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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_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 : {
51 491 : 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 6 : 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 6 : 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 : #endif /* __LINUX_OVERFLOW_H */
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