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. Though using literals for arguments as
63 cases is to refactor the code as suggested bel !! 63 suggested below is also harmless.)
64 arithmetic.) <<
65 64
66 For example, do not use ``count * size`` as an 65 For example, do not use ``count * size`` as an argument, as in::
67 66
68 foo = kmalloc(count * size, GFP_KERNEL 67 foo = kmalloc(count * size, GFP_KERNEL);
69 68
70 Instead, the 2-factor form of the allocator sh 69 Instead, the 2-factor form of the allocator should be used::
71 70
72 foo = kmalloc_array(count, size, GFP_K 71 foo = kmalloc_array(count, size, GFP_KERNEL);
73 72
74 Specifically, kmalloc() can be replaced with k 73 Specifically, kmalloc() can be replaced with kmalloc_array(), and
75 kzalloc() can be replaced with kcalloc(). 74 kzalloc() can be replaced with kcalloc().
76 75
77 If no 2-factor form is available, the saturate 76 If no 2-factor form is available, the saturate-on-overflow helpers should
78 be used:: 77 be used::
79 78
80 bar = dma_alloc_coherent(dev, array_si !! 79 bar = vmalloc(array_size(count, size));
81 80
82 Another common case to avoid is calculating th 81 Another common case to avoid is calculating the size of a structure with
83 a trailing array of others structures, as in:: 82 a trailing array of others structures, as in::
84 83
85 header = kzalloc(sizeof(*header) + cou 84 header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
86 GFP_KERNEL); 85 GFP_KERNEL);
87 86
88 Instead, use the helper:: 87 Instead, use the helper::
89 88
90 header = kzalloc(struct_size(header, i 89 header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
91 90
92 .. note:: If you are using struct_size() on a 91 .. note:: If you are using struct_size() on a structure containing a zero-length
93 or a one-element array as a trailing a 92 or a one-element array as a trailing array member, please refactor such
94 array usage and switch to a `flexible 93 array usage and switch to a `flexible array member
95 <#zero-length-and-one-element-arrays>` 94 <#zero-length-and-one-element-arrays>`_ instead.
96 95
97 For other calculations, please compose the use 96 For other calculations, please compose the use of the size_mul(),
98 size_add(), and size_sub() helpers. For exampl 97 size_add(), and size_sub() helpers. For example, in the case of::
99 98
100 foo = krealloc(current_size + chunk_si 99 foo = krealloc(current_size + chunk_size * (count - 3), GFP_KERNEL);
101 100
102 Instead, use the helpers:: 101 Instead, use the helpers::
103 102
104 foo = krealloc(size_add(current_size, 103 foo = krealloc(size_add(current_size,
105 size_mul(chunk 104 size_mul(chunk_size,
106 size_ 105 size_sub(count, 3))), GFP_KERNEL);
107 106
108 For more details, also see array3_size() and f 107 For more details, also see array3_size() and flex_array_size(),
109 as well as the related check_mul_overflow(), c 108 as well as the related check_mul_overflow(), check_add_overflow(),
110 check_sub_overflow(), and check_shl_overflow() 109 check_sub_overflow(), and check_shl_overflow() family of functions.
111 110
112 simple_strtol(), simple_strtoll(), simple_strt 111 simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
113 ---------------------------------------------- 112 ----------------------------------------------------------------------
114 The simple_strtol(), simple_strtoll(), 113 The simple_strtol(), simple_strtoll(),
115 simple_strtoul(), and simple_strtoull() functi 114 simple_strtoul(), and simple_strtoull() functions
116 explicitly ignore overflows, which may lead to 115 explicitly ignore overflows, which may lead to unexpected results
117 in callers. The respective kstrtol(), kstrtoll 116 in callers. The respective kstrtol(), kstrtoll(),
118 kstrtoul(), and kstrtoull() functions tend to 117 kstrtoul(), and kstrtoull() functions tend to be the
119 correct replacements, though note that those r 118 correct replacements, though note that those require the string to be
120 NUL or newline terminated. 119 NUL or newline terminated.
121 120
122 strcpy() 121 strcpy()
123 -------- 122 --------
124 strcpy() performs no bounds checking on the de 123 strcpy() performs no bounds checking on the destination buffer. This
125 could result in linear overflows beyond the en 124 could result in linear overflows beyond the end of the buffer, leading to
126 all kinds of misbehaviors. While `CONFIG_FORTI 125 all kinds of misbehaviors. While `CONFIG_FORTIFY_SOURCE=y` and various
127 compiler flags help reduce the risk of using t 126 compiler flags help reduce the risk of using this function, there is
128 no good reason to add new uses of this functio 127 no good reason to add new uses of this function. The safe replacement
129 is strscpy(), though care must be given to any 128 is strscpy(), though care must be given to any cases where the return
130 value of strcpy() was used, since strscpy() do 129 value of strcpy() was used, since strscpy() does not return a pointer to
131 the destination, but rather a count of non-NUL 130 the destination, but rather a count of non-NUL bytes copied (or negative
132 errno when it truncates). 131 errno when it truncates).
133 132
134 strncpy() on NUL-terminated strings 133 strncpy() on NUL-terminated strings
135 ----------------------------------- 134 -----------------------------------
136 Use of strncpy() does not guarantee that the d 135 Use of strncpy() does not guarantee that the destination buffer will
137 be NUL terminated. This can lead to various li 136 be NUL terminated. This can lead to various linear read overflows and
138 other misbehavior due to the missing terminati 137 other misbehavior due to the missing termination. It also NUL-pads
139 the destination buffer if the source contents 138 the destination buffer if the source contents are shorter than the
140 destination buffer size, which may be a needle 139 destination buffer size, which may be a needless performance penalty
141 for callers using only NUL-terminated strings. !! 140 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 141 strscpy(), though care must be given to any cases where the return value
145 of strncpy() was used, since strscpy() does no 142 of strncpy() was used, since strscpy() does not return a pointer to the
146 destination, but rather a count of non-NUL byt 143 destination, but rather a count of non-NUL bytes copied (or negative
147 errno when it truncates). Any cases still need 144 errno when it truncates). Any cases still needing NUL-padding should
148 instead use strscpy_pad(). 145 instead use strscpy_pad().
149 146
150 If a caller is using non-NUL-terminated string !! 147 If a caller is using non-NUL-terminated strings, strncpy() can
151 used, and the destinations should be marked wi !! 148 still be used, but destinations should be marked with the `__nonstring
152 <https://gcc.gnu.org/onlinedocs/gcc/Common-Var 149 <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
153 attribute to avoid future compiler warnings. F !! 150 attribute to avoid future compiler warnings.
154 NUL-padding, strtomem_pad() can be used. <<
155 151
156 strlcpy() 152 strlcpy()
157 --------- 153 ---------
158 strlcpy() reads the entire source buffer first 154 strlcpy() reads the entire source buffer first (since the return value
159 is meant to match that of strlen()). This read 155 is meant to match that of strlen()). This read may exceed the destination
160 size limit. This is both inefficient and can l 156 size limit. This is both inefficient and can lead to linear read overflows
161 if a source string is not NUL-terminated. The 157 if a source string is not NUL-terminated. The safe replacement is strscpy(),
162 though care must be given to any cases where t 158 though care must be given to any cases where the return value of strlcpy()
163 is used, since strscpy() will return negative 159 is used, since strscpy() will return negative errno values when it truncates.
164 160
165 %p format specifier 161 %p format specifier
166 ------------------- 162 -------------------
167 Traditionally, using "%p" in format strings wo 163 Traditionally, using "%p" in format strings would lead to regular address
168 exposure flaws in dmesg, proc, sysfs, etc. Ins 164 exposure flaws in dmesg, proc, sysfs, etc. Instead of leaving these to
169 be exploitable, all "%p" uses in the kernel ar 165 be exploitable, all "%p" uses in the kernel are being printed as a hashed
170 value, rendering them unusable for addressing. 166 value, rendering them unusable for addressing. New uses of "%p" should not
171 be added to the kernel. For text addresses, us 167 be added to the kernel. For text addresses, using "%pS" is likely better,
172 as it produces the more useful symbol name ins 168 as it produces the more useful symbol name instead. For nearly everything
173 else, just do not add "%p" at all. 169 else, just do not add "%p" at all.
174 170
175 Paraphrasing Linus's current `guidance <https:/ 171 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/>`_:
176 172
177 - If the hashed "%p" value is pointless, ask y 173 - If the hashed "%p" value is pointless, ask yourself whether the pointer
178 itself is important. Maybe it should be remo 174 itself is important. Maybe it should be removed entirely?
179 - If you really think the true pointer value i 175 - If you really think the true pointer value is important, why is some
180 system state or user privilege level conside 176 system state or user privilege level considered "special"? If you think
181 you can justify it (in comments and commit l 177 you can justify it (in comments and commit log) well enough to stand
182 up to Linus's scrutiny, maybe you can use "% 178 up to Linus's scrutiny, maybe you can use "%px", along with making sure
183 you have sensible permissions. 179 you have sensible permissions.
184 180
185 If you are debugging something where "%p" hash 181 If you are debugging something where "%p" hashing is causing problems,
186 you can temporarily boot with the debug flag " 182 you can temporarily boot with the debug flag "`no_hash_pointers
187 <https://git.kernel.org/linus/5ead723a20e0447b 183 <https://git.kernel.org/linus/5ead723a20e0447bc7db33dc3070b420e5f80aa6>`_".
188 184
189 Variable Length Arrays (VLAs) 185 Variable Length Arrays (VLAs)
190 ----------------------------- 186 -----------------------------
191 Using stack VLAs produces much worse machine c 187 Using stack VLAs produces much worse machine code than statically
192 sized stack arrays. While these non-trivial `p 188 sized stack arrays. While these non-trivial `performance issues
193 <https://git.kernel.org/linus/02361bc77888>`_ 189 <https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
194 eliminate VLAs, they are also a security risk. 190 eliminate VLAs, they are also a security risk. Dynamic growth of a stack
195 array may exceed the remaining memory in the s 191 array may exceed the remaining memory in the stack segment. This could
196 lead to a crash, possible overwriting sensitiv 192 lead to a crash, possible overwriting sensitive contents at the end of the
197 stack (when built without `CONFIG_THREAD_INFO_ 193 stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
198 memory adjacent to the stack (when built witho 194 memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
199 195
200 Implicit switch case fall-through 196 Implicit switch case fall-through
201 --------------------------------- 197 ---------------------------------
202 The C language allows switch cases to fall thr 198 The C language allows switch cases to fall through to the next case
203 when a "break" statement is missing at the end 199 when a "break" statement is missing at the end of a case. This, however,
204 introduces ambiguity in the code, as it's not 200 introduces ambiguity in the code, as it's not always clear if the missing
205 break is intentional or a bug. For example, it 201 break is intentional or a bug. For example, it's not obvious just from
206 looking at the code if `STATE_ONE` is intentio 202 looking at the code if `STATE_ONE` is intentionally designed to fall
207 through into `STATE_TWO`:: 203 through into `STATE_TWO`::
208 204
209 switch (value) { 205 switch (value) {
210 case STATE_ONE: 206 case STATE_ONE:
211 do_something(); 207 do_something();
212 case STATE_TWO: 208 case STATE_TWO:
213 do_other(); 209 do_other();
214 break; 210 break;
215 default: 211 default:
216 WARN("unknown state"); 212 WARN("unknown state");
217 } 213 }
218 214
219 As there have been a long list of flaws `due t 215 As there have been a long list of flaws `due to missing "break" statements
220 <https://cwe.mitre.org/data/definitions/484.ht 216 <https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
221 implicit fall-through. In order to identify in 217 implicit fall-through. In order to identify intentional fall-through
222 cases, we have adopted a pseudo-keyword macro 218 cases, we have adopted a pseudo-keyword macro "fallthrough" which
223 expands to gcc's extension `__attribute__((__f 219 expands to gcc's extension `__attribute__((__fallthrough__))
224 <https://gcc.gnu.org/onlinedocs/gcc/Statement- 220 <https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_.
225 (When the C17/C18 `[[fallthrough]]` syntax is 221 (When the C17/C18 `[[fallthrough]]` syntax is more commonly supported by
226 C compilers, static analyzers, and IDEs, we ca 222 C compilers, static analyzers, and IDEs, we can switch to using that syntax
227 for the macro pseudo-keyword.) 223 for the macro pseudo-keyword.)
228 224
229 All switch/case blocks must end in one of: 225 All switch/case blocks must end in one of:
230 226
231 * break; 227 * break;
232 * fallthrough; 228 * fallthrough;
233 * continue; 229 * continue;
234 * goto <label>; 230 * goto <label>;
235 * return [expression]; 231 * return [expression];
236 232
237 Zero-length and one-element arrays 233 Zero-length and one-element arrays
238 ---------------------------------- 234 ----------------------------------
239 There is a regular need in the kernel to provi 235 There is a regular need in the kernel to provide a way to declare having
240 a dynamically sized set of trailing elements i 236 a dynamically sized set of trailing elements in a structure. Kernel code
241 should always use `"flexible array members" <h 237 should always use `"flexible array members" <https://en.wikipedia.org/wiki/Flexible_array_member>`_
242 for these cases. The older style of one-elemen 238 for these cases. The older style of one-element or zero-length arrays should
243 no longer be used. 239 no longer be used.
244 240
245 In older C code, dynamically sized trailing el 241 In older C code, dynamically sized trailing elements were done by specifying
246 a one-element array at the end of a structure: 242 a one-element array at the end of a structure::
247 243
248 struct something { 244 struct something {
249 size_t count; 245 size_t count;
250 struct foo items[1]; 246 struct foo items[1];
251 }; 247 };
252 248
253 This led to fragile size calculations via size 249 This led to fragile size calculations via sizeof() (which would need to
254 remove the size of the single trailing element 250 remove the size of the single trailing element to get a correct size of
255 the "header"). A `GNU C extension <https://gcc 251 the "header"). A `GNU C extension <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_
256 was introduced to allow for zero-length arrays 252 was introduced to allow for zero-length arrays, to avoid these kinds of
257 size problems:: 253 size problems::
258 254
259 struct something { 255 struct something {
260 size_t count; 256 size_t count;
261 struct foo items[0]; 257 struct foo items[0];
262 }; 258 };
263 259
264 But this led to other problems, and didn't sol 260 But this led to other problems, and didn't solve some problems shared by
265 both styles, like not being able to detect whe 261 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 262 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 263 when such a struct was in unions, structs of structs, etc).
268 264
269 C99 introduced "flexible array members", which 265 C99 introduced "flexible array members", which lacks a numeric size for
270 the array declaration entirely:: 266 the array declaration entirely::
271 267
272 struct something { 268 struct something {
273 size_t count; 269 size_t count;
274 struct foo items[]; 270 struct foo items[];
275 }; 271 };
276 272
277 This is the way the kernel expects dynamically 273 This is the way the kernel expects dynamically sized trailing elements
278 to be declared. It allows the compiler to gene 274 to be declared. It allows the compiler to generate errors when the
279 flexible array does not occur last in the stru 275 flexible array does not occur last in the structure, which helps to prevent
280 some kind of `undefined behavior 276 some kind of `undefined behavior
281 <https://git.kernel.org/linus/76497732932f15e7 277 <https://git.kernel.org/linus/76497732932f15e7323dc805e8ea8dc11bb587cf>`_
282 bugs from being inadvertently introduced to th 278 bugs from being inadvertently introduced to the codebase. It also allows
283 the compiler to correctly analyze array sizes 279 the compiler to correctly analyze array sizes (via sizeof(),
284 `CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOU 280 `CONFIG_FORTIFY_SOURCE`, and `CONFIG_UBSAN_BOUNDS`). For instance,
285 there is no mechanism that warns us that the f 281 there is no mechanism that warns us that the following application of the
286 sizeof() operator to a zero-length array alway 282 sizeof() operator to a zero-length array always results in zero::
287 283
288 struct something { 284 struct something {
289 size_t count; 285 size_t count;
290 struct foo items[0]; 286 struct foo items[0];
291 }; 287 };
292 288
293 struct something *instance; 289 struct something *instance;
294 290
295 instance = kmalloc(struct_size(instanc 291 instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
296 instance->count = count; 292 instance->count = count;
297 293
298 size = sizeof(instance->items) * insta 294 size = sizeof(instance->items) * instance->count;
299 memcpy(instance->items, source, size); 295 memcpy(instance->items, source, size);
300 296
301 At the last line of code above, ``size`` turns 297 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 298 have thought it represents the total size in bytes of the dynamic memory recently
303 allocated for the trailing array ``items``. He 299 allocated for the trailing array ``items``. Here are a couple examples of this
304 issue: `link 1 300 issue: `link 1
305 <https://git.kernel.org/linus/f2cd32a443da694a 301 <https://git.kernel.org/linus/f2cd32a443da694ac4e28fbf4ac6f9d5cc63a539>`_,
306 `link 2 302 `link 2
307 <https://git.kernel.org/linus/ab91c2a89f86be28 303 <https://git.kernel.org/linus/ab91c2a89f86be2898cee208d492816ec238b2cf>`_.
308 Instead, `flexible array members have incomple 304 Instead, `flexible array members have incomplete type, and so the sizeof()
309 operator may not be applied <https://gcc.gnu.o 305 operator may not be applied <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
310 so any misuse of such operators will be immedi 306 so any misuse of such operators will be immediately noticed at build time.
311 307
312 With respect to one-element arrays, one has to 308 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 309 occupy at least as much space as a single object of the type
314 <https://gcc.gnu.org/onlinedocs/gcc/Zero-Lengt 310 <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
315 hence they contribute to the size of the enclo 311 hence they contribute to the size of the enclosing structure. This is prone
316 to error every time people want to calculate t 312 to error every time people want to calculate the total size of dynamic memory
317 to allocate for a structure containing an arra 313 to allocate for a structure containing an array of this kind as a member::
318 314
319 struct something { 315 struct something {
320 size_t count; 316 size_t count;
321 struct foo items[1]; 317 struct foo items[1];
322 }; 318 };
323 319
324 struct something *instance; 320 struct something *instance;
325 321
326 instance = kmalloc(struct_size(instanc 322 instance = kmalloc(struct_size(instance, items, count - 1), GFP_KERNEL);
327 instance->count = count; 323 instance->count = count;
328 324
329 size = sizeof(instance->items) * insta 325 size = sizeof(instance->items) * instance->count;
330 memcpy(instance->items, source, size); 326 memcpy(instance->items, source, size);
331 327
332 In the example above, we had to remember to ca 328 In the example above, we had to remember to calculate ``count - 1`` when using
333 the struct_size() helper, otherwise we would h 329 the struct_size() helper, otherwise we would have --unintentionally-- allocated
334 memory for one too many ``items`` objects. The 330 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 331 to implement this is through the use of a `flexible array member`, together with
336 struct_size() and flex_array_size() helpers:: 332 struct_size() and flex_array_size() helpers::
337 333
338 struct something { 334 struct something {
339 size_t count; 335 size_t count;
340 struct foo items[]; 336 struct foo items[];
341 }; 337 };
342 338
343 struct something *instance; 339 struct something *instance;
344 340
345 instance = kmalloc(struct_size(instanc 341 instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
346 instance->count = count; 342 instance->count = count;
347 343
348 memcpy(instance->items, source, flex_a 344 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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