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 | |