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 */
51static 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 */
266static 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 */
285static 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 */
306static 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 arbitrary trailing expressions
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 * @trailer: Trailing expressions for attributes and/or initializers.
400 */
401#define __DEFINE_FLEX(type, name, member, count, trailer...) \
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 trailer; \
408 type *name = (type *)&name##_u
409
410/**
411 * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
412 * Enables caller macro to pass (different) initializer.
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 * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
419 */
420#define _DEFINE_FLEX(type, name, member, count, initializer...) \
421 __DEFINE_FLEX(type, name, member, count, = { .obj initializer })
422
423/**
424 * DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
425 * flexible array member, when it does not have a __counted_by annotation.
426 *
427 * @type: structure type name, including "struct" keyword.
428 * @name: Name for a variable to define.
429 * @member: Name of the array member.
430 * @count: Number of elements in the array; must be compile-time const.
431 *
432 * Define a zeroed, on-stack, instance of @type structure with a trailing
433 * flexible array member.
434 * Use __struct_size(@name) to get compile-time size of it afterwards.
435 * Use __member_size(@name->member) to get compile-time size of @name members.
436 * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of
437 * elements in array @member.
438 */
439#define DEFINE_RAW_FLEX(type, name, member, count) \
440 __DEFINE_FLEX(type, name, member, count, = { })
441
442/**
443 * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing
444 * flexible array member.
445 *
446 * @TYPE: structure type name, including "struct" keyword.
447 * @NAME: Name for a variable to define.
448 * @MEMBER: Name of the array member.
449 * @COUNTER: Name of the __counted_by member.
450 * @COUNT: Number of elements in the array; must be compile-time const.
451 *
452 * Define a zeroed, on-stack, instance of @TYPE structure with a trailing
453 * flexible array member.
454 * Use __struct_size(@NAME) to get compile-time size of it afterwards.
455 * Use __member_size(@NAME->member) to get compile-time size of @NAME members.
456 * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of
457 * elements in array @member.
458 */
459#define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
460 _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
461
462/**
463 * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.
464 * Returns the number of elements in @array.
465 *
466 * @name: Name for a variable defined in DEFINE_RAW_FLEX()/DEFINE_FLEX().
467 * @array: Name of the array member.
468 */
469#define STACK_FLEX_ARRAY_SIZE(name, array) \
470 (__member_size((name)->array) / sizeof(*(name)->array) + \
471 __must_be_array((name)->array))
472
473#endif /* __LINUX_OVERFLOW_H */
474

source code of linux/include/linux/overflow.h

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