1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 .. _deprecated: 3 .. _deprecated:
4 4
5 ============================================== 5 =====================================================================
6 Deprecated Interfaces, Language Features, Attr 6 Deprecated Interfaces, Language Features, Attributes, and Conventions
7 ============================================== 7 =====================================================================
8 8
9 In a perfect world, it would be possible to co 9 In a perfect world, it would be possible to convert all instances of
10 some deprecated API into the new API and entir 10 some deprecated API into the new API and entirely remove the old API in
11 a single development cycle. However, due to th 11 a single development cycle. However, due to the size of the kernel, the
12 maintainership hierarchy, and timing, it's not 12 maintainership hierarchy, and timing, it's not always feasible to do these
13 kinds of conversions at once. This means that 13 kinds of conversions at once. This means that new instances may sneak into
14 the kernel while old ones are being removed, o 14 the kernel while old ones are being removed, only making the amount of
15 work to remove the API grow. In order to educa 15 work to remove the API grow. In order to educate developers about what
16 has been deprecated and why, this list has bee 16 has been deprecated and why, this list has been created as a place to
17 point when uses of deprecated things are propo 17 point when uses of deprecated things are proposed for inclusion in the
18 kernel. 18 kernel.
19 19
20 __deprecated 20 __deprecated
21 ------------ 21 ------------
22 While this attribute does visually mark an int 22 While this attribute does visually mark an interface as deprecated,
23 it `does not produce warnings during builds an 23 it `does not produce warnings during builds any more
24 <https://git.kernel.org/linus/771c035372a036f8 24 <https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
25 because one of the standing goals of the kerne 25 because one of the standing goals of the kernel is to build without
26 warnings and no one was actually doing anythin 26 warnings and no one was actually doing anything to remove these deprecated
27 interfaces. While using `__deprecated` is nice 27 interfaces. While using `__deprecated` is nice to note an old API in
28 a header file, it isn't the full solution. Suc 28 a header file, it isn't the full solution. Such interfaces must either
29 be fully removed from the kernel, or added to 29 be fully removed from the kernel, or added to this file to discourage
30 others from using them in the future. 30 others from using them in the future.
31 31
32 BUG() and BUG_ON() 32 BUG() and BUG_ON()
33 ------------------ 33 ------------------
34 Use WARN() and WARN_ON() instead, and handle t 34 Use WARN() and WARN_ON() instead, and handle the "impossible"
35 error condition as gracefully as possible. Whi 35 error condition as gracefully as possible. While the BUG()-family
36 of APIs were originally designed to act as an 36 of APIs were originally designed to act as an "impossible situation"
37 assert and to kill a kernel thread "safely", t 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 38 too risky. (e.g. "In what order do locks need to be released? Have
39 various states been restored?") Very commonly, 39 various states been restored?") Very commonly, using BUG() will
40 destabilize a system or entirely break it, whi 40 destabilize a system or entirely break it, which makes it impossible
41 to debug or even get viable crash reports. Lin 41 to debug or even get viable crash reports. Linus has `very strong
42 <https://lore.kernel.org/lkml/CA+55aFy6jNLsywVY 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 43 feelings `about this
44 <https://lore.kernel.org/lkml/CAHk-=whDHsbK3HTO 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 45
46 Note that the WARN()-family should only be use 46 Note that the WARN()-family should only be used for "expected to
47 be unreachable" situations. If you want to war 47 be unreachable" situations. If you want to warn about "reachable
48 but undesirable" situations, please use the pr 48 but undesirable" situations, please use the pr_warn()-family of
49 functions. System owners may have set the *pan 49 functions. System owners may have set the *panic_on_warn* sysctl,
50 to make sure their systems do not continue run 50 to make sure their systems do not continue running in the face of
51 "unreachable" conditions. (For example, see co 51 "unreachable" conditions. (For example, see commits like `this one
52 <https://git.kernel.org/linus/d4689846881d160a 52 <https://git.kernel.org/linus/d4689846881d160a4d12a514e991a740bcb5d65a>`_.)
53 53
54 open-coded arithmetic in allocator arguments 54 open-coded arithmetic in allocator arguments
55 -------------------------------------------- 55 --------------------------------------------
56 Dynamic size calculations (especially multipli 56 Dynamic size calculations (especially multiplication) should not be
57 performed in memory allocator (or similar) fun 57 performed in memory allocator (or similar) function arguments due to the
58 risk of them overflowing. This could lead to v 58 risk of them overflowing. This could lead to values wrapping around and a
59 smaller allocation being made than the caller 59 smaller allocation being made than the caller was expecting. Using those
60 allocations could lead to linear overflows of 60 allocations could lead to linear overflows of heap memory and other
61 misbehaviors. (One exception to this is litera 61 misbehaviors. (One exception to this is literal values where the compiler
62 can warn if they might overflow. However, the 62 can warn if they might overflow. However, the preferred way in these
63 cases is to refactor the code as suggested bel 63 cases is to refactor the code as suggested below to avoid the open-coded
64 arithmetic.) 64 arithmetic.)
65 65
66 For example, do not use ``count * size`` as an 66 For example, do not use ``count * size`` as an argument, as in::
67 67
68 foo = kmalloc(count * size, GFP_KERNEL 68 foo = kmalloc(count * size, GFP_KERNEL);
69 69
70 Instead, the 2-factor form of the allocator sh 70 Instead, the 2-factor form of the allocator should be used::
71 71
72 foo = kmalloc_array(count, size, GFP_K 72 foo = kmalloc_array(count, size, GFP_KERNEL);
73 73
74 Specifically, kmalloc() can be replaced with k <<
75 kzalloc() can be replaced with kcalloc(). <<
76 <<
77 If no 2-factor form is available, the saturate 74 If no 2-factor form is available, the saturate-on-overflow helpers should
78 be used:: 75 be used::
79 76
80 bar = dma_alloc_coherent(dev, array_si !! 77 bar = vmalloc(array_size(count, size));
81 78
82 Another common case to avoid is calculating th 79 Another common case to avoid is calculating the size of a structure with
83 a trailing array of others structures, as in:: 80 a trailing array of others structures, as in::
84 81
85 header = kzalloc(sizeof(*header) + cou 82 header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
86 GFP_KERNEL); 83 GFP_KERNEL);
87 84
88 Instead, use the helper:: 85 Instead, use the helper::
89 86
90 header = kzalloc(struct_size(header, i 87 header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
91 88
92 .. note:: If you are using struct_size() on a 89 .. note:: If you are using struct_size() on a structure containing a zero-length
93 or a one-element array as a trailing a 90 or a one-element array as a trailing array member, please refactor such
94 array usage and switch to a `flexible 91 array usage and switch to a `flexible array member
95 <#zero-length-and-one-element-arrays>` 92 <#zero-length-and-one-element-arrays>`_ instead.
96 93
97 For other calculations, please compose the use !! 94 See array_size(), array3_size(), and struct_size(),
98 size_add(), and size_sub() helpers. For exampl !! 95 for more details as well as the related check_add_overflow() and
99 !! 96 check_mul_overflow() family of functions.
100 foo = krealloc(current_size + chunk_si <<
101 <<
102 Instead, use the helpers:: <<
103 <<
104 foo = krealloc(size_add(current_size, <<
105 size_mul(chunk <<
106 size_ <<
107 <<
108 For more details, also see array3_size() and f <<
109 as well as the related check_mul_overflow(), c <<
110 check_sub_overflow(), and check_shl_overflow() <<
111 97
112 simple_strtol(), simple_strtoll(), simple_strt 98 simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
113 ---------------------------------------------- 99 ----------------------------------------------------------------------
114 The simple_strtol(), simple_strtoll(), 100 The simple_strtol(), simple_strtoll(),
115 simple_strtoul(), and simple_strtoull() functi 101 simple_strtoul(), and simple_strtoull() functions
116 explicitly ignore overflows, which may lead to 102 explicitly ignore overflows, which may lead to unexpected results
117 in callers. The respective kstrtol(), kstrtoll 103 in callers. The respective kstrtol(), kstrtoll(),
118 kstrtoul(), and kstrtoull() functions tend to 104 kstrtoul(), and kstrtoull() functions tend to be the
119 correct replacements, though note that those r 105 correct replacements, though note that those require the string to be
120 NUL or newline terminated. 106 NUL or newline terminated.
121 107
122 strcpy() 108 strcpy()
123 -------- 109 --------
124 strcpy() performs no bounds checking on the de 110 strcpy() performs no bounds checking on the destination buffer. This
125 could result in linear overflows beyond the en 111 could result in linear overflows beyond the end of the buffer, leading to
126 all kinds of misbehaviors. While `CONFIG_FORTI 112 all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various
127 compiler flags help reduce the risk of using t 113 compiler flags help reduce the risk of using this function, there is
128 no good reason to add new uses of this functio 114 no good reason to add new uses of this function. The safe replacement
129 is strscpy(), though care must be given to any 115 is strscpy(), though care must be given to any cases where the return
130 value of strcpy() was used, since strscpy() do 116 value of strcpy() was used, since strscpy() does not return a pointer to
131 the destination, but rather a count of non-NUL 117 the destination, but rather a count of non-NUL bytes copied (or negative
132 errno when it truncates). 118 errno when it truncates).
133 119
134 strncpy() on NUL-terminated strings 120 strncpy() on NUL-terminated strings
135 ----------------------------------- 121 -----------------------------------
136 Use of strncpy() does not guarantee that the d 122 Use of strncpy() does not guarantee that the destination buffer will
137 be NUL terminated. This can lead to various li 123 be NUL terminated. This can lead to various linear read overflows and
138 other misbehavior due to the missing terminati 124 other misbehavior due to the missing termination. It also NUL-pads
139 the destination buffer if the source contents 125 the destination buffer if the source contents are shorter than the
140 destination buffer size, which may be a needle 126 destination buffer size, which may be a needless performance penalty
141 for callers using only NUL-terminated strings. !! 127 for callers using only NUL-terminated strings. The safe replacement is
142 <<
143 When the destination is required to be NUL-ter <<
144 strscpy(), though care must be given to any ca 128 strscpy(), though care must be given to any cases where the return value
145 of strncpy() was used, since strscpy() does no 129 of strncpy() was used, since strscpy() does not return a pointer to the
146 destination, but rather a count of non-NUL byt 130 destination, but rather a count of non-NUL bytes copied (or negative
147 errno when it truncates). Any cases still need 131 errno when it truncates). Any cases still needing NUL-padding should
148 instead use strscpy_pad(). 132 instead use strscpy_pad().
149 133
150 If a caller is using non-NUL-terminated string !! 134 If a caller is using non-NUL-terminated strings, strncpy() can
151 used, and the destinations should be marked wi !! 135 still be used, but destinations should be marked with the `__nonstring
152 <https://gcc.gnu.org/onlinedocs/gcc/Common-Var 136 <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
153 attribute to avoid future compiler warnings. F !! 137 attribute to avoid future compiler warnings.
154 NUL-padding, strtomem_pad() can be used. <<
155 138
156 strlcpy() 139 strlcpy()
157 --------- 140 ---------
158 strlcpy() reads the entire source buffer first 141 strlcpy() reads the entire source buffer first (since the return value
159 is meant to match that of strlen()). This read 142 is meant to match that of strlen()). This read may exceed the destination
160 size limit. This is both inefficient and can l 143 size limit. This is both inefficient and can lead to linear read overflows
161 if a source string is not NUL-terminated. The 144 if a source string is not NUL-terminated. The safe replacement is strscpy(),
162 though care must be given to any cases where t 145 though care must be given to any cases where the return value of strlcpy()
163 is used, since strscpy() will return negative 146 is used, since strscpy() will return negative errno values when it truncates.
164 147
165 %p format specifier 148 %p format specifier
166 ------------------- 149 -------------------
167 Traditionally, using "%p" in format strings wo 150 Traditionally, using "%p" in format strings would lead to regular address
168 exposure flaws in dmesg, proc, sysfs, etc. Ins 151 exposure flaws in dmesg, proc, sysfs, etc. Instead of leaving these to
169 be exploitable, all "%p" uses in the kernel ar 152 be exploitable, all "%p" uses in the kernel are being printed as a hashed
170 value, rendering them unusable for addressing. 153 value, rendering them unusable for addressing. New uses of "%p" should not
171 be added to the kernel. For text addresses, us 154 be added to the kernel. For text addresses, using "%pS" is likely better,
172 as it produces the more useful symbol name ins 155 as it produces the more useful symbol name instead. For nearly everything
173 else, just do not add "%p" at all. 156 else, just do not add "%p" at all.
174 157
175 Paraphrasing Linus's current `guidance <https:/ 158 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/>`_:
176 159
177 - If the hashed "%p" value is pointless, ask y 160 - If the hashed "%p" value is pointless, ask yourself whether the pointer
178 itself is important. Maybe it should be remo 161 itself is important. Maybe it should be removed entirely?
179 - If you really think the true pointer value i 162 - If you really think the true pointer value is important, why is some
180 system state or user privilege level conside 163 system state or user privilege level considered "special"? If you think
181 you can justify it (in comments and commit l 164 you can justify it (in comments and commit log) well enough to stand
182 up to Linus's scrutiny, maybe you can use "% 165 up to Linus's scrutiny, maybe you can use "%px", along with making sure
183 you have sensible permissions. 166 you have sensible permissions.
184 167
185 If you are debugging something where "%p" hash 168 If you are debugging something where "%p" hashing is causing problems,
186 you can temporarily boot with the debug flag " 169 you can temporarily boot with the debug flag "`no_hash_pointers
187 <https://git.kernel.org/linus/5ead723a20e0447b 170 <https://git.kernel.org/linus/5ead723a20e0447bc7db33dc3070b420e5f80aa6>`_".
188 171
189 Variable Length Arrays (VLAs) 172 Variable Length Arrays (VLAs)
190 ----------------------------- 173 -----------------------------
191 Using stack VLAs produces much worse machine c 174 Using stack VLAs produces much worse machine code than statically
192 sized stack arrays. While these non-trivial `p 175 sized stack arrays. While these non-trivial `performance issues
193 <https://git.kernel.org/linus/02361bc77888>`_ 176 <https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
194 eliminate VLAs, they are also a security risk. 177 eliminate VLAs, they are also a security risk. Dynamic growth of a stack
195 array may exceed the remaining memory in the s 178 array may exceed the remaining memory in the stack segment. This could
196 lead to a crash, possible overwriting sensitiv 179 lead to a crash, possible overwriting sensitive contents at the end of the
197 stack (when built without `CONFIG_THREAD_INFO_ 180 stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
198 memory adjacent to the stack (when built witho 181 memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
199 182
200 Implicit switch case fall-through 183 Implicit switch case fall-through
201 --------------------------------- 184 ---------------------------------
202 The C language allows switch cases to fall thr 185 The C language allows switch cases to fall through to the next case
203 when a "break" statement is missing at the end 186 when a "break" statement is missing at the end of a case. This, however,
204 introduces ambiguity in the code, as it's not 187 introduces ambiguity in the code, as it's not always clear if the missing
205 break is intentional or a bug. For example, it 188 break is intentional or a bug. For example, it's not obvious just from
206 looking at the code if `STATE_ONE` is intentio 189 looking at the code if `STATE_ONE` is intentionally designed to fall
207 through into `STATE_TWO`:: 190 through into `STATE_TWO`::
208 191
209 switch (value) { 192 switch (value) {
210 case STATE_ONE: 193 case STATE_ONE:
211 do_something(); 194 do_something();
212 case STATE_TWO: 195 case STATE_TWO:
213 do_other(); 196 do_other();
214 break; 197 break;
215 default: 198 default:
216 WARN("unknown state"); 199 WARN("unknown state");
217 } 200 }
218 201
219 As there have been a long list of flaws `due t 202 As there have been a long list of flaws `due to missing "break" statements
220 <https://cwe.mitre.org/data/definitions/484.ht 203 <https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
221 implicit fall-through. In order to identify in 204 implicit fall-through. In order to identify intentional fall-through
222 cases, we have adopted a pseudo-keyword macro 205 cases, we have adopted a pseudo-keyword macro "fallthrough" which
223 expands to gcc's extension `__attribute__((__f 206 expands to gcc's extension `__attribute__((__fallthrough__))
224 <https://gcc.gnu.org/onlinedocs/gcc/Statement- 207 <https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_.
225 (When the C17/C18 `[[fallthrough]]` syntax is 208 (When the C17/C18 `[[fallthrough]]` syntax is more commonly supported by
226 C compilers, static analyzers, and IDEs, we ca 209 C compilers, static analyzers, and IDEs, we can switch to using that syntax
227 for the macro pseudo-keyword.) 210 for the macro pseudo-keyword.)
228 211
229 All switch/case blocks must end in one of: 212 All switch/case blocks must end in one of:
230 213
231 * break; 214 * break;
232 * fallthrough; 215 * fallthrough;
233 * continue; 216 * continue;
234 * goto <label>; 217 * goto <label>;
235 * return [expression]; 218 * return [expression];
236 219
237 Zero-length and one-element arrays 220 Zero-length and one-element arrays
238 ---------------------------------- 221 ----------------------------------
239 There is a regular need in the kernel to provi 222 There is a regular need in the kernel to provide a way to declare having
240 a dynamically sized set of trailing elements i 223 a dynamically sized set of trailing elements in a structure. Kernel code
241 should always use `"flexible array members" <h 224 should always use `"flexible array members" <https://en.wikipedia.org/wiki/Flexible_array_member>`_
242 for these cases. The older style of one-elemen 225 for these cases. The older style of one-element or zero-length arrays should
243 no longer be used. 226 no longer be used.
244 227
245 In older C code, dynamically sized trailing el 228 In older C code, dynamically sized trailing elements were done by specifying
246 a one-element array at the end of a structure: 229 a one-element array at the end of a structure::
247 230
248 struct something { 231 struct something {
249 size_t count; 232 size_t count;
250 struct foo items[1]; 233 struct foo items[1];
251 }; 234 };
252 235
253 This led to fragile size calculations via size 236 This led to fragile size calculations via sizeof() (which would need to
254 remove the size of the single trailing element 237 remove the size of the single trailing element to get a correct size of
255 the "header"). A `GNU C extension <https://gcc 238 the "header"). A `GNU C extension <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_
256 was introduced to allow for zero-length arrays 239 was introduced to allow for zero-length arrays, to avoid these kinds of
257 size problems:: 240 size problems::
258 241
259 struct something { 242 struct something {
260 size_t count; 243 size_t count;
261 struct foo items[0]; 244 struct foo items[0];
262 }; 245 };
263 246
264 But this led to other problems, and didn't sol 247 But this led to other problems, and didn't solve some problems shared by
265 both styles, like not being able to detect whe 248 both styles, like not being able to detect when such an array is accidentally
266 being used _not_ at the end of a structure (wh 249 being used _not_ at the end of a structure (which could happen directly, or
267 when such a struct was in unions, structs of s 250 when such a struct was in unions, structs of structs, etc).
268 251
269 C99 introduced "flexible array members", which 252 C99 introduced "flexible array members", which lacks a numeric size for
270 the array declaration entirely:: 253 the array declaration entirely::
271 254
272 struct something { 255 struct something {
273 size_t count; 256 size_t count;
274 struct foo items[]; 257 struct foo items[];
275 }; 258 };
276 259
277 This is the way the kernel expects dynamically 260 This is the way the kernel expects dynamically sized trailing elements
278 to be declared. It allows the compiler to gene 261 to be declared. It allows the compiler to generate errors when the
279 flexible array does not occur last in the stru 262 flexible array does not occur last in the structure, which helps to prevent
280 some kind of `undefined behavior 263 some kind of `undefined behavior
281 <https://git.kernel.org/linus/76497732932f15e7 264 <https://git.kernel.org/linus/76497732932f15e7323dc805e8ea8dc11bb587cf>`_
282 bugs from being inadvertently introduced to th 265 bugs from being inadvertently introduced to the codebase. It also allows
283 the compiler to correctly analyze array sizes 266 the compiler to correctly analyze array sizes (via sizeof(),
284 `CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOU 267 `CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOUNDS`). For instance,
285 there is no mechanism that warns us that the f 268 there is no mechanism that warns us that the following application of the
286 sizeof() operator to a zero-length array alway 269 sizeof() operator to a zero-length array always results in zero::
287 270
288 struct something { 271 struct something {
289 size_t count; 272 size_t count;
290 struct foo items[0]; 273 struct foo items[0];
291 }; 274 };
292 275
293 struct something *instance; 276 struct something *instance;
294 277
295 instance = kmalloc(struct_size(instanc 278 instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
296 instance->count = count; 279 instance->count = count;
297 280
298 size = sizeof(instance->items) * insta 281 size = sizeof(instance->items) * instance->count;
299 memcpy(instance->items, source, size); 282 memcpy(instance->items, source, size);
300 283
301 At the last line of code above, ``size`` turns 284 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 b 285 have thought it represents the total size in bytes of the dynamic memory recently
303 allocated for the trailing array ``items``. He 286 allocated for the trailing array ``items``. Here are a couple examples of this
304 issue: `link 1 287 issue: `link 1
305 <https://git.kernel.org/linus/f2cd32a443da694a 288 <https://git.kernel.org/linus/f2cd32a443da694ac4e28fbf4ac6f9d5cc63a539>`_,
306 `link 2 289 `link 2
307 <https://git.kernel.org/linus/ab91c2a89f86be28 290 <https://git.kernel.org/linus/ab91c2a89f86be2898cee208d492816ec238b2cf>`_.
308 Instead, `flexible array members have incomple 291 Instead, `flexible array members have incomplete type, and so the sizeof()
309 operator may not be applied <https://gcc.gnu.o 292 operator may not be applied <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
310 so any misuse of such operators will be immedi 293 so any misuse of such operators will be immediately noticed at build time.
311 294
312 With respect to one-element arrays, one has to 295 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 obje 296 occupy at least as much space as a single object of the type
314 <https://gcc.gnu.org/onlinedocs/gcc/Zero-Lengt 297 <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
315 hence they contribute to the size of the enclo 298 hence they contribute to the size of the enclosing structure. This is prone
316 to error every time people want to calculate t 299 to error every time people want to calculate the total size of dynamic memory
317 to allocate for a structure containing an arra 300 to allocate for a structure containing an array of this kind as a member::
318 301
319 struct something { 302 struct something {
320 size_t count; 303 size_t count;
321 struct foo items[1]; 304 struct foo items[1];
322 }; 305 };
323 306
324 struct something *instance; 307 struct something *instance;
325 308
326 instance = kmalloc(struct_size(instanc 309 instance = kmalloc(struct_size(instance, items, count - 1), GFP_KERNEL);
327 instance->count = count; 310 instance->count = count;
328 311
329 size = sizeof(instance->items) * insta 312 size = sizeof(instance->items) * instance->count;
330 memcpy(instance->items, source, size); 313 memcpy(instance->items, source, size);
331 314
332 In the example above, we had to remember to ca 315 In the example above, we had to remember to calculate ``count - 1`` when using
333 the struct_size() helper, otherwise we would h 316 the struct_size() helper, otherwise we would have --unintentionally-- allocated
334 memory for one too many ``items`` objects. The 317 memory for one too many ``items`` objects. The cleanest and least error-prone way
335 to implement this is through the use of a `fle 318 to implement this is through the use of a `flexible array member`, together with
336 struct_size() and flex_array_size() helpers:: 319 struct_size() and flex_array_size() helpers::
337 320
338 struct something { 321 struct something {
339 size_t count; 322 size_t count;
340 struct foo items[]; 323 struct foo items[];
341 }; 324 };
342 325
343 struct something *instance; 326 struct something *instance;
344 327
345 instance = kmalloc(struct_size(instanc 328 instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
346 instance->count = count; 329 instance->count = count;
347 330
348 memcpy(instance->items, source, flex_a 331 memcpy(instance->items, source, flex_array_size(instance, items, instance->count));
349 <<
350 There are two special cases of replacement whe <<
351 helper needs to be used. (Note that it is name <<
352 use in UAPI headers.) Those cases are when the <<
353 alone in a struct or is part of a union. These <<
354 specification, but for no technical reason (as <<
355 existing use of such arrays in those places an <<
356 DECLARE_FLEX_ARRAY() uses). For example, to co <<
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(str <<
372 DECLARE_FLEX_ARRAY(str <<
373 }; <<
374 }; <<
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