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 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))) 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 */ 210 211 /** 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 = \ 236 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ 237 *_d = (_a_full << _to_shift); \ 238 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ 239 (*_d >> _to_shift) != _a); \ 240 }) 241 242 /** 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 } 286 287 /* 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) 292 { 293 size_t bytes; 294 295 if (check_mul_overflow(a, b, &bytes)) 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))) 318 319 #endif /* __LINUX_OVERFLOW_H */ 320