1 /* SPDX-License-Identifier: GPL-2.0 OR MIT */
2 #ifndef _LINUXKPI_LINUX_OVERFLOW_H
3 #define _LINUXKPI_LINUX_OVERFLOW_H
4
5 #include <linux/compiler.h>
6 #include <linux/limits.h>
7 #ifdef __linux__
8 #include <linux/const.h>
9 #endif
10
11 /*
12 * We need to compute the minimum and maximum values representable in a given
13 * type. These macros may also be useful elsewhere. It would seem more obvious
14 * 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 __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_max(t) __type_max(typeof(t))
38 #define __type_min(T) ((T)((T)-type_max(T)-(T)1))
39 #define type_min(t) __type_min(typeof(t))
40
41 /*
42 * Avoids triggering -Wtype-limits compilation warning,
43 * while using unsigned data types to check a < 0.
44 */
45 #define is_non_negative(a) ((a) > 0 || (a) == 0)
46 #define is_negative(a) (!(is_non_negative(a)))
47
48 /*
49 * Allows for effectively applying __must_check to a macro so we can have
50 * both the type-agnostic benefits of the macros while also being able to
51 * enforce that the return value is, in fact, checked.
52 */
__must_check_overflow(bool overflow)53 static inline bool __must_check __must_check_overflow(bool overflow)
54 {
55 return unlikely(overflow);
56 }
57
58 /**
59 * check_add_overflow() - Calculate addition with overflow checking
60 * @a: first addend
61 * @b: second addend
62 * @d: pointer to store sum
63 *
64 * Returns true on wrap-around, false otherwise.
65 *
66 * *@d holds the results of the attempted addition, regardless of whether
67 * wrap-around occurred.
68 */
69 #define check_add_overflow(a, b, d) \
70 __must_check_overflow(__builtin_add_overflow(a, b, d))
71
72 /**
73 * wrapping_add() - Intentionally perform a wrapping addition
74 * @type: type for result of calculation
75 * @a: first addend
76 * @b: second addend
77 *
78 * Return the potentially wrapped-around addition without
79 * tripping any wrap-around sanitizers that may be enabled.
80 */
81 #define wrapping_add(type, a, b) \
82 ({ \
83 type __val; \
84 __builtin_add_overflow(a, b, &__val); \
85 __val; \
86 })
87
88 /**
89 * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
90 * @var: variable to be incremented
91 * @offset: amount to add
92 *
93 * Increments @var by @offset with wrap-around. Returns the resulting
94 * value of @var. Will not trip any wrap-around sanitizers.
95 *
96 * Returns the new value of @var.
97 */
98 #define wrapping_assign_add(var, offset) \
99 ({ \
100 typeof(var) *__ptr = &(var); \
101 *__ptr = wrapping_add(typeof(var), *__ptr, offset); \
102 })
103
104 /**
105 * check_sub_overflow() - Calculate subtraction with overflow checking
106 * @a: minuend; value to subtract from
107 * @b: subtrahend; value to subtract from @a
108 * @d: pointer to store difference
109 *
110 * Returns true on wrap-around, false otherwise.
111 *
112 * *@d holds the results of the attempted subtraction, regardless of whether
113 * wrap-around occurred.
114 */
115 #define check_sub_overflow(a, b, d) \
116 __must_check_overflow(__builtin_sub_overflow(a, b, d))
117
118 /**
119 * wrapping_sub() - Intentionally perform a wrapping subtraction
120 * @type: type for result of calculation
121 * @a: minuend; value to subtract from
122 * @b: subtrahend; value to subtract from @a
123 *
124 * Return the potentially wrapped-around subtraction without
125 * tripping any wrap-around sanitizers that may be enabled.
126 */
127 #define wrapping_sub(type, a, b) \
128 ({ \
129 type __val; \
130 __builtin_sub_overflow(a, b, &__val); \
131 __val; \
132 })
133
134 /**
135 * wrapping_assign_sub() - Intentionally perform a wrapping decrement assign
136 * @var: variable to be decremented
137 * @offset: amount to subtract
138 *
139 * Decrements @var by @offset with wrap-around. Returns the resulting
140 * value of @var. Will not trip any wrap-around sanitizers.
141 *
142 * Returns the new value of @var.
143 */
144 #define wrapping_assign_sub(var, offset) \
145 ({ \
146 typeof(var) *__ptr = &(var); \
147 *__ptr = wrapping_sub(typeof(var), *__ptr, offset); \
148 })
149
150 /**
151 * check_mul_overflow() - Calculate multiplication with overflow checking
152 * @a: first factor
153 * @b: second factor
154 * @d: pointer to store product
155 *
156 * Returns true on wrap-around, false otherwise.
157 *
158 * *@d holds the results of the attempted multiplication, regardless of whether
159 * wrap-around occurred.
160 */
161 #define check_mul_overflow(a, b, d) \
162 __must_check_overflow(__builtin_mul_overflow(a, b, d))
163
164 /**
165 * wrapping_mul() - Intentionally perform a wrapping multiplication
166 * @type: type for result of calculation
167 * @a: first factor
168 * @b: second factor
169 *
170 * Return the potentially wrapped-around multiplication without
171 * tripping any wrap-around sanitizers that may be enabled.
172 */
173 #define wrapping_mul(type, a, b) \
174 ({ \
175 type __val; \
176 __builtin_mul_overflow(a, b, &__val); \
177 __val; \
178 })
179
180 /**
181 * check_shl_overflow() - Calculate a left-shifted value and check overflow
182 * @a: Value to be shifted
183 * @s: How many bits left to shift
184 * @d: Pointer to where to store the result
185 *
186 * Computes *@d = (@a << @s)
187 *
188 * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't
189 * make sense. Example conditions:
190 *
191 * - '@a << @s' causes bits to be lost when stored in *@d.
192 * - '@s' is garbage (e.g. negative) or so large that the result of
193 * '@a << @s' is guaranteed to be 0.
194 * - '@a' is negative.
195 * - '@a << @s' sets the sign bit, if any, in '*@d'.
196 *
197 * '*@d' will hold the results of the attempted shift, but is not
198 * considered "safe for use" if true is returned.
199 */
200 #define check_shl_overflow(a, s, d) __must_check_overflow(({ \
201 typeof(a) _a = a; \
202 typeof(s) _s = s; \
203 typeof(d) _d = d; \
204 unsigned long long _a_full = _a; \
205 unsigned int _to_shift = \
206 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
207 *_d = (_a_full << _to_shift); \
208 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
209 (*_d >> _to_shift) != _a); \
210 }))
211
212 #define __overflows_type_constexpr(x, T) ( \
213 is_unsigned_type(typeof(x)) ? \
214 (x) > type_max(T) : \
215 is_unsigned_type(typeof(T)) ? \
216 (x) < 0 || (x) > type_max(T) : \
217 (x) < type_min(T) || (x) > type_max(T))
218
219 #define __overflows_type(x, T) ({ \
220 typeof(T) v = 0; \
221 check_add_overflow((x), v, &v); \
222 })
223
224 /**
225 * overflows_type - helper for checking the overflows between value, variables,
226 * or data type
227 *
228 * @n: source constant value or variable to be checked
229 * @T: destination variable or data type proposed to store @x
230 *
231 * Compares the @x expression for whether or not it can safely fit in
232 * the storage of the type in @T. @x and @T can have different types.
233 * If @x is a constant expression, this will also resolve to a constant
234 * expression.
235 *
236 * Returns: true if overflow can occur, false otherwise.
237 */
238 #define overflows_type(n, T) \
239 __builtin_choose_expr(__is_constexpr(n), \
240 __overflows_type_constexpr(n, T), \
241 __overflows_type(n, T))
242
243 /**
244 * castable_to_type - like __same_type(), but also allows for casted literals
245 *
246 * @n: variable or constant value
247 * @T: variable or data type
248 *
249 * Unlike the __same_type() macro, this allows a constant value as the
250 * first argument. If this value would not overflow into an assignment
251 * of the second argument's type, it returns true. Otherwise, this falls
252 * back to __same_type().
253 */
254 #define castable_to_type(n, T) \
255 __builtin_choose_expr(__is_constexpr(n), \
256 !__overflows_type_constexpr(n, T), \
257 __same_type(n, T))
258
259 /**
260 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
261 * @factor1: first factor
262 * @factor2: second factor
263 *
264 * Returns: calculate @factor1 * @factor2, both promoted to size_t,
265 * with any overflow causing the return value to be SIZE_MAX. The
266 * lvalue must be size_t to avoid implicit type conversion.
267 */
size_mul(size_t factor1,size_t factor2)268 static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
269 {
270 size_t bytes;
271
272 if (check_mul_overflow(factor1, factor2, &bytes))
273 return SIZE_MAX;
274
275 return bytes;
276 }
277
278 /**
279 * size_add() - Calculate size_t addition with saturation at SIZE_MAX
280 * @addend1: first addend
281 * @addend2: second addend
282 *
283 * Returns: calculate @addend1 + @addend2, both promoted to size_t,
284 * with any overflow causing the return value to be SIZE_MAX. The
285 * lvalue must be size_t to avoid implicit type conversion.
286 */
size_add(size_t addend1,size_t addend2)287 static inline size_t __must_check size_add(size_t addend1, size_t addend2)
288 {
289 size_t bytes;
290
291 if (check_add_overflow(addend1, addend2, &bytes))
292 return SIZE_MAX;
293
294 return bytes;
295 }
296
297 /**
298 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
299 * @minuend: value to subtract from
300 * @subtrahend: value to subtract from @minuend
301 *
302 * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
303 * with any overflow causing the return value to be SIZE_MAX. For
304 * composition with the size_add() and size_mul() helpers, neither
305 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
306 * The lvalue must be size_t to avoid implicit type conversion.
307 */
size_sub(size_t minuend,size_t subtrahend)308 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
309 {
310 size_t bytes;
311
312 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
313 check_sub_overflow(minuend, subtrahend, &bytes))
314 return SIZE_MAX;
315
316 return bytes;
317 }
318
319 /**
320 * array_size() - Calculate size of 2-dimensional array.
321 * @a: dimension one
322 * @b: dimension two
323 *
324 * Calculates size of 2-dimensional array: @a * @b.
325 *
326 * Returns: number of bytes needed to represent the array or SIZE_MAX on
327 * overflow.
328 */
329 #define array_size(a, b) size_mul(a, b)
330
331 /**
332 * array3_size() - Calculate size of 3-dimensional array.
333 * @a: dimension one
334 * @b: dimension two
335 * @c: dimension three
336 *
337 * Calculates size of 3-dimensional array: @a * @b * @c.
338 *
339 * Returns: number of bytes needed to represent the array or SIZE_MAX on
340 * overflow.
341 */
342 #define array3_size(a, b, c) size_mul(size_mul(a, b), c)
343
344 /**
345 * flex_array_size() - Calculate size of a flexible array member
346 * within an enclosing structure.
347 * @p: Pointer to the structure.
348 * @member: Name of the flexible array member.
349 * @count: Number of elements in the array.
350 *
351 * Calculates size of a flexible array of @count number of @member
352 * elements, at the end of structure @p.
353 *
354 * Return: number of bytes needed or SIZE_MAX on overflow.
355 */
356 #define flex_array_size(p, member, count) \
357 __builtin_choose_expr(__is_constexpr(count), \
358 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \
359 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
360
361 /**
362 * struct_size() - Calculate size of structure with trailing flexible array.
363 * @p: Pointer to the structure.
364 * @member: Name of the array member.
365 * @count: Number of elements in the array.
366 *
367 * Calculates size of memory needed for structure of @p followed by an
368 * array of @count number of @member elements.
369 *
370 * Return: number of bytes needed or SIZE_MAX on overflow.
371 */
372 #define struct_size(p, member, count) \
373 __builtin_choose_expr(__is_constexpr(count), \
374 sizeof(*(p)) + flex_array_size(p, member, count), \
375 size_add(sizeof(*(p)), flex_array_size(p, member, count)))
376
377 /**
378 * struct_size_t() - Calculate size of structure with trailing flexible array
379 * @type: structure type name.
380 * @member: Name of the array member.
381 * @count: Number of elements in the array.
382 *
383 * Calculates size of memory needed for structure @type followed by an
384 * array of @count number of @member elements. Prefer using struct_size()
385 * when possible instead, to keep calculations associated with a specific
386 * instance variable of type @type.
387 *
388 * Return: number of bytes needed or SIZE_MAX on overflow.
389 */
390 #define struct_size_t(type, member, count) \
391 struct_size((type *)NULL, member, count)
392
393 /**
394 * __DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
395 * Enables caller macro to pass arbitrary trailing expressions
396 *
397 * @type: structure type name, including "struct" keyword.
398 * @name: Name for a variable to define.
399 * @member: Name of the array member.
400 * @count: Number of elements in the array; must be compile-time const.
401 * @trailer: Trailing expressions for attributes and/or initializers.
402 */
403 #define __DEFINE_FLEX(type, name, member, count, trailer...) \
404 _Static_assert(__builtin_constant_p(count), \
405 "onstack flex array members require compile-time const count"); \
406 union { \
407 u8 bytes[struct_size_t(type, member, count)]; \
408 type obj; \
409 } name##_u trailer; \
410 type *name = (type *)&name##_u
411
412 /**
413 * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family.
414 * Enables caller macro to pass (different) initializer.
415 *
416 * @type: structure type name, including "struct" keyword.
417 * @name: Name for a variable to define.
418 * @member: Name of the array member.
419 * @count: Number of elements in the array; must be compile-time const.
420 * @initializer: Initializer expression (e.g., pass `= { }` at minimum).
421 */
422 #define _DEFINE_FLEX(type, name, member, count, initializer...) \
423 __DEFINE_FLEX(type, name, member, count, = { .obj initializer })
424
425 /**
426 * DEFINE_RAW_FLEX() - Define an on-stack instance of structure with a trailing
427 * flexible array member, when it does not have a __counted_by annotation.
428 *
429 * @type: structure type name, including "struct" keyword.
430 * @name: Name for a variable to define.
431 * @member: Name of the array member.
432 * @count: Number of elements in the array; must be compile-time const.
433 *
434 * Define a zeroed, on-stack, instance of @type structure with a trailing
435 * flexible array member.
436 * Use __struct_size(@name) to get compile-time size of it afterwards.
437 * Use __member_size(@name->member) to get compile-time size of @name members.
438 * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of
439 * elements in array @member.
440 */
441 #define DEFINE_RAW_FLEX(type, name, member, count) \
442 __DEFINE_FLEX(type, name, member, count, = { })
443
444 /**
445 * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing
446 * flexible array member.
447 *
448 * @TYPE: structure type name, including "struct" keyword.
449 * @NAME: Name for a variable to define.
450 * @MEMBER: Name of the array member.
451 * @COUNTER: Name of the __counted_by member.
452 * @COUNT: Number of elements in the array; must be compile-time const.
453 *
454 * Define a zeroed, on-stack, instance of @TYPE structure with a trailing
455 * flexible array member.
456 * Use __struct_size(@NAME) to get compile-time size of it afterwards.
457 * Use __member_size(@NAME->member) to get compile-time size of @NAME members.
458 * Use STACK_FLEX_ARRAY_SIZE(@name, @member) to get compile-time number of
459 * elements in array @member.
460 */
461 #define DEFINE_FLEX(TYPE, NAME, MEMBER, COUNTER, COUNT) \
462 _DEFINE_FLEX(TYPE, NAME, MEMBER, COUNT, = { .COUNTER = COUNT, })
463
464 /**
465 * STACK_FLEX_ARRAY_SIZE() - helper macro for DEFINE_FLEX() family.
466 * Returns the number of elements in @array.
467 *
468 * @name: Name for a variable defined in DEFINE_RAW_FLEX()/DEFINE_FLEX().
469 * @array: Name of the array member.
470 */
471 #define STACK_FLEX_ARRAY_SIZE(name, array) \
472 (__member_size((name)->array) / sizeof(*(name)->array) + \
473 __must_be_array((name)->array))
474
475 #endif /* _LINUXKPI_LINUX_OVERFLOW_H */
476