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 * Avoids triggering -Wtype-limits compilation warning,
41 * while using unsigned data types to check a < 0.
42 */
43 #define is_non_negative(a) ((a) > 0 || (a) == 0)
44 #define is_negative(a) (!(is_non_negative(a)))
45
46 #ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
47 /*
48 * For simplicity and code hygiene, the fallback code below insists on
49 * a, b and *d having the same type (similar to the min() and max()
50 * macros), whereas gcc's type-generic overflow checkers accept
51 * different types. Hence we don't just make check_add_overflow an
52 * alias for __builtin_add_overflow, but add type checks similar to
53 * below.
54 */
55 #define check_add_overflow(a, b, d) ({ \
56 typeof(a) __a = (a); \
57 typeof(b) __b = (b); \
58 typeof(d) __d = (d); \
59 (void) (&__a == &__b); \
60 (void) (&__a == __d); \
61 __builtin_add_overflow(__a, __b, __d); \
62 })
63
64 #define check_sub_overflow(a, b, d) ({ \
65 typeof(a) __a = (a); \
66 typeof(b) __b = (b); \
67 typeof(d) __d = (d); \
68 (void) (&__a == &__b); \
69 (void) (&__a == __d); \
70 __builtin_sub_overflow(__a, __b, __d); \
71 })
72
73 #define check_mul_overflow(a, b, d) ({ \
74 typeof(a) __a = (a); \
75 typeof(b) __b = (b); \
76 typeof(d) __d = (d); \
77 (void) (&__a == &__b); \
78 (void) (&__a == __d); \
79 __builtin_mul_overflow(__a, __b, __d); \
80 })
81
82 #else
83
84
85 /* Checking for unsigned overflow is relatively easy without causing UB. */
86 #define __unsigned_add_overflow(a, b, d) ({ \
87 typeof(a) __a = (a); \
88 typeof(b) __b = (b); \
89 typeof(d) __d = (d); \
90 (void) (&__a == &__b); \
91 (void) (&__a == __d); \
92 *__d = __a + __b; \
93 *__d < __a; \
94 })
95 #define __unsigned_sub_overflow(a, b, d) ({ \
96 typeof(a) __a = (a); \
97 typeof(b) __b = (b); \
98 typeof(d) __d = (d); \
99 (void) (&__a == &__b); \
100 (void) (&__a == __d); \
101 *__d = __a - __b; \
102 __a < __b; \
103 })
104 /*
105 * If one of a or b is a compile-time constant, this avoids a division.
106 */
107 #define __unsigned_mul_overflow(a, b, d) ({ \
108 typeof(a) __a = (a); \
109 typeof(b) __b = (b); \
110 typeof(d) __d = (d); \
111 (void) (&__a == &__b); \
112 (void) (&__a == __d); \
113 *__d = __a * __b; \
114 __builtin_constant_p(__b) ? \
115 __b > 0 && __a > type_max(typeof(__a)) / __b : \
116 __a > 0 && __b > type_max(typeof(__b)) / __a; \
117 })
118
119 /*
120 * For signed types, detecting overflow is much harder, especially if
121 * we want to avoid UB. But the interface of these macros is such that
122 * we must provide a result in *d, and in fact we must produce the
123 * result promised by gcc's builtins, which is simply the possibly
124 * wrapped-around value. Fortunately, we can just formally do the
125 * operations in the widest relevant unsigned type (u64) and then
126 * truncate the result - gcc is smart enough to generate the same code
127 * with and without the (u64) casts.
128 */
129
130 /*
131 * Adding two signed integers can overflow only if they have the same
132 * sign, and overflow has happened iff the result has the opposite
133 * sign.
134 */
135 #define __signed_add_overflow(a, b, d) ({ \
136 typeof(a) __a = (a); \
137 typeof(b) __b = (b); \
138 typeof(d) __d = (d); \
139 (void) (&__a == &__b); \
140 (void) (&__a == __d); \
141 *__d = (u64)__a + (u64)__b; \
142 (((~(__a ^ __b)) & (*__d ^ __a)) \
143 & type_min(typeof(__a))) != 0; \
144 })
145
146 /*
147 * Subtraction is similar, except that overflow can now happen only
148 * when the signs are opposite. In this case, overflow has happened if
149 * the result has the opposite sign of a.
150 */
151 #define __signed_sub_overflow(a, b, d) ({ \
152 typeof(a) __a = (a); \
153 typeof(b) __b = (b); \
154 typeof(d) __d = (d); \
155 (void) (&__a == &__b); \
156 (void) (&__a == __d); \
157 *__d = (u64)__a - (u64)__b; \
158 ((((__a ^ __b)) & (*__d ^ __a)) \
159 & type_min(typeof(__a))) != 0; \
160 })
161
162 /*
163 * Signed multiplication is rather hard. gcc always follows C99, so
164 * division is truncated towards 0. This means that we can write the
165 * overflow check like this:
166 *
167 * (a > 0 && (b > MAX/a || b < MIN/a)) ||
168 * (a < -1 && (b > MIN/a || b < MAX/a) ||
169 * (a == -1 && b == MIN)
170 *
171 * The redundant casts of -1 are to silence an annoying -Wtype-limits
172 * (included in -Wextra) warning: When the type is u8 or u16, the
173 * __b_c_e in check_mul_overflow obviously selects
174 * __unsigned_mul_overflow, but unfortunately gcc still parses this
175 * code and warns about the limited range of __b.
176 */
177
178 #define __signed_mul_overflow(a, b, d) ({ \
179 typeof(a) __a = (a); \
180 typeof(b) __b = (b); \
181 typeof(d) __d = (d); \
182 typeof(a) __tmax = type_max(typeof(a)); \
183 typeof(a) __tmin = type_min(typeof(a)); \
184 (void) (&__a == &__b); \
185 (void) (&__a == __d); \
186 *__d = (u64)__a * (u64)__b; \
187 (__b > 0 && (__a > __tmax/__b || __a < __tmin/__b)) || \
188 (__b < (typeof(__b))-1 && (__a > __tmin/__b || __a < __tmax/__b)) || \
189 (__b == (typeof(__b))-1 && __a == __tmin); \
190 })
191
192
193 #define check_add_overflow(a, b, d) \
194 __builtin_choose_expr(is_signed_type(typeof(a)), \
195 __signed_add_overflow(a, b, d), \
196 __unsigned_add_overflow(a, b, d))
197
198 #define check_sub_overflow(a, b, d) \
199 __builtin_choose_expr(is_signed_type(typeof(a)), \
200 __signed_sub_overflow(a, b, d), \
201 __unsigned_sub_overflow(a, b, d))
202
203 #define check_mul_overflow(a, b, d) \
204 __builtin_choose_expr(is_signed_type(typeof(a)), \
205 __signed_mul_overflow(a, b, d), \
206 __unsigned_mul_overflow(a, b, d))
207
208
209 #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
210
211 /** check_shl_overflow() - Calculate a left-shifted value and check overflow
212 *
213 * @a: Value to be shifted
214 * @s: How many bits left to shift
215 * @d: Pointer to where to store the result
216 *
217 * Computes *@d = (@a << @s)
218 *
219 * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
220 * make sense. Example conditions:
221 * - 'a << s' causes bits to be lost when stored in *d.
222 * - 's' is garbage (e.g. negative) or so large that the result of
223 * 'a << s' is guaranteed to be 0.
224 * - 'a' is negative.
225 * - 'a << s' sets the sign bit, if any, in '*d'.
226 *
227 * '*d' will hold the results of the attempted shift, but is not
228 * considered "safe for use" if false is returned.
229 */
230 #define check_shl_overflow(a, s, d) ({ \
231 typeof(a) _a = a; \
232 typeof(s) _s = s; \
233 typeof(d) _d = d; \
234 u64 _a_full = _a; \
235 unsigned int _to_shift = \
236 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
237 *_d = (_a_full << _to_shift); \
238 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
239 (*_d >> _to_shift) != _a); \
240 })
241
242 /**
243 * array_size() - Calculate size of 2-dimensional array.
244 *
245 * @a: dimension one
246 * @b: dimension two
247 *
248 * Calculates size of 2-dimensional array: @a * @b.
249 *
250 * Returns: number of bytes needed to represent the array or SIZE_MAX on
251 * overflow.
252 */
253 static inline __must_check size_t array_size(size_t a, size_t b)
254 {
255 size_t bytes;
256
257 if (check_mul_overflow(a, b, &bytes))
258 return SIZE_MAX;
259
260 return bytes;
261 }
262
263 /**
264 * array3_size() - Calculate size of 3-dimensional array.
265 *
266 * @a: dimension one
267 * @b: dimension two
268 * @c: dimension three
269 *
270 * Calculates size of 3-dimensional array: @a * @b * @c.
271 *
272 * Returns: number of bytes needed to represent the array or SIZE_MAX on
273 * overflow.
274 */
275 static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
276 {
277 size_t bytes;
278
279 if (check_mul_overflow(a, b, &bytes))
280 return SIZE_MAX;
281 if (check_mul_overflow(bytes, c, &bytes))
282 return SIZE_MAX;
283
284 return bytes;
285 }
286
287 /*
288 * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for
289 * struct_size() below.
290 */
291 static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
292 {
293 size_t bytes;
294
295 if (check_mul_overflow(a, b, &bytes))
296 return SIZE_MAX;
297 if (check_add_overflow(bytes, c, &bytes))
298 return SIZE_MAX;
299
300 return bytes;
301 }
302
303 /**
304 * struct_size() - Calculate size of structure with trailing array.
305 * @p: Pointer to the structure.
306 * @member: Name of the array member.
307 * @n: Number of elements in the array.
308 *
309 * Calculates size of memory needed for structure @p followed by an
310 * array of @n @member elements.
311 *
312 * Return: number of bytes needed or SIZE_MAX on overflow.
313 */
314 #define struct_size(p, member, n) \
315 __ab_c_size(n, \
316 sizeof(*(p)->member) + __must_be_array((p)->member),\
317 sizeof(*(p)))
318
319 #endif /* __LINUX_OVERFLOW_H */