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 */
247static 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 */
269static 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
281static 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