "safely", they turn out to just be
38 too risky. (e.g. "In what order do locks need to be released? Have
39 various states been restored?") Very commonly, using BUG() will
40 destabilize a system or entirely break it, which makes it impossible
41 to debug or even get viable crash reports. Linus has `very strong
42 <https://lore.kernel.org/lkml/CA+55aFy6jNLsywVYdGp83AMrXBo_P-pkjkphPGrO=82SPKCpLQ@mail.gmail.com/">https://lore.kernel.org/lkml/CA+55aFy6jNLsywVYdGp83AMrXBo_P-pkjkphPGrO=82SPKCpLQ@mail.gmail.com/>`_
43 feelings `about this
44 <https://lore.kernel.org/lkml/CAHk-=whDHsbK3HTOpTF=ue_o04onRwTEaK_ZoJp_fjbqq4+=Jw@mail.gmail.com/">https://lore.kernel.org/lkml/CAHk-=whDHsbK3HTOpTF=ue_o04onRwTEaK_ZoJp_fjbqq4+=Jw@mail.gmail.com/>`_.
45
46 Note that the WARN()-family should only be used for "expected to
47 be unreachable" situations. If you want to warn about "reachable
48 but undesirable" situations, please use the pr_warn()-family of
49 functions. System owners may have set the *panic_on_warn* sysctl,
50 to make sure their systems do not continue running in the face of
51 "unreachable" conditions. (For example, see commits like `this one
52 <https://git.kernel.org/linus/d4689846881d160a4d12a514e991a740bcb5d65a>`_.)
53
54 open-coded arithmetic in allocator arguments
55 --------------------------------------------
56 Dynamic size calculations (especially multiplication) should not be
57 performed in memory allocator (or similar) function arguments due to the
58 risk of them overflowing. This could lead to values wrapping around and a
59 smaller allocation being made than the caller was expecting. Using those
60 allocations could lead to linear overflows of heap memory and other
61 misbehaviors. (One exception to this is literal values where the compiler
62 can warn if they might overflow. However, the preferred way in these
63 cases is to refactor the code as suggested below to avoid the open-coded
64 arithmetic.)
65
66 For example, do not use ``count * size`` as an argument, as in::
67
68 foo = kmalloc(count * size, GFP_KERNEL);
69
70 Instead, the 2-factor form of the allocator should be used::
71
72 foo = kmalloc_array(count, size, GFP_KERNEL);
73
74 Specifically, kmalloc() can be replaced with kmalloc_array(), and
75 kzalloc() can be replaced with kcalloc().
76
77 If no 2-factor form is available, the saturate-on-overflow helpers should
78 be used::
79
80 bar = dma_alloc_coherent(dev, array_size(count, size), &dma, GFP_KERNEL);
81
82 Another common case to avoid is calculating the size of a structure with
83 a trailing array of others structures, as in::
84
85 header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
86 GFP_KERNEL);
87
88 Instead, use the helper::
89
90 header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
91
92 .. note:: If you are using struct_size() on a structure containing a zero-length
93 or a one-element array as a trailing array member, please refactor such
94 array usage and switch to a `flexible array member
95 <#zero-length-and-one-element-arrays>`_ instead.
96
97 For other calculations, please compose the use of the size_mul(),
98 size_add(), and size_sub() helpers. For example, in the case of::
99
100 foo = krealloc(current_size + chunk_size * (count - 3), GFP_KERNEL);
101
102 Instead, use the helpers::
103
104 foo = krealloc(size_add(current_size,
105 size_mul(chunk_size,
106 size_sub(count, 3))), GFP_KERNEL);
107
108 For more details, also see array3_size() and flex_array_size(),
109 as well as the related check_mul_overflow(), check_add_overflow(),
110 check_sub_overflow(), and check_shl_overflow() family of functions.
111
112 simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
113 ----------------------------------------------------------------------
114 The simple_strtol(), simple_strtoll(),
115 simple_strtoul(), and simple_strtoull() functions
116 explicitly ignore overflows, which may lead to unexpected results
117 in callers. The respective kstrtol(), kstrtoll(),
118 kstrtoul(), and kstrtoull() functions tend to be the
119 correct replacements, though note that those require the string to be
120 NUL or newline terminated.
121
122 strcpy()
123 --------
124 strcpy() performs no bounds checking on the destination buffer. This
125 could result in linear overflows beyond the end of the buffer, leading to
126 all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various
127 compiler flags help reduce the risk of using this function, there is
128 no good reason to add new uses of this function. The safe replacement
129 is strscpy(), though care must be given to any cases where the return
130 value of strcpy() was used, since strscpy() does not return a pointer to
131 the destination, but rather a count of non-NUL bytes copied (or negative
132 errno when it truncates).
133
134 strncpy() on NUL-terminated strings
135 -----------------------------------
136 Use of strncpy() does not guarantee that the destination buffer will
137 be NUL terminated. This can lead to various linear read overflows and
138 other misbehavior due to the missing termination. It also NUL-pads
139 the destination buffer if the source contents are shorter than the
140 destination buffer size, which may be a needless performance penalty
141 for callers using only NUL-terminated strings.
142
143 When the destination is required to be NUL-terminated, the replacement is
144 strscpy(), though care must be given to any cases where the return value
145 of strncpy() was used, since strscpy() does not return a pointer to the
146 destination, but rather a count of non-NUL bytes copied (or negative
147 errno when it truncates). Any cases still needing NUL-padding should
148 instead use strscpy_pad().
149
150 If a caller is using non-NUL-terminated strings, strtomem() should be
151 used, and the destinations should be marked with the `__nonstring
152 <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
153 attribute to avoid future compiler warnings. For cases still needing
154 NUL-padding, strtomem_pad() can be used.
155
156 strlcpy()
157 ---------
158 strlcpy() reads the entire source buffer first (since the return value
159 is meant to match that of strlen()). This read may exceed the destination
160 size limit. This is both inefficient and can lead to linear read overflows
161 if a source string is not NUL-terminated. The safe replacement is strscpy(),
162 though care must be given to any cases where the return value of strlcpy()
163 is used, since strscpy() will return negative errno values when it truncates.
164
165 %p format specifier
166 -------------------
167 Traditionally, using "%p" in format strings would lead to regular address
168 exposure flaws in dmesg, proc, sysfs, etc. Instead of leaving these to
169 be exploitable, all "%p" uses in the kernel are being printed as a hashed
170 value, rendering them unusable for addressing. New uses of "%p" should not
171 be added to the kernel. For text addresses, using "%pS" is likely better,
172 as it produces the more useful symbol name instead. For nearly everything
173 else, just do not add "%p" at all.
174
175 Paraphrasing Linus's current `guidance <https://lore.kernel.org/lkml/CA+55aFwQEd_d40g4mUCSsVRZzrFPUJt74vc6PPpb675hYNXcKw@mail.gmail.com/">https://lore.kernel.org/lkml/CA+55aFwQEd_d40g4mUCSsVRZzrFPUJt74vc6PPpb675hYNXcKw@mail.gmail.com/>`_:
176
177 - If the hashed "%p" value is pointless, ask yourself whether the pointer
178 itself is important. Maybe it should be removed entirely?
179 - If you really think the true pointer value is important, why is some
180 system state or user privilege level considered "special"? If you think
181 you can justify it (in comments and commit log) well enough to stand
182 up to Linus's scrutiny, maybe you can use "%px", along with making sure
183 you have sensible permissions.
184
185 If you are debugging something where "%p" hashing is causing problems,
186 you can temporarily boot with the debug flag "`no_hash_pointers
187 <https://git.kernel.org/linus/5ead723a20e0447bc7db33dc3070b420e5f80aa6>`_".
188
189 Variable Length Arrays (VLAs)
190 -----------------------------
191 Using stack VLAs produces much worse machine code than statically
192 sized stack arrays. While these non-trivial `performance issues
193 <https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
194 eliminate VLAs, they are also a security risk. Dynamic growth of a stack
195 array may exceed the remaining memory in the stack segment. This could
196 lead to a crash, possible overwriting sensitive contents at the end of the
197 stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
198 memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
199
200 Implicit switch case fall-through
201 ---------------------------------
202 The C language allows switch cases to fall through to the next case
203 when a "break" statement is missing at the end of a case. This, however,
204 introduces ambiguity in the code, as it's not always clear if the missing
205 break is intentional or a bug. For example, it's not obvious just from
206 looking at the code if `STATE_ONE` is intentionally designed to fall
207 through into `STATE_TWO`::
208
209 switch (value) {
210 case STATE_ONE:
211 do_something();
212 case STATE_TWO:
213 do_other();
214 break;
215 default:
216 WARN("unknown state");
217 }
218
219 As there have been a long list of flaws `due to missing "break" statements
220 <https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
221 implicit fall-through. In order to identify intentional fall-through
222 cases, we have adopted a pseudo-keyword macro "fallthrough" which
223 expands to gcc's extension `__attribute__((__fallthrough__))
224 <https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_.
225 (When the C17/C18 `[[fallthrough]]` syntax is more commonly supported by
226 C compilers, static analyzers, and IDEs, we can switch to using that syntax
227 for the macro pseudo-keyword.)
228
229 All switch/case blocks must end in one of:
230
231 * break;
232 * fallthrough;
233 * continue;
234 * goto <label>;
235 * return [expression];
236
237 Zero-length and one-element arrays
238 ----------------------------------
239 There is a regular need in the kernel to provide a way to declare having
240 a dynamically sized set of trailing elements in a structure. Kernel code
241 should always use `"flexible array members" <https://en.wikipedia.org/wiki/Flexible_array_member>`_
242 for these cases. The older style of one-element or zero-length arrays should
243 no longer be used.
244
245 In older C code, dynamically sized trailing elements were done by specifying
246 a one-element array at the end of a structure::
247
248 struct something {
249 size_t count;
250 struct foo items[1];
251 };
252
253 This led to fragile size calculations via sizeof() (which would need to
254 remove the size of the single trailing element to get a correct size of
255 the "header"). A `GNU C extension <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_
256 was introduced to allow for zero-length arrays, to avoid these kinds of
257 size problems::
258
259 struct something {
260 size_t count;
261 struct foo items[0];
262 };
263
264 But this led to other problems, and didn't solve some problems shared by
265 both styles, like not being able to detect when such an array is accidentally
266 being used _not_ at the end of a structure (which could happen directly, or
267 when such a struct was in unions, structs of structs, etc).
268
269 C99 introduced "flexible array members", which lacks a numeric size for
270 the array declaration entirely::
271
272 struct something {
273 size_t count;
274 struct foo items[];
275 };
276
277 This is the way the kernel expects dynamically sized trailing elements
278 to be declared. It allows the compiler to generate errors when the
279 flexible array does not occur last in the structure, which helps to prevent
280 some kind of `undefined behavior
281 <https://git.kernel.org/linus/76497732932f15e7323dc805e8ea8dc11bb587cf>`_
282 bugs from being inadvertently introduced to the codebase. It also allows
283 the compiler to correctly analyze array sizes (via sizeof(),
284 `CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOUNDS`). For instance,
285 there is no mechanism that warns us that the following application of the
286 sizeof() operator to a zero-length array always results in zero::
287
288 struct something {
289 size_t count;
290 struct foo items[0];
291 };
292
293 struct something *instance;
294
295 instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
296 instance->count = count;
297
298 size = sizeof(instance->items) * instance->count;
299 memcpy(instance->items, source, size);
300
301 At the last line of code above, ``size`` turns out to be ``zero``, when one might
302 have thought it represents the total size in bytes of the dynamic memory recently
303 allocated for the trailing array ``items``. Here are a couple examples of this
304 issue: `link 1
305 <https://git.kernel.org/linus/f2cd32a443da694ac4e28fbf4ac6f9d5cc63a539>`_,
306 `link 2
307 <https://git.kernel.org/linus/ab91c2a89f86be2898cee208d492816ec238b2cf>`_.
308 Instead, `flexible array members have incomplete type, and so the sizeof()
309 operator may not be applied <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
310 so any misuse of such operators will be immediately noticed at build time.
311
312 With respect to one-element arrays, one has to be acutely aware that `such arrays
313 occupy at least as much space as a single object of the type
314 <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
315 hence they contribute to the size of the enclosing structure. This is prone
316 to error every time people want to calculate the total size of dynamic memory
317 to allocate for a structure containing an array of this kind as a member::
318
319 struct something {
320 size_t count;
321 struct foo items[1];
322 };
323
324 struct something *instance;
325
326 instance = kmalloc(struct_size(instance, items, count - 1), GFP_KERNEL);
327 instance->count = count;
328
329 size = sizeof(instance->items) * instance->count;
330 memcpy(instance->items, source, size);
331
332 In the example above, we had to remember to calculate ``count - 1`` when using
333 the struct_size() helper, otherwise we would have --unintentionally-- allocated
334 memory for one too many ``items`` objects. The cleanest and least error-prone way
335 to implement this is through the use of a `flexible array member`, together with
336 struct_size() and flex_array_size() helpers::
337
338 struct something {
339 size_t count;
340 struct foo items[];
341 };
342
343 struct something *instance;
344
345 instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
346 instance->count = count;
347
348 memcpy(instance->items, source, flex_array_size(instance, items, instance->count));
349
350 There are two special cases of replacement where the DECLARE_FLEX_ARRAY()
351 helper needs to be used. (Note that it is named __DECLARE_FLEX_ARRAY() for
352 use in UAPI headers.) Those cases are when the flexible array is either
353 alone in a struct or is part of a union. These are disallowed by the C99
354 specification, but for no technical reason (as can be seen by both the
355 existing use of such arrays in those places and the work-around that
356 DECLARE_FLEX_ARRAY() uses). For example, to convert this::
357
358 struct something {
359 ...
360 union {
361 struct type1 one[0];
362 struct type2 two[0];
363 };
364 };
365
366 The helper must be used::
367
368 struct something {
369 ...
370 union {
371 DECLARE_FLEX_ARRAY(struct type1, one);
372 DECLARE_FLEX_ARRAY(struct type2, two);
373 };
374 };
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