]>
Commit | Line | Data |
---|---|---|
a8b18c24 RV |
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 | #endif /* __LINUX_OVERFLOW_H */ |