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 | /** check_shl_overflow() - Calculate a left-shifted value and check overflow |

206 | * |

207 | * @a: Value to be shifted |

208 | * @s: How many bits left to shift |

209 | * @d: Pointer to where to store the result |

210 | * |

211 | * Computes *@d = (@a << @s) |

212 | * |

213 | * Returns true if '*d' cannot hold the result or when 'a << s' doesn't |

214 | * make sense. Example conditions: |

215 | * - 'a << s' causes bits to be lost when stored in *d. |

216 | * - 's' is garbage (e.g. negative) or so large that the result of |

217 | * 'a << s' is guaranteed to be 0. |

218 | * - 'a' is negative. |

219 | * - 'a << s' sets the sign bit, if any, in '*d'. |

220 | * |

221 | * '*d' will hold the results of the attempted shift, but is not |

222 | * considered "safe for use" if false is returned. |

223 | */ |

224 | #define check_shl_overflow(a, s, d) ({ \ |

225 | typeof(a) _a = a; \ |

226 | typeof(s) _s = s; \ |

227 | typeof(d) _d = d; \ |

228 | u64 _a_full = _a; \ |

229 | unsigned int _to_shift = \ |

230 | _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \ |

231 | *_d = (_a_full << _to_shift); \ |

232 | (_to_shift != _s || *_d < 0 || _a < 0 || \ |

233 | (*_d >> _to_shift) != _a); \ |

234 | }) |

235 | |

236 | /** |

237 | * array_size() - Calculate size of 2-dimensional array. |

238 | * |

239 | * @a: dimension one |

240 | * @b: dimension two |

241 | * |

242 | * Calculates size of 2-dimensional array: @a * @b. |

243 | * |

244 | * Returns: number of bytes needed to represent the array or SIZE_MAX on |

245 | * overflow. |

246 | */ |

247 | static inline __must_check size_t array_size(size_t a, size_t b) |

248 | { |

249 | size_t bytes; |

250 | |

251 | if (check_mul_overflow(a, b, &bytes)) |

252 | return SIZE_MAX; |

253 | |

254 | return bytes; |

255 | } |

256 | |

257 | /** |

258 | * array3_size() - Calculate size of 3-dimensional array. |

259 | * |

260 | * @a: dimension one |

261 | * @b: dimension two |

262 | * @c: dimension three |

263 | * |

264 | * Calculates size of 3-dimensional array: @a * @b * @c. |

265 | * |

266 | * Returns: number of bytes needed to represent the array or SIZE_MAX on |

267 | * overflow. |

268 | */ |

269 | static inline __must_check size_t array3_size(size_t a, size_t b, size_t c) |

270 | { |

271 | size_t bytes; |

272 | |

273 | if (check_mul_overflow(a, b, &bytes)) |

274 | return SIZE_MAX; |

275 | if (check_mul_overflow(bytes, c, &bytes)) |

276 | return SIZE_MAX; |

277 | |

278 | return bytes; |

279 | } |

280 | |

281 | static inline __must_check size_t __ab_c_size(size_t n, size_t size, size_t c) |

282 | { |

283 | size_t bytes; |

284 | |

285 | if (check_mul_overflow(n, size, &bytes)) |

286 | return SIZE_MAX; |

287 | if (check_add_overflow(bytes, c, &bytes)) |

288 | return SIZE_MAX; |

289 | |

290 | return bytes; |

291 | } |

292 | |

293 | /** |

294 | * struct_size() - Calculate size of structure with trailing array. |

295 | * @p: Pointer to the structure. |

296 | * @member: Name of the array member. |

297 | * @n: Number of elements in the array. |

298 | * |

299 | * Calculates size of memory needed for structure @p followed by an |

300 | * array of @n @member elements. |

301 | * |

302 | * Return: number of bytes needed or SIZE_MAX on overflow. |

303 | */ |

304 | #define struct_size(p, member, n) \ |

305 | __ab_c_size(n, \ |

306 | sizeof(*(p)->member) + __must_be_array((p)->member),\ |

307 | sizeof(*(p))) |

308 | |

309 | #endif /* __LINUX_OVERFLOW_H */ |

310 |