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f0907827 RV |
1 | /* SPDX-License-Identifier: GPL-2.0 OR MIT */ |
2 | #ifndef __LINUX_OVERFLOW_H | |
3 | #define __LINUX_OVERFLOW_H | |
4 | ||
5 | #include <linux/compiler.h> | |
6 | ||
7 | /* | |
8 | * In the fallback code below, we need to compute the minimum and | |
9 | * maximum values representable in a given type. These macros may also | |
10 | * be useful elsewhere, so we provide them outside the | |
11 | * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block. | |
12 | * | |
13 | * It would seem more obvious to do something like | |
14 | * | |
15 | * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0) | |
16 | * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0) | |
17 | * | |
18 | * Unfortunately, the middle expressions, strictly speaking, have | |
19 | * undefined behaviour, and at least some versions of gcc warn about | |
20 | * the type_max expression (but not if -fsanitize=undefined is in | |
21 | * effect; in that case, the warning is deferred to runtime...). | |
22 | * | |
23 | * The slightly excessive casting in type_min is to make sure the | |
24 | * macros also produce sensible values for the exotic type _Bool. [The | |
25 | * overflow checkers only almost work for _Bool, but that's | |
26 | * a-feature-not-a-bug, since people shouldn't be doing arithmetic on | |
27 | * _Bools. Besides, the gcc builtins don't allow _Bool* as third | |
28 | * argument.] | |
29 | * | |
30 | * Idea stolen from | |
31 | * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html - | |
32 | * credit to Christian Biere. | |
33 | */ | |
34 | #define is_signed_type(type) (((type)(-1)) < (type)1) | |
35 | #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type))) | |
36 | #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T))) | |
37 | #define type_min(T) ((T)((T)-type_max(T)-(T)1)) | |
38 | ||
dc7fe518 LR |
39 | /* |
40 | * Avoids triggering -Wtype-limits compilation warning, | |
41 | * while using unsigned data types to check a < 0. | |
42 | */ | |
43 | #define is_non_negative(a) ((a) > 0 || (a) == 0) | |
44 | #define is_negative(a) (!(is_non_negative(a))) | |
f0907827 RV |
45 | |
46 | #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW | |
47 | /* | |
48 | * For simplicity and code hygiene, the fallback code below insists on | |
49 | * a, b and *d having the same type (similar to the min() and max() | |
50 | * macros), whereas gcc's type-generic overflow checkers accept | |
51 | * different types. Hence we don't just make check_add_overflow an | |
52 | * alias for __builtin_add_overflow, but add type checks similar to | |
53 | * below. | |
54 | */ | |
55 | #define check_add_overflow(a, b, d) ({ \ | |
56 | typeof(a) __a = (a); \ | |
57 | typeof(b) __b = (b); \ | |
58 | typeof(d) __d = (d); \ | |
59 | (void) (&__a == &__b); \ | |
60 | (void) (&__a == __d); \ | |
61 | __builtin_add_overflow(__a, __b, __d); \ | |
62 | }) | |
63 | ||
64 | #define check_sub_overflow(a, b, d) ({ \ | |
65 | typeof(a) __a = (a); \ | |
66 | typeof(b) __b = (b); \ | |
67 | typeof(d) __d = (d); \ | |
68 | (void) (&__a == &__b); \ | |
69 | (void) (&__a == __d); \ | |
70 | __builtin_sub_overflow(__a, __b, __d); \ | |
71 | }) | |
72 | ||
73 | #define check_mul_overflow(a, b, d) ({ \ | |
74 | typeof(a) __a = (a); \ | |
75 | typeof(b) __b = (b); \ | |
76 | typeof(d) __d = (d); \ | |
77 | (void) (&__a == &__b); \ | |
78 | (void) (&__a == __d); \ | |
79 | __builtin_mul_overflow(__a, __b, __d); \ | |
80 | }) | |
81 | ||
82 | #else | |
83 | ||
84 | ||
85 | /* Checking for unsigned overflow is relatively easy without causing UB. */ | |
86 | #define __unsigned_add_overflow(a, b, d) ({ \ | |
87 | typeof(a) __a = (a); \ | |
88 | typeof(b) __b = (b); \ | |
89 | typeof(d) __d = (d); \ | |
90 | (void) (&__a == &__b); \ | |
91 | (void) (&__a == __d); \ | |
92 | *__d = __a + __b; \ | |
93 | *__d < __a; \ | |
94 | }) | |
95 | #define __unsigned_sub_overflow(a, b, d) ({ \ | |
96 | typeof(a) __a = (a); \ | |
97 | typeof(b) __b = (b); \ | |
98 | typeof(d) __d = (d); \ | |
99 | (void) (&__a == &__b); \ | |
100 | (void) (&__a == __d); \ | |
101 | *__d = __a - __b; \ | |
102 | __a < __b; \ | |
103 | }) | |
104 | /* | |
105 | * If one of a or b is a compile-time constant, this avoids a division. | |
106 | */ | |
107 | #define __unsigned_mul_overflow(a, b, d) ({ \ | |
108 | typeof(a) __a = (a); \ | |
109 | typeof(b) __b = (b); \ | |
110 | typeof(d) __d = (d); \ | |
111 | (void) (&__a == &__b); \ | |
112 | (void) (&__a == __d); \ | |
113 | *__d = __a * __b; \ | |
114 | __builtin_constant_p(__b) ? \ | |
115 | __b > 0 && __a > type_max(typeof(__a)) / __b : \ | |
116 | __a > 0 && __b > type_max(typeof(__b)) / __a; \ | |
117 | }) | |
118 | ||
119 | /* | |
120 | * For signed types, detecting overflow is much harder, especially if | |
121 | * we want to avoid UB. But the interface of these macros is such that | |
122 | * we must provide a result in *d, and in fact we must produce the | |
123 | * result promised by gcc's builtins, which is simply the possibly | |
124 | * wrapped-around value. Fortunately, we can just formally do the | |
125 | * operations in the widest relevant unsigned type (u64) and then | |
126 | * truncate the result - gcc is smart enough to generate the same code | |
127 | * with and without the (u64) casts. | |
128 | */ | |
129 | ||
130 | /* | |
131 | * Adding two signed integers can overflow only if they have the same | |
132 | * sign, and overflow has happened iff the result has the opposite | |
133 | * sign. | |
134 | */ | |
135 | #define __signed_add_overflow(a, b, d) ({ \ | |
136 | typeof(a) __a = (a); \ | |
137 | typeof(b) __b = (b); \ | |
138 | typeof(d) __d = (d); \ | |
139 | (void) (&__a == &__b); \ | |
140 | (void) (&__a == __d); \ | |
141 | *__d = (u64)__a + (u64)__b; \ | |
142 | (((~(__a ^ __b)) & (*__d ^ __a)) \ | |
143 | & type_min(typeof(__a))) != 0; \ | |
144 | }) | |
145 | ||
146 | /* | |
147 | * Subtraction is similar, except that overflow can now happen only | |
148 | * when the signs are opposite. In this case, overflow has happened if | |
149 | * the result has the opposite sign of a. | |
150 | */ | |
151 | #define __signed_sub_overflow(a, b, d) ({ \ | |
152 | typeof(a) __a = (a); \ | |
153 | typeof(b) __b = (b); \ | |
154 | typeof(d) __d = (d); \ | |
155 | (void) (&__a == &__b); \ | |
156 | (void) (&__a == __d); \ | |
157 | *__d = (u64)__a - (u64)__b; \ | |
158 | ((((__a ^ __b)) & (*__d ^ __a)) \ | |
159 | & type_min(typeof(__a))) != 0; \ | |
160 | }) | |
161 | ||
162 | /* | |
163 | * Signed multiplication is rather hard. gcc always follows C99, so | |
164 | * division is truncated towards 0. This means that we can write the | |
165 | * overflow check like this: | |
166 | * | |
167 | * (a > 0 && (b > MAX/a || b < MIN/a)) || | |
168 | * (a < -1 && (b > MIN/a || b < MAX/a) || | |
169 | * (a == -1 && b == MIN) | |
170 | * | |
171 | * The redundant casts of -1 are to silence an annoying -Wtype-limits | |
172 | * (included in -Wextra) warning: When the type is u8 or u16, the | |
173 | * __b_c_e in check_mul_overflow obviously selects | |
174 | * __unsigned_mul_overflow, but unfortunately gcc still parses this | |
175 | * code and warns about the limited range of __b. | |
176 | */ | |
177 | ||
178 | #define __signed_mul_overflow(a, b, d) ({ \ | |
179 | typeof(a) __a = (a); \ | |
180 | typeof(b) __b = (b); \ | |
181 | typeof(d) __d = (d); \ | |
182 | typeof(a) __tmax = type_max(typeof(a)); \ | |
183 | typeof(a) __tmin = type_min(typeof(a)); \ | |
184 | (void) (&__a == &__b); \ | |
185 | (void) (&__a == __d); \ | |
186 | *__d = (u64)__a * (u64)__b; \ | |
187 | (__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) || \ | |
188 | (__b < (typeof(__b))-1 && (__a > __tmin/__b || __a < __tmax/__b)) || \ | |
189 | (__b == (typeof(__b))-1 && __a == __tmin); \ | |
190 | }) | |
191 | ||
192 | ||
193 | #define check_add_overflow(a, b, d) \ | |
194 | __builtin_choose_expr(is_signed_type(typeof(a)), \ | |
195 | __signed_add_overflow(a, b, d), \ | |
196 | __unsigned_add_overflow(a, b, d)) | |
197 | ||
198 | #define check_sub_overflow(a, b, d) \ | |
199 | __builtin_choose_expr(is_signed_type(typeof(a)), \ | |
200 | __signed_sub_overflow(a, b, d), \ | |
201 | __unsigned_sub_overflow(a, b, d)) | |
202 | ||
203 | #define check_mul_overflow(a, b, d) \ | |
204 | __builtin_choose_expr(is_signed_type(typeof(a)), \ | |
205 | __signed_mul_overflow(a, b, d), \ | |
206 | __unsigned_mul_overflow(a, b, d)) | |
207 | ||
208 | ||
209 | #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ | |
210 | ||
0c668477 JG |
211 | /** check_shl_overflow() - Calculate a left-shifted value and check overflow |
212 | * | |
213 | * @a: Value to be shifted | |
214 | * @s: How many bits left to shift | |
215 | * @d: Pointer to where to store the result | |
216 | * | |
217 | * Computes *@d = (@a << @s) | |
218 | * | |
219 | * Returns true if '*d' cannot hold the result or when 'a << s' doesn't | |
220 | * make sense. Example conditions: | |
221 | * - 'a << s' causes bits to be lost when stored in *d. | |
222 | * - 's' is garbage (e.g. negative) or so large that the result of | |
223 | * 'a << s' is guaranteed to be 0. | |
224 | * - 'a' is negative. | |
225 | * - 'a << s' sets the sign bit, if any, in '*d'. | |
226 | * | |
227 | * '*d' will hold the results of the attempted shift, but is not | |
228 | * considered "safe for use" if false is returned. | |
229 | */ | |
230 | #define check_shl_overflow(a, s, d) ({ \ | |
231 | typeof(a) _a = a; \ | |
232 | typeof(s) _s = s; \ | |
233 | typeof(d) _d = d; \ | |
234 | u64 _a_full = _a; \ | |
235 | unsigned int _to_shift = \ | |
dc7fe518 | 236 | is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ |
0c668477 | 237 | *_d = (_a_full << _to_shift); \ |
dc7fe518 LR |
238 | (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ |
239 | (*_d >> _to_shift) != _a); \ | |
0c668477 JG |
240 | }) |
241 | ||
610b15c5 KC |
242 | /** |
243 | * array_size() - Calculate size of 2-dimensional array. | |
244 | * | |
245 | * @a: dimension one | |
246 | * @b: dimension two | |
247 | * | |
248 | * Calculates size of 2-dimensional array: @a * @b. | |
249 | * | |
250 | * Returns: number of bytes needed to represent the array or SIZE_MAX on | |
251 | * overflow. | |
252 | */ | |
253 | static inline __must_check size_t array_size(size_t a, size_t b) | |
254 | { | |
255 | size_t bytes; | |
256 | ||
257 | if (check_mul_overflow(a, b, &bytes)) | |
258 | return SIZE_MAX; | |
259 | ||
260 | return bytes; | |
261 | } | |
262 | ||
263 | /** | |
264 | * array3_size() - Calculate size of 3-dimensional array. | |
265 | * | |
266 | * @a: dimension one | |
267 | * @b: dimension two | |
268 | * @c: dimension three | |
269 | * | |
270 | * Calculates size of 3-dimensional array: @a * @b * @c. | |
271 | * | |
272 | * Returns: number of bytes needed to represent the array or SIZE_MAX on | |
273 | * overflow. | |
274 | */ | |
275 | static inline __must_check size_t array3_size(size_t a, size_t b, size_t c) | |
276 | { | |
277 | size_t bytes; | |
278 | ||
279 | if (check_mul_overflow(a, b, &bytes)) | |
280 | return SIZE_MAX; | |
281 | if (check_mul_overflow(bytes, c, &bytes)) | |
282 | return SIZE_MAX; | |
283 | ||
284 | return bytes; | |
285 | } | |
286 | ||
e0478542 RV |
287 | /* |
288 | * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for | |
289 | * struct_size() below. | |
290 | */ | |
291 | static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c) | |
610b15c5 KC |
292 | { |
293 | size_t bytes; | |
294 | ||
e0478542 | 295 | if (check_mul_overflow(a, b, &bytes)) |
610b15c5 KC |
296 | return SIZE_MAX; |
297 | if (check_add_overflow(bytes, c, &bytes)) | |
298 | return SIZE_MAX; | |
299 | ||
300 | return bytes; | |
301 | } | |
302 | ||
303 | /** | |
304 | * struct_size() - Calculate size of structure with trailing array. | |
305 | * @p: Pointer to the structure. | |
306 | * @member: Name of the array member. | |
307 | * @n: Number of elements in the array. | |
308 | * | |
309 | * Calculates size of memory needed for structure @p followed by an | |
310 | * array of @n @member elements. | |
311 | * | |
312 | * Return: number of bytes needed or SIZE_MAX on overflow. | |
313 | */ | |
314 | #define struct_size(p, member, n) \ | |
315 | __ab_c_size(n, \ | |
316 | sizeof(*(p)->member) + __must_be_array((p)->member),\ | |
317 | sizeof(*(p))) | |
318 | ||
f0907827 | 319 | #endif /* __LINUX_OVERFLOW_H */ |