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 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW 41 /* 42 * For simplicity and code hygiene, the fallback code below insists on 43 * a, b and *d having the same type (similar to the min() and max() 44 * macros), whereas gcc's type-generic overflow checkers accept 45 * different types. Hence we don't just make check_add_overflow an 46 * alias for __builtin_add_overflow, but add type checks similar to 47 * below. 48 */ 49 #define check_add_overflow(a, b, d) ({ \ 50 typeof(a) __a = (a); \ 51 typeof(b) __b = (b); \ 52 typeof(d) __d = (d); \ 53 (void) (&__a == &__b); \ 54 (void) (&__a == __d); \ 55 __builtin_add_overflow(__a, __b, __d); \ 56 }) 57 58 #define check_sub_overflow(a, b, d) ({ \ 59 typeof(a) __a = (a); \ 60 typeof(b) __b = (b); \ 61 typeof(d) __d = (d); \ 62 (void) (&__a == &__b); \ 63 (void) (&__a == __d); \ 64 __builtin_sub_overflow(__a, __b, __d); \ 65 }) 66 67 #define check_mul_overflow(a, b, d) ({ \ 68 typeof(a) __a = (a); \ 69 typeof(b) __b = (b); \ 70 typeof(d) __d = (d); \ 71 (void) (&__a == &__b); \ 72 (void) (&__a == __d); \ 73 __builtin_mul_overflow(__a, __b, __d); \ 74 }) 75 76 #else 77 78 79 /* Checking for unsigned overflow is relatively easy without causing UB. */ 80 #define __unsigned_add_overflow(a, b, d) ({ \ 81 typeof(a) __a = (a); \ 82 typeof(b) __b = (b); \ 83 typeof(d) __d = (d); \ 84 (void) (&__a == &__b); \ 85 (void) (&__a == __d); \ 86 *__d = __a + __b; \ 87 *__d < __a; \ 88 }) 89 #define __unsigned_sub_overflow(a, b, d) ({ \ 90 typeof(a) __a = (a); \ 91 typeof(b) __b = (b); \ 92 typeof(d) __d = (d); \ 93 (void) (&__a == &__b); \ 94 (void) (&__a == __d); \ 95 *__d = __a - __b; \ 96 __a < __b; \ 97 }) 98 /* 99 * If one of a or b is a compile-time constant, this avoids a division. 100 */ 101 #define __unsigned_mul_overflow(a, b, d) ({ \ 102 typeof(a) __a = (a); \ 103 typeof(b) __b = (b); \ 104 typeof(d) __d = (d); \ 105 (void) (&__a == &__b); \ 106 (void) (&__a == __d); \ 107 *__d = __a * __b; \ 108 __builtin_constant_p(__b) ? \ 109 __b > 0 && __a > type_max(typeof(__a)) / __b : \ 110 __a > 0 && __b > type_max(typeof(__b)) / __a; \ 111 }) 112 113 /* 114 * For signed types, detecting overflow is much harder, especially if 115 * we want to avoid UB. But the interface of these macros is such that 116 * we must provide a result in *d, and in fact we must produce the 117 * result promised by gcc's builtins, which is simply the possibly 118 * wrapped-around value. Fortunately, we can just formally do the 119 * operations in the widest relevant unsigned type (u64) and then 120 * truncate the result - gcc is smart enough to generate the same code 121 * with and without the (u64) casts. 122 */ 123 124 /* 125 * Adding two signed integers can overflow only if they have the same 126 * sign, and overflow has happened iff the result has the opposite 127 * sign. 128 */ 129 #define __signed_add_overflow(a, b, d) ({ \ 130 typeof(a) __a = (a); \ 131 typeof(b) __b = (b); \ 132 typeof(d) __d = (d); \ 133 (void) (&__a == &__b); \ 134 (void) (&__a == __d); \ 135 *__d = (u64)__a + (u64)__b; \ 136 (((~(__a ^ __b)) & (*__d ^ __a)) \ 137 & type_min(typeof(__a))) != 0; \ 138 }) 139 140 /* 141 * Subtraction is similar, except that overflow can now happen only 142 * when the signs are opposite. In this case, overflow has happened if 143 * the result has the opposite sign of a. 144 */ 145 #define __signed_sub_overflow(a, b, d) ({ \ 146 typeof(a) __a = (a); \ 147 typeof(b) __b = (b); \ 148 typeof(d) __d = (d); \ 149 (void) (&__a == &__b); \ 150 (void) (&__a == __d); \ 151 *__d = (u64)__a - (u64)__b; \ 152 ((((__a ^ __b)) & (*__d ^ __a)) \ 153 & type_min(typeof(__a))) != 0; \ 154 }) 155 156 /* 157 * Signed multiplication is rather hard. gcc always follows C99, so 158 * division is truncated towards 0. This means that we can write the 159 * overflow check like this: 160 * 161 * (a > 0 && (b > MAX/a || b < MIN/a)) || 162 * (a < -1 && (b > MIN/a || b < MAX/a) || 163 * (a == -1 && b == MIN) 164 * 165 * The redundant casts of -1 are to silence an annoying -Wtype-limits 166 * (included in -Wextra) warning: When the type is u8 or u16, the 167 * __b_c_e in check_mul_overflow obviously selects 168 * __unsigned_mul_overflow, but unfortunately gcc still parses this 169 * code and warns about the limited range of __b. 170 */ 171 172 #define __signed_mul_overflow(a, b, d) ({ \ 173 typeof(a) __a = (a); \ 174 typeof(b) __b = (b); \ 175 typeof(d) __d = (d); \ 176 typeof(a) __tmax = type_max(typeof(a)); \ 177 typeof(a) __tmin = type_min(typeof(a)); \ 178 (void) (&__a == &__b); \ 179 (void) (&__a == __d); \ 180 *__d = (u64)__a * (u64)__b; \ 181 (__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) || \ 182 (__b < (typeof(__b))-1 && (__a > __tmin/__b || __a < __tmax/__b)) || \ 183 (__b == (typeof(__b))-1 && __a == __tmin); \ 184 }) 185 186 187 #define check_add_overflow(a, b, d) \ 188 __builtin_choose_expr(is_signed_type(typeof(a)), \ 189 __signed_add_overflow(a, b, d), \ 190 __unsigned_add_overflow(a, b, d)) 191 192 #define check_sub_overflow(a, b, d) \ 193 __builtin_choose_expr(is_signed_type(typeof(a)), \ 194 __signed_sub_overflow(a, b, d), \ 195 __unsigned_sub_overflow(a, b, d)) 196 197 #define check_mul_overflow(a, b, d) \ 198 __builtin_choose_expr(is_signed_type(typeof(a)), \ 199 __signed_mul_overflow(a, b, d), \ 200 __unsigned_mul_overflow(a, b, d)) 201 202 203 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ 204 205 /** 206 * array_size() - Calculate size of 2-dimensional array. 207 * 208 * @a: dimension one 209 * @b: dimension two 210 * 211 * Calculates size of 2-dimensional array: @a * @b. 212 * 213 * Returns: number of bytes needed to represent the array or SIZE_MAX on 214 * overflow. 215 */ 216 static inline __must_check size_t array_size(size_t a, size_t b) 217 { 218 size_t bytes; 219 220 if (check_mul_overflow(a, b, &bytes)) 221 return SIZE_MAX; 222 223 return bytes; 224 } 225 226 /** 227 * array3_size() - Calculate size of 3-dimensional array. 228 * 229 * @a: dimension one 230 * @b: dimension two 231 * @c: dimension three 232 * 233 * Calculates size of 3-dimensional array: @a * @b * @c. 234 * 235 * Returns: number of bytes needed to represent the array or SIZE_MAX on 236 * overflow. 237 */ 238 static inline __must_check size_t array3_size(size_t a, size_t b, size_t c) 239 { 240 size_t bytes; 241 242 if (check_mul_overflow(a, b, &bytes)) 243 return SIZE_MAX; 244 if (check_mul_overflow(bytes, c, &bytes)) 245 return SIZE_MAX; 246 247 return bytes; 248 } 249 250 static inline __must_check size_t __ab_c_size(size_t n, size_t size, size_t c) 251 { 252 size_t bytes; 253 254 if (check_mul_overflow(n, size, &bytes)) 255 return SIZE_MAX; 256 if (check_add_overflow(bytes, c, &bytes)) 257 return SIZE_MAX; 258 259 return bytes; 260 } 261 262 /** 263 * struct_size() - Calculate size of structure with trailing array. 264 * @p: Pointer to the structure. 265 * @member: Name of the array member. 266 * @n: Number of elements in the array. 267 * 268 * Calculates size of memory needed for structure @p followed by an 269 * array of @n @member elements. 270 * 271 * Return: number of bytes needed or SIZE_MAX on overflow. 272 */ 273 #define struct_size(p, member, n) \ 274 __ab_c_size(n, \ 275 sizeof(*(p)->member) + __must_be_array((p)->member),\ 276 sizeof(*(p))) 277 278 #endif /* __LINUX_OVERFLOW_H */ 279