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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 #include <linux/limits.h>
  7 #include <linux/const.h>
  8 
  9 /*
 10  * We need to compute the minimum and maximum values representable in a given
 11  * type. These macros may also be useful elsewhere. It would seem more obvious
 12  * to do something like:
 13  *
 14  * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
 15  * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
 16  *
 17  * Unfortunately, the middle expressions, strictly speaking, have
 18  * undefined behaviour, and at least some versions of gcc warn about
 19  * the type_max expression (but not if -fsanitize=undefined is in
 20  * effect; in that case, the warning is deferred to runtime...).
 21  *
 22  * The slightly excessive casting in type_min is to make sure the
 23  * macros also produce sensible values for the exotic type _Bool. [The
 24  * overflow checkers only almost work for _Bool, but that's
 25  * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
 26  * _Bools. Besides, the gcc builtins don't allow _Bool* as third
 27  * argument.]
 28  *
 29  * Idea stolen from
 30  * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
 31  * credit to Christian Biere.
 32  */
 33 #define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
 34 #define __type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
 35 #define type_max(t)     __type_max(typeof(t))
 36 #define __type_min(T) ((T)((T)-type_max(T)-(T)1))
 37 #define type_min(t)     __type_min(typeof(t))
 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 /*
 47  * Allows for effectively applying __must_check to a macro so we can have
 48  * both the type-agnostic benefits of the macros while also being able to
 49  * enforce that the return value is, in fact, checked.
 50  */
 51 static inline bool __must_check __must_check_overflow(bool overflow)
 52 {
 53         return unlikely(overflow);
 54 }
 55 
 56 /**
 57  * check_add_overflow() - Calculate addition with overflow checking
 58  * @a: first addend
 59  * @b: second addend
 60  * @d: pointer to store sum
 61  *
 62  * Returns true on wrap-around, false otherwise.
 63  *
 64  * *@d holds the results of the attempted addition, regardless of whether
 65  * wrap-around occurred.
 66  */
 67 #define check_add_overflow(a, b, d)     \
 68         __must_check_overflow(__builtin_add_overflow(a, b, d))
 69 
 70 /**
 71  * wrapping_add() - Intentionally perform a wrapping addition
 72  * @type: type for result of calculation
 73  * @a: first addend
 74  * @b: second addend
 75  *
 76  * Return the potentially wrapped-around addition without
 77  * tripping any wrap-around sanitizers that may be enabled.
 78  */
 79 #define wrapping_add(type, a, b)                                \
 80         ({                                                      \
 81                 type __val;                                     \
 82                 __builtin_add_overflow(a, b, &__val);           \
 83                 __val;                                          \
 84         })
 85 
 86 /**
 87  * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
 88  * @var: variable to be incremented
 89  * @offset: amount to add
 90  *
 91  * Increments @var by @offset with wrap-around. Returns the resulting
 92  * value of @var. Will not trip any wrap-around sanitizers.
 93  *
 94  * Returns the new value of @var.
 95  */
 96 #define wrapping_assign_add(var, offset)                                \
 97         ({                                                              \
 98                 typeof(var) *__ptr = &(var);                            \
 99                 *__ptr = wrapping_add(typeof(var), *__ptr, offset);     \
100         })
101 
102 /**
103  * check_sub_overflow() - Calculate subtraction with overflow checking
104  * @a: minuend; value to subtract from
105  * @b: subtrahend; value to subtract from @a
106  * @d: pointer to store difference
107  *
108  * Returns true on wrap-around, false otherwise.
109  *
110  * *@d holds the results of the attempted subtraction, regardless of whether
111  * wrap-around occurred.
112  */
113 #define check_sub_overflow(a, b, d)     \
114         __must_check_overflow(__builtin_sub_overflow(a, b, d))
115 
116 /**
117  * wrapping_sub() - Intentionally perform a wrapping subtraction
118  * @type: type for result of calculation
119  * @a: minuend; value to subtract from
120  * @b: subtrahend; value to subtract from @a
121  *
122  * Return the potentially wrapped-around subtraction without
123  * tripping any wrap-around sanitizers that may be enabled.
124  */
125 #define wrapping_sub(type, a, b)                                \
126         ({                                                      \
127                 type __val;                                     \
128                 __builtin_sub_overflow(a, b, &__val);           \
129                 __val;                                          \
130         })
131 
132 /**
133  * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
134  * @var: variable to be decremented
135  * @offset: amount to subtract
136  *
137  * Decrements @var by @offset with wrap-around. Returns the resulting
138  * value of @var. Will not trip any wrap-around sanitizers.
139  *
140  * Returns the new value of @var.
141  */
142 #define wrapping_assign_sub(var, offset)                                \
143         ({                                                              \
144                 typeof(var) *__ptr = &(var);                            \
145                 *__ptr = wrapping_sub(typeof(var), *__ptr, offset);     \
146         })
147 
148 /**
149  * check_mul_overflow() - Calculate multiplication with overflow checking
150  * @a: first factor
151  * @b: second factor
152  * @d: pointer to store product
153  *
154  * Returns true on wrap-around, false otherwise.
155  *
156  * *@d holds the results of the attempted multiplication, regardless of whether
157  * wrap-around occurred.
158  */
159 #define check_mul_overflow(a, b, d)     \
160         __must_check_overflow(__builtin_mul_overflow(a, b, d))
161 
162 /**
163  * wrapping_mul() - Intentionally perform a wrapping multiplication
164  * @type: type for result of calculation
165  * @a: first factor
166  * @b: second factor
167  *
168  * Return the potentially wrapped-around multiplication without
169  * tripping any wrap-around sanitizers that may be enabled.
170  */
171 #define wrapping_mul(type, a, b)                                \
172         ({                                                      \
173                 type __val;                                     \
174                 __builtin_mul_overflow(a, b, &__val);           \
175                 __val;                                          \
176         })
177 
178 /**
179  * check_shl_overflow() - Calculate a left-shifted value and check overflow
180  * @a: Value to be shifted
181  * @s: How many bits left to shift
182  * @d: Pointer to where to store the result
183  *
184  * Computes *@d = (@a << @s)
185  *
186  * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't
187  * make sense. Example conditions:
188  *
189  * - '@a << @s' causes bits to be lost when stored in *@d.
190  * - '@s' is garbage (e.g. negative) or so large that the result of
191  *   '@a << @s' is guaranteed to be 0.
192  * - '@a' is negative.
193  * - '@a << @s' sets the sign bit, if any, in '*@d'.
194  *
195  * '*@d' will hold the results of the attempted shift, but is not
196  * considered "safe for use" if true is returned.
197  */
198 #define check_shl_overflow(a, s, d) __must_check_overflow(({            \
199         typeof(a) _a = a;                                               \
200         typeof(s) _s = s;                                               \
201         typeof(d) _d = d;                                               \
202         unsigned long long _a_full = _a;                                \
203         unsigned int _to_shift =                                        \
204                 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0;    \
205         *_d = (_a_full << _to_shift);                                   \
206         (_to_shift != _s || is_negative(*_d) || is_negative(_a) ||      \
207         (*_d >> _to_shift) != _a);                                      \
208 }))
209 
210 #define __overflows_type_constexpr(x, T) (                      \
211         is_unsigned_type(typeof(x)) ?                           \
212                 (x) > type_max(T) :                             \
213         is_unsigned_type(typeof(T)) ?                           \
214                 (x) < 0 || (x) > type_max(T) :                  \
215         (x) < type_min(T) || (x) > type_max(T))
216 
217 #define __overflows_type(x, T)          ({      \
218         typeof(T) v = 0;                        \
219         check_add_overflow((x), v, &v);         \
220 })
221 
222 /**
223  * overflows_type - helper for checking the overflows between value, variables,
224  *                  or data type
225  *
226  * @n: source constant value or variable to be checked
227  * @T: destination variable or data type proposed to store @x
228  *
229  * Compares the @x expression for whether or not it can safely fit in
230  * the storage of the type in @T. @x and @T can have different types.
231  * If @x is a constant expression, this will also resolve to a constant
232  * expression.
233  *
234  * Returns: true if overflow can occur, false otherwise.
235  */
236 #define overflows_type(n, T)                                    \
237         __builtin_choose_expr(__is_constexpr(n),                \
238                               __overflows_type_constexpr(n, T), \
239                               __overflows_type(n, T))
240 
241 /**
242  * castable_to_type - like __same_type(), but also allows for casted literals
243  *
244  * @n: variable or constant value
245  * @T: variable or data type
246  *
247  * Unlike the __same_type() macro, this allows a constant value as the
248  * first argument. If this value would not overflow into an assignment
249  * of the second argument's type, it returns true. Otherwise, this falls
250  * back to __same_type().
251  */
252 #define castable_to_type(n, T)                                          \
253         __builtin_choose_expr(__is_constexpr(n),                        \
254                               !__overflows_type_constexpr(n, T),        \
255                               __same_type(n, T))
256 
257 /**
258  * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
259  * @factor1: first factor
260  * @factor2: second factor
261  *
262  * Returns: calculate @factor1 * @factor2, both promoted to size_t,
263  * with any overflow causing the return value to be SIZE_MAX. The
264  * lvalue must be size_t to avoid implicit type conversion.
265  */
266 static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
267 {
268         size_t bytes;
269 
270         if (check_mul_overflow(factor1, factor2, &bytes))
271                 return SIZE_MAX;
272 
273         return bytes;
274 }
275 
276 /**
277  * size_add() - Calculate size_t addition with saturation at SIZE_MAX
278  * @addend1: first addend
279  * @addend2: second addend
280  *
281  * Returns: calculate @addend1 + @addend2, both promoted to size_t,
282  * with any overflow causing the return value to be SIZE_MAX. The
283  * lvalue must be size_t to avoid implicit type conversion.
284  */
285 static inline size_t __must_check size_add(size_t addend1, size_t addend2)
286 {
287         size_t bytes;
288 
289         if (check_add_overflow(addend1, addend2, &bytes))
290                 return SIZE_MAX;
291 
292         return bytes;
293 }
294 
295 /**
296  * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
297  * @minuend: value to subtract from
298  * @subtrahend: value to subtract from @minuend
299  *
300  * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
301  * with any overflow causing the return value to be SIZE_MAX. For
302  * composition with the size_add() and size_mul() helpers, neither
303  * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
304  * The lvalue must be size_t to avoid implicit type conversion.
305  */
306 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
307 {
308         size_t bytes;
309 
310         if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
311             check_sub_overflow(minuend, subtrahend, &bytes))
312                 return SIZE_MAX;
313 
314         return bytes;
315 }
316 
317 /**
318  * array_size() - Calculate size of 2-dimensional array.
319  * @a: dimension one
320  * @b: dimension two
321  *
322  * Calculates size of 2-dimensional array: @a * @b.
323  *
324  * Returns: number of bytes needed to represent the array or SIZE_MAX on
325  * overflow.
326  */
327 #define array_size(a, b)        size_mul(a, b)
328 
329 /**
330  * array3_size() - Calculate size of 3-dimensional array.
331  * @a: dimension one
332  * @b: dimension two
333  * @c: dimension three
334  *
335  * Calculates size of 3-dimensional array: @a * @b * @c.
336  *
337  * Returns: number of bytes needed to represent the array or SIZE_MAX on
338  * overflow.
339  */
340 #define array3_size(a, b, c)    size_mul(size_mul(a, b), c)
341 
342 /**
343  * flex_array_size() - Calculate size of a flexible array member
344  *                     within an enclosing structure.
345  * @p: Pointer to the structure.
346  * @member: Name of the flexible array member.
347  * @count: Number of elements in the array.
348  *
349  * Calculates size of a flexible array of @count number of @member
350  * elements, at the end of structure @p.
351  *
352  * Return: number of bytes needed or SIZE_MAX on overflow.
353  */
354 #define flex_array_size(p, member, count)                               \
355         __builtin_choose_expr(__is_constexpr(count),                    \
356                 (count) * sizeof(*(p)->member) + __must_be_array((p)->member),  \
357                 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
358 
359 /**
360  * struct_size() - Calculate size of structure with trailing flexible array.
361  * @p: Pointer to the structure.
362  * @member: Name of the array member.
363  * @count: Number of elements in the array.
364  *
365  * Calculates size of memory needed for structure of @p followed by an
366  * array of @count number of @member elements.
367  *
368  * Return: number of bytes needed or SIZE_MAX on overflow.
369  */
370 #define struct_size(p, member, count)                                   \
371         __builtin_choose_expr(__is_constexpr(count),                    \
372                 sizeof(*(p)) + flex_array_size(p, member, count),       \
373                 size_add(sizeof(*(p)), flex_array_size(p, member, count)))
374 
375 /**
376  * struct_size_t() - Calculate size of structure with trailing flexible array
377  * @type: structure type name.
378  * @member: Name of the array member.
379  * @count: Number of elements in the array.
380  *
381  * Calculates size of memory needed for structure @type followed by an
382  * array of @count number of @member elements. Prefer using struct_size()
383  * when possible instead, to keep calculations associated with a specific
384  * instance variable of type @type.
385  *
386  * Return: number of bytes needed or SIZE_MAX on overflow.
387  */
388 #define struct_size_t(type, member, count)                                      \
389         struct_size((type *)NULL, member, count)
390 
391 /**
392  * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
393  * Enables caller macro to pass (different) initializer.
394  *
395  * @type: structure type name, including "struct" keyword.
396  * @name: Name for a variable to define.
397  * @member: Name of the array member.
398  * @count: Number of elements in the array; must be compile-time const.
399  * @initializer: initializer expression (could be empty for no init).
400  */
401 #define _DEFINE_FLEX(type, name, member, count, initializer...)                 \
402         _Static_assert(__builtin_constant_p(count),                             \
403                        "onstack flex array members require compile-time const count"); \
404         union {                                                                 \
405                 u8 bytes[struct_size_t(type, member, count)];                   \
406                 type obj;                                                       \
407         } name##_u initializer;                                                 \
408         type *name = (type *)&name##_u
409 
410 /**
411  * DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
412  * flexible array member, when it does not have a __counted_by annotation.
413  *
414  * @type: structure type name, including "struct" keyword.
415  * @name: Name for a variable to define.
416  * @member: Name of the array member.
417  * @count: Number of elements in the array; must be compile-time const.
418  *
419  * Define a zeroed, on-stack, instance of @type structure with a trailing
420  * flexible array member.
421  * Use __struct_size(@name) to get compile-time size of it afterwards.
422  */
423 #define DEFINE_RAW_FLEX(type, name, member, count)      \
424         _DEFINE_FLEX(type, name, member, count, = {})
425 
426 /**
427  * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing
428  * flexible array member.
429  *
430  * @TYPE: structure type name, including "struct" keyword.
431  * @NAME: Name for a variable to define.
432  * @MEMBER: Name of the array member.
433  * @COUNTER: Name of the __counted_by member.
434  * @COUNT: Number of elements in the array; must be compile-time const.
435  *
436  * Define a zeroed, on-stack, instance of @TYPE structure with a trailing
437  * flexible array member.
438  * Use __struct_size(@NAME) to get compile-time size of it afterwards.
439  */
440 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
441         _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .obj.COUNTER = COUNT, })
442 
443 #endif /* __LINUX_OVERFLOW_H */
444 

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