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TOMOYO Linux Cross Reference
Linux/include/linux/overflow.h

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  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 static inline __must_check size_t __ab_c_size(size_t n, size_t size, size_t c)
288 {
289         size_t bytes;
290 
291         if (check_mul_overflow(n, size, &bytes))
292                 return SIZE_MAX;
293         if (check_add_overflow(bytes, c, &bytes))
294                 return SIZE_MAX;
295 
296         return bytes;
297 }
298 
299 /**
300  * struct_size() - Calculate size of structure with trailing array.
301  * @p: Pointer to the structure.
302  * @member: Name of the array member.
303  * @n: Number of elements in the array.
304  *
305  * Calculates size of memory needed for structure @p followed by an
306  * array of @n @member elements.
307  *
308  * Return: number of bytes needed or SIZE_MAX on overflow.
309  */
310 #define struct_size(p, member, n)                                       \
311         __ab_c_size(n,                                                  \
312                     sizeof(*(p)->member) + __must_be_array((p)->member),\
313                     sizeof(*(p)))
314 
315 #endif /* __LINUX_OVERFLOW_H */
316 

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