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_min(T) ((T)((T)-type_max(T)-(T)1)) |
36 | |
37 | /* |
38 | * Avoids triggering -Wtype-limits compilation warning, |
39 | * while using unsigned data types to check a < 0. |
40 | */ |
41 | #define is_non_negative(a) ((a) > 0 || (a) == 0) |
42 | #define is_negative(a) (!(is_non_negative(a))) |
43 | |
44 | /* |
45 | * Allows for effectively applying __must_check to a macro so we can have |
46 | * both the type-agnostic benefits of the macros while also being able to |
47 | * enforce that the return value is, in fact, checked. |
48 | */ |
49 | static inline bool __must_check __must_check_overflow(bool overflow) |
50 | { |
51 | return unlikely(overflow); |
52 | } |
53 | |
54 | /** |
55 | * check_add_overflow() - Calculate addition with overflow checking |
56 | * @a: first addend |
57 | * @b: second addend |
58 | * @d: pointer to store sum |
59 | * |
60 | * Returns 0 on success. |
61 | * |
62 | * *@d holds the results of the attempted addition, but is not considered |
63 | * "safe for use" on a non-zero return value, which indicates that the |
64 | * sum has overflowed or been truncated. |
65 | */ |
66 | #define check_add_overflow(a, b, d) \ |
67 | __must_check_overflow(__builtin_add_overflow(a, b, d)) |
68 | |
69 | /** |
70 | * check_sub_overflow() - Calculate subtraction with overflow checking |
71 | * @a: minuend; value to subtract from |
72 | * @b: subtrahend; value to subtract from @a |
73 | * @d: pointer to store difference |
74 | * |
75 | * Returns 0 on success. |
76 | * |
77 | * *@d holds the results of the attempted subtraction, but is not considered |
78 | * "safe for use" on a non-zero return value, which indicates that the |
79 | * difference has underflowed or been truncated. |
80 | */ |
81 | #define check_sub_overflow(a, b, d) \ |
82 | __must_check_overflow(__builtin_sub_overflow(a, b, d)) |
83 | |
84 | /** |
85 | * check_mul_overflow() - Calculate multiplication with overflow checking |
86 | * @a: first factor |
87 | * @b: second factor |
88 | * @d: pointer to store product |
89 | * |
90 | * Returns 0 on success. |
91 | * |
92 | * *@d holds the results of the attempted multiplication, but is not |
93 | * considered "safe for use" on a non-zero return value, which indicates |
94 | * that the product has overflowed or been truncated. |
95 | */ |
96 | #define check_mul_overflow(a, b, d) \ |
97 | __must_check_overflow(__builtin_mul_overflow(a, b, d)) |
98 | |
99 | /** |
100 | * check_shl_overflow() - Calculate a left-shifted value and check overflow |
101 | * @a: Value to be shifted |
102 | * @s: How many bits left to shift |
103 | * @d: Pointer to where to store the result |
104 | * |
105 | * Computes *@d = (@a << @s) |
106 | * |
107 | * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't |
108 | * make sense. Example conditions: |
109 | * |
110 | * - '@a << @s' causes bits to be lost when stored in *@d. |
111 | * - '@s' is garbage (e.g. negative) or so large that the result of |
112 | * '@a << @s' is guaranteed to be 0. |
113 | * - '@a' is negative. |
114 | * - '@a << @s' sets the sign bit, if any, in '*@d'. |
115 | * |
116 | * '*@d' will hold the results of the attempted shift, but is not |
117 | * considered "safe for use" if true is returned. |
118 | */ |
119 | #define check_shl_overflow(a, s, d) __must_check_overflow(({ \ |
120 | typeof(a) _a = a; \ |
121 | typeof(s) _s = s; \ |
122 | typeof(d) _d = d; \ |
123 | u64 _a_full = _a; \ |
124 | unsigned int _to_shift = \ |
125 | is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \ |
126 | *_d = (_a_full << _to_shift); \ |
127 | (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \ |
128 | (*_d >> _to_shift) != _a); \ |
129 | })) |
130 | |
131 | #define __overflows_type_constexpr(x, T) ( \ |
132 | is_unsigned_type(typeof(x)) ? \ |
133 | (x) > type_max(typeof(T)) : \ |
134 | is_unsigned_type(typeof(T)) ? \ |
135 | (x) < 0 || (x) > type_max(typeof(T)) : \ |
136 | (x) < type_min(typeof(T)) || (x) > type_max(typeof(T))) |
137 | |
138 | #define __overflows_type(x, T) ({ \ |
139 | typeof(T) v = 0; \ |
140 | check_add_overflow((x), v, &v); \ |
141 | }) |
142 | |
143 | /** |
144 | * overflows_type - helper for checking the overflows between value, variables, |
145 | * or data type |
146 | * |
147 | * @n: source constant value or variable to be checked |
148 | * @T: destination variable or data type proposed to store @x |
149 | * |
150 | * Compares the @x expression for whether or not it can safely fit in |
151 | * the storage of the type in @T. @x and @T can have different types. |
152 | * If @x is a constant expression, this will also resolve to a constant |
153 | * expression. |
154 | * |
155 | * Returns: true if overflow can occur, false otherwise. |
156 | */ |
157 | #define overflows_type(n, T) \ |
158 | __builtin_choose_expr(__is_constexpr(n), \ |
159 | __overflows_type_constexpr(n, T), \ |
160 | __overflows_type(n, T)) |
161 | |
162 | /** |
163 | * castable_to_type - like __same_type(), but also allows for casted literals |
164 | * |
165 | * @n: variable or constant value |
166 | * @T: variable or data type |
167 | * |
168 | * Unlike the __same_type() macro, this allows a constant value as the |
169 | * first argument. If this value would not overflow into an assignment |
170 | * of the second argument's type, it returns true. Otherwise, this falls |
171 | * back to __same_type(). |
172 | */ |
173 | #define castable_to_type(n, T) \ |
174 | __builtin_choose_expr(__is_constexpr(n), \ |
175 | !__overflows_type_constexpr(n, T), \ |
176 | __same_type(n, T)) |
177 | |
178 | /** |
179 | * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX |
180 | * @factor1: first factor |
181 | * @factor2: second factor |
182 | * |
183 | * Returns: calculate @factor1 * @factor2, both promoted to size_t, |
184 | * with any overflow causing the return value to be SIZE_MAX. The |
185 | * lvalue must be size_t to avoid implicit type conversion. |
186 | */ |
187 | static inline size_t __must_check size_mul(size_t factor1, size_t factor2) |
188 | { |
189 | size_t bytes; |
190 | |
191 | if (check_mul_overflow(factor1, factor2, &bytes)) |
192 | return SIZE_MAX; |
193 | |
194 | return bytes; |
195 | } |
196 | |
197 | /** |
198 | * size_add() - Calculate size_t addition with saturation at SIZE_MAX |
199 | * @addend1: first addend |
200 | * @addend2: second addend |
201 | * |
202 | * Returns: calculate @addend1 + @addend2, both promoted to size_t, |
203 | * with any overflow causing the return value to be SIZE_MAX. The |
204 | * lvalue must be size_t to avoid implicit type conversion. |
205 | */ |
206 | static inline size_t __must_check size_add(size_t addend1, size_t addend2) |
207 | { |
208 | size_t bytes; |
209 | |
210 | if (check_add_overflow(addend1, addend2, &bytes)) |
211 | return SIZE_MAX; |
212 | |
213 | return bytes; |
214 | } |
215 | |
216 | /** |
217 | * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX |
218 | * @minuend: value to subtract from |
219 | * @subtrahend: value to subtract from @minuend |
220 | * |
221 | * Returns: calculate @minuend - @subtrahend, both promoted to size_t, |
222 | * with any overflow causing the return value to be SIZE_MAX. For |
223 | * composition with the size_add() and size_mul() helpers, neither |
224 | * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX). |
225 | * The lvalue must be size_t to avoid implicit type conversion. |
226 | */ |
227 | static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) |
228 | { |
229 | size_t bytes; |
230 | |
231 | if (minuend == SIZE_MAX || subtrahend == SIZE_MAX || |
232 | check_sub_overflow(minuend, subtrahend, &bytes)) |
233 | return SIZE_MAX; |
234 | |
235 | return bytes; |
236 | } |
237 | |
238 | /** |
239 | * array_size() - Calculate size of 2-dimensional array. |
240 | * @a: dimension one |
241 | * @b: dimension two |
242 | * |
243 | * Calculates size of 2-dimensional array: @a * @b. |
244 | * |
245 | * Returns: number of bytes needed to represent the array or SIZE_MAX on |
246 | * overflow. |
247 | */ |
248 | #define array_size(a, b) size_mul(a, b) |
249 | |
250 | /** |
251 | * array3_size() - Calculate size of 3-dimensional array. |
252 | * @a: dimension one |
253 | * @b: dimension two |
254 | * @c: dimension three |
255 | * |
256 | * Calculates size of 3-dimensional array: @a * @b * @c. |
257 | * |
258 | * Returns: number of bytes needed to represent the array or SIZE_MAX on |
259 | * overflow. |
260 | */ |
261 | #define array3_size(a, b, c) size_mul(size_mul(a, b), c) |
262 | |
263 | /** |
264 | * flex_array_size() - Calculate size of a flexible array member |
265 | * within an enclosing structure. |
266 | * @p: Pointer to the structure. |
267 | * @member: Name of the flexible array member. |
268 | * @count: Number of elements in the array. |
269 | * |
270 | * Calculates size of a flexible array of @count number of @member |
271 | * elements, at the end of structure @p. |
272 | * |
273 | * Return: number of bytes needed or SIZE_MAX on overflow. |
274 | */ |
275 | #define flex_array_size(p, member, count) \ |
276 | __builtin_choose_expr(__is_constexpr(count), \ |
277 | (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \ |
278 | size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member))) |
279 | |
280 | /** |
281 | * struct_size() - Calculate size of structure with trailing flexible array. |
282 | * @p: Pointer to the structure. |
283 | * @member: Name of the array member. |
284 | * @count: Number of elements in the array. |
285 | * |
286 | * Calculates size of memory needed for structure of @p followed by an |
287 | * array of @count number of @member elements. |
288 | * |
289 | * Return: number of bytes needed or SIZE_MAX on overflow. |
290 | */ |
291 | #define struct_size(p, member, count) \ |
292 | __builtin_choose_expr(__is_constexpr(count), \ |
293 | sizeof(*(p)) + flex_array_size(p, member, count), \ |
294 | size_add(sizeof(*(p)), flex_array_size(p, member, count))) |
295 | |
296 | /** |
297 | * struct_size_t() - Calculate size of structure with trailing flexible array |
298 | * @type: structure type name. |
299 | * @member: Name of the array member. |
300 | * @count: Number of elements in the array. |
301 | * |
302 | * Calculates size of memory needed for structure @type followed by an |
303 | * array of @count number of @member elements. Prefer using struct_size() |
304 | * when possible instead, to keep calculations associated with a specific |
305 | * instance variable of type @type. |
306 | * |
307 | * Return: number of bytes needed or SIZE_MAX on overflow. |
308 | */ |
309 | #define struct_size_t(type, member, count) \ |
310 | struct_size((type *)NULL, member, count) |
311 | |
312 | /** |
313 | * _DEFINE_FLEX() - helper macro for DEFINE_FLEX() family. |
314 | * Enables caller macro to pass (different) initializer. |
315 | * |
316 | * @type: structure type name, including "struct" keyword. |
317 | * @name: Name for a variable to define. |
318 | * @member: Name of the array member. |
319 | * @count: Number of elements in the array; must be compile-time const. |
320 | * @initializer: initializer expression (could be empty for no init). |
321 | */ |
322 | #define _DEFINE_FLEX(type, name, member, count, initializer) \ |
323 | _Static_assert(__builtin_constant_p(count), \ |
324 | "onstack flex array members require compile-time const count"); \ |
325 | union { \ |
326 | u8 bytes[struct_size_t(type, member, count)]; \ |
327 | type obj; \ |
328 | } name##_u initializer; \ |
329 | type *name = (type *)&name##_u |
330 | |
331 | /** |
332 | * DEFINE_FLEX() - Define an on-stack instance of structure with a trailing |
333 | * flexible array member. |
334 | * |
335 | * @type: structure type name, including "struct" keyword. |
336 | * @name: Name for a variable to define. |
337 | * @member: Name of the array member. |
338 | * @count: Number of elements in the array; must be compile-time const. |
339 | * |
340 | * Define a zeroed, on-stack, instance of @type structure with a trailing |
341 | * flexible array member. |
342 | * Use __struct_size(@name) to get compile-time size of it afterwards. |
343 | */ |
344 | #define DEFINE_FLEX(type, name, member, count) \ |
345 | _DEFINE_FLEX(type, name, member, count, = {}) |
346 | |
347 | #endif /* __LINUX_OVERFLOW_H */ |
348 | |