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 #include <linux/limits.h>
7
8 /*
9 * In the fallback code below, we need to compute the minimum and
10 * maximum values representable in a given type. These macros may also
11 * be useful elsewhere, so we provide them outside the
12 * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
13 *
14 * It would seem more obvious to do something like
15 *
16 * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
17 * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
18 *
19 * Unfortunately, the middle expressions, strictly speaking, have
20 * undefined behaviour, and at least some versions of gcc warn about
21 * the type_max expression (but not if -fsanitize=undefined is in
22 * effect; in that case, the warning is deferred to runtime...).
23 *
24 * The slightly excessive casting in type_min is to make sure the
25 * macros also produce sensible values for the exotic type _Bool. [The
26 * overflow checkers only almost work for _Bool, but that's
27 * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
28 * _Bools. Besides, the gcc builtins don't allow _Bool* as third
29 * argument.]
30 *
31 * Idea stolen from
32 * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
33 * credit to Christian Biere.
34 */
35 #define is_signed_type(type) (((type)(-1)) < (type)1)
36 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
37 #define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
38 #define type_min(T) ((T)((T)-type_max(T)-(T)1))
39
40 /*
41 * Avoids triggering -Wtype-limits compilation warning,
42 * while using unsigned data types to check a < 0.
43 */
44 #define is_non_negative(a) ((a) > 0 || (a) == 0)
45 #define is_negative(a) (!(is_non_negative(a)))
46
47 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
48 /*
49 * For simplicity and code hygiene, the fallback code below insists on
50 * a, b and *d having the same type (similar to the min() and max()
51 * macros), whereas gcc's type-generic overflow checkers accept
52 * different types. Hence we don't just make check_add_overflow an
53 * alias for __builtin_add_overflow, but add type checks similar to
54 * below.
55 */
56 #define check_add_overflow(a, b, d) ({ \
57 typeof(a) __a = (a); \
58 typeof(b) __b = (b); \
59 typeof(d) __d = (d); \
60 (void) (&__a == &__b); \
61 (void) (&__a == __d); \
62 __builtin_add_overflow(__a, __b, __d); \
63 })
64
65 #define check_sub_overflow(a, b, d) ({ \
66 typeof(a) __a = (a); \
67 typeof(b) __b = (b); \
68 typeof(d) __d = (d); \
69 (void) (&__a == &__b); \
70 (void) (&__a == __d); \
71 __builtin_sub_overflow(__a, __b, __d); \
72 })
73
74 #define check_mul_overflow(a, b, d) ({ \
75 typeof(a) __a = (a); \
76 typeof(b) __b = (b); \
77 typeof(d) __d = (d); \
78 (void) (&__a == &__b); \
79 (void) (&__a == __d); \
80 __builtin_mul_overflow(__a, __b, __d); \
81 })
82
83 #else
84
85
86 /* Checking for unsigned overflow is relatively easy without causing UB. */
87 #define __unsigned_add_overflow(a, b, d) ({ \
88 typeof(a) __a = (a); \
89 typeof(b) __b = (b); \
90 typeof(d) __d = (d); \
91 (void) (&__a == &__b); \
92 (void) (&__a == __d); \
93 *__d = __a + __b; \
94 *__d < __a; \
95 })
96 #define __unsigned_sub_overflow(a, b, d) ({ \
97 typeof(a) __a = (a); \
98 typeof(b) __b = (b); \
99 typeof(d) __d = (d); \
100 (void) (&__a == &__b); \
101 (void) (&__a == __d); \
102 *__d = __a - __b; \
103 __a < __b; \
104 })
105 /*
106 * If one of a or b is a compile-time constant, this avoids a division.
107 */
108 #define __unsigned_mul_overflow(a, b, d) ({ \
109 typeof(a) __a = (a); \
110 typeof(b) __b = (b); \
111 typeof(d) __d = (d); \
112 (void) (&__a == &__b); \
113 (void) (&__a == __d); \
114 *__d = __a * __b; \
115 __builtin_constant_p(__b) ? \
116 __b > 0 && __a > type_max(typeof(__a)) / __b : \
117 __a > 0 && __b > type_max(typeof(__b)) / __a; \
118 })
119
120 /*
121 * For signed types, detecting overflow is much harder, especially if
122 * we want to avoid UB. But the interface of these macros is such that
123 * we must provide a result in *d, and in fact we must produce the
124 * result promised by gcc's builtins, which is simply the possibly
125 * wrapped-around value. Fortunately, we can just formally do the
126 * operations in the widest relevant unsigned type (u64) and then
127 * truncate the result - gcc is smart enough to generate the same code
128 * with and without the (u64) casts.
129 */
130
131 /*
132 * Adding two signed integers can overflow only if they have the same
133 * sign, and overflow has happened iff the result has the opposite
134 * sign.
135 */
136 #define __signed_add_overflow(a, b, d) ({ \
137 typeof(a) __a = (a); \
138 typeof(b) __b = (b); \
139 typeof(d) __d = (d); \
140 (void) (&__a == &__b); \
141 (void) (&__a == __d); \
142 *__d = (u64)__a + (u64)__b; \
143 (((~(__a ^ __b)) & (*__d ^ __a)) \
144 & type_min(typeof(__a))) != 0; \
145 })
146
147 /*
148 * Subtraction is similar, except that overflow can now happen only
149 * when the signs are opposite. In this case, overflow has happened if
150 * the result has the opposite sign of a.
151 */
152 #define __signed_sub_overflow(a, b, d) ({ \
153 typeof(a) __a = (a); \
154 typeof(b) __b = (b); \
155 typeof(d) __d = (d); \
156 (void) (&__a == &__b); \
157 (void) (&__a == __d); \
158 *__d = (u64)__a - (u64)__b; \
159 ((((__a ^ __b)) & (*__d ^ __a)) \
160 & type_min(typeof(__a))) != 0; \
161 })
162
163 /*
164 * Signed multiplication is rather hard. gcc always follows C99, so
165 * division is truncated towards 0. This means that we can write the
166 * overflow check like this:
167 *
168 * (a > 0 && (b > MAX/a || b < MIN/a)) ||
169 * (a < -1 && (b > MIN/a || b < MAX/a) ||
170 * (a == -1 && b == MIN)
171 *
172 * The redundant casts of -1 are to silence an annoying -Wtype-limits
173 * (included in -Wextra) warning: When the type is u8 or u16, the
174 * __b_c_e in check_mul_overflow obviously selects
175 * __unsigned_mul_overflow, but unfortunately gcc still parses this
176 * code and warns about the limited range of __b.
177 */
178
179 #define __signed_mul_overflow(a, b, d) ({ \
180 typeof(a) __a = (a); \
181 typeof(b) __b = (b); \
182 typeof(d) __d = (d); \
183 typeof(a) __tmax = type_max(typeof(a)); \
184 typeof(a) __tmin = type_min(typeof(a)); \
185 (void) (&__a == &__b); \
186 (void) (&__a == __d); \
187 *__d = (u64)__a * (u64)__b; \
188 (__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) || \
189 (__b < (typeof(__b))-1 && (__a > __tmin/__b || __a < __tmax/__b)) || \
190 (__b == (typeof(__b))-1 && __a == __tmin); \
191 })
192
193
194 #define check_add_overflow(a, b, d) \
195 __builtin_choose_expr(is_signed_type(typeof(a)), \
196 __signed_add_overflow(a, b, d), \
197 __unsigned_add_overflow(a, b, d))
198
199 #define check_sub_overflow(a, b, d) \
200 __builtin_choose_expr(is_signed_type(typeof(a)), \
201 __signed_sub_overflow(a, b, d), \
202 __unsigned_sub_overflow(a, b, d))
203
204 #define check_mul_overflow(a, b, d) \
205 __builtin_choose_expr(is_signed_type(typeof(a)), \
206 __signed_mul_overflow(a, b, d), \
207 __unsigned_mul_overflow(a, b, d))
208
209
210 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
211
212 /** check_shl_overflow() - Calculate a left-shifted value and check overflow
213 *
214 * @a: Value to be shifted
215 * @s: How many bits left to shift
216 * @d: Pointer to where to store the result
217 *
218 * Computes *@d = (@a << @s)
219 *
220 * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
221 * make sense. Example conditions:
222 * - 'a << s' causes bits to be lost when stored in *d.
223 * - 's' is garbage (e.g. negative) or so large that the result of
224 * 'a << s' is guaranteed to be 0.
225 * - 'a' is negative.
226 * - 'a << s' sets the sign bit, if any, in '*d'.
227 *
228 * '*d' will hold the results of the attempted shift, but is not
229 * considered "safe for use" if false is returned.
230 */
231 #define check_shl_overflow(a, s, d) ({ \
232 typeof(a) _a = a; \
233 typeof(s) _s = s; \
234 typeof(d) _d = d; \
235 u64 _a_full = _a; \
236 unsigned int _to_shift = \
237 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
238 *_d = (_a_full << _to_shift); \
239 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
240 (*_d >> _to_shift) != _a); \
241 })
242
243 /**
244 * array_size() - Calculate size of 2-dimensional array.
245 *
246 * @a: dimension one
247 * @b: dimension two
248 *
249 * Calculates size of 2-dimensional array: @a * @b.
250 *
251 * Returns: number of bytes needed to represent the array or SIZE_MAX on
252 * overflow.
253 */
array_size(size_t a,size_t b)254 static inline __must_check size_t array_size(size_t a, size_t b)
255 {
256 size_t bytes;
257
258 if (check_mul_overflow(a, b, &bytes))
259 return SIZE_MAX;
260
261 return bytes;
262 }
263
264 /**
265 * array3_size() - Calculate size of 3-dimensional array.
266 *
267 * @a: dimension one
268 * @b: dimension two
269 * @c: dimension three
270 *
271 * Calculates size of 3-dimensional array: @a * @b * @c.
272 *
273 * Returns: number of bytes needed to represent the array or SIZE_MAX on
274 * overflow.
275 */
array3_size(size_t a,size_t b,size_t c)276 static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
277 {
278 size_t bytes;
279
280 if (check_mul_overflow(a, b, &bytes))
281 return SIZE_MAX;
282 if (check_mul_overflow(bytes, c, &bytes))
283 return SIZE_MAX;
284
285 return bytes;
286 }
287
__ab_c_size(size_t n,size_t size,size_t c)288 static inline __must_check size_t __ab_c_size(size_t n, size_t size, size_t c)
289 {
290 size_t bytes;
291
292 if (check_mul_overflow(n, size, &bytes))
293 return SIZE_MAX;
294 if (check_add_overflow(bytes, c, &bytes))
295 return SIZE_MAX;
296
297 return bytes;
298 }
299
300 /**
301 * struct_size() - Calculate size of structure with trailing array.
302 * @p: Pointer to the structure.
303 * @member: Name of the array member.
304 * @n: Number of elements in the array.
305 *
306 * Calculates size of memory needed for structure @p followed by an
307 * array of @n @member elements.
308 *
309 * Return: number of bytes needed or SIZE_MAX on overflow.
310 */
311 #define struct_size(p, member, n) \
312 __ab_c_size(n, \
313 sizeof(*(p)->member) + __must_be_array((p)->member),\
314 sizeof(*(p)))
315
316 #endif /* __LINUX_OVERFLOW_H */
317