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 */
__must_check_overflow(bool overflow)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 0 on success.
63 *
64 * *@d holds the results of the attempted addition, but is not considered
65 * "safe for use" on a non-zero return value, which indicates that the
66 * sum has overflowed or been truncated.
67 */
68 #define check_add_overflow(a, b, d) \
69 __must_check_overflow(__builtin_add_overflow(a, b, d))
70
71 /**
72 * check_sub_overflow() - Calculate subtraction with overflow checking
73 * @a: minuend; value to subtract from
74 * @b: subtrahend; value to subtract from @a
75 * @d: pointer to store difference
76 *
77 * Returns 0 on success.
78 *
79 * *@d holds the results of the attempted subtraction, but is not considered
80 * "safe for use" on a non-zero return value, which indicates that the
81 * difference has underflowed or been truncated.
82 */
83 #define check_sub_overflow(a, b, d) \
84 __must_check_overflow(__builtin_sub_overflow(a, b, d))
85
86 /**
87 * check_mul_overflow() - Calculate multiplication with overflow checking
88 * @a: first factor
89 * @b: second factor
90 * @d: pointer to store product
91 *
92 * Returns 0 on success.
93 *
94 * *@d holds the results of the attempted multiplication, but is not
95 * considered "safe for use" on a non-zero return value, which indicates
96 * that the product has overflowed or been truncated.
97 */
98 #define check_mul_overflow(a, b, d) \
99 __must_check_overflow(__builtin_mul_overflow(a, b, d))
100
101 /**
102 * check_shl_overflow() - Calculate a left-shifted value and check overflow
103 * @a: Value to be shifted
104 * @s: How many bits left to shift
105 * @d: Pointer to where to store the result
106 *
107 * Computes *@d = (@a << @s)
108 *
109 * Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't
110 * make sense. Example conditions:
111 *
112 * - '@a << @s' causes bits to be lost when stored in *@d.
113 * - '@s' is garbage (e.g. negative) or so large that the result of
114 * '@a << @s' is guaranteed to be 0.
115 * - '@a' is negative.
116 * - '@a << @s' sets the sign bit, if any, in '*@d'.
117 *
118 * '*@d' will hold the results of the attempted shift, but is not
119 * considered "safe for use" if true is returned.
120 */
121 #define check_shl_overflow(a, s, d) __must_check_overflow(({ \
122 typeof(a) _a = a; \
123 typeof(s) _s = s; \
124 typeof(d) _d = d; \
125 u64 _a_full = _a; \
126 unsigned int _to_shift = \
127 is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
128 *_d = (_a_full << _to_shift); \
129 (_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
130 (*_d >> _to_shift) != _a); \
131 }))
132
133 #define __overflows_type_constexpr(x, T) ( \
134 is_unsigned_type(typeof(x)) ? \
135 (x) > type_max(T) : \
136 is_unsigned_type(typeof(T)) ? \
137 (x) < 0 || (x) > type_max(T) : \
138 (x) < type_min(T) || (x) > type_max(T))
139
140 #define __overflows_type(x, T) ({ \
141 typeof(T) v = 0; \
142 check_add_overflow((x), v, &v); \
143 })
144
145 /**
146 * overflows_type - helper for checking the overflows between value, variables,
147 * or data type
148 *
149 * @n: source constant value or variable to be checked
150 * @T: destination variable or data type proposed to store @x
151 *
152 * Compares the @x expression for whether or not it can safely fit in
153 * the storage of the type in @T. @x and @T can have different types.
154 * If @x is a constant expression, this will also resolve to a constant
155 * expression.
156 *
157 * Returns: true if overflow can occur, false otherwise.
158 */
159 #define overflows_type(n, T) \
160 __builtin_choose_expr(__is_constexpr(n), \
161 __overflows_type_constexpr(n, T), \
162 __overflows_type(n, T))
163
164 /**
165 * castable_to_type - like __same_type(), but also allows for casted literals
166 *
167 * @n: variable or constant value
168 * @T: variable or data type
169 *
170 * Unlike the __same_type() macro, this allows a constant value as the
171 * first argument. If this value would not overflow into an assignment
172 * of the second argument's type, it returns true. Otherwise, this falls
173 * back to __same_type().
174 */
175 #define castable_to_type(n, T) \
176 __builtin_choose_expr(__is_constexpr(n), \
177 !__overflows_type_constexpr(n, T), \
178 __same_type(n, T))
179
180 /**
181 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
182 * @factor1: first factor
183 * @factor2: second factor
184 *
185 * Returns: calculate @factor1 * @factor2, both promoted to size_t,
186 * with any overflow causing the return value to be SIZE_MAX. The
187 * lvalue must be size_t to avoid implicit type conversion.
188 */
size_mul(size_t factor1,size_t factor2)189 static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
190 {
191 size_t bytes;
192
193 if (check_mul_overflow(factor1, factor2, &bytes))
194 return SIZE_MAX;
195
196 return bytes;
197 }
198
199 /**
200 * size_add() - Calculate size_t addition with saturation at SIZE_MAX
201 * @addend1: first addend
202 * @addend2: second addend
203 *
204 * Returns: calculate @addend1 + @addend2, both promoted to size_t,
205 * with any overflow causing the return value to be SIZE_MAX. The
206 * lvalue must be size_t to avoid implicit type conversion.
207 */
size_add(size_t addend1,size_t addend2)208 static inline size_t __must_check size_add(size_t addend1, size_t addend2)
209 {
210 size_t bytes;
211
212 if (check_add_overflow(addend1, addend2, &bytes))
213 return SIZE_MAX;
214
215 return bytes;
216 }
217
218 /**
219 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
220 * @minuend: value to subtract from
221 * @subtrahend: value to subtract from @minuend
222 *
223 * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
224 * with any overflow causing the return value to be SIZE_MAX. For
225 * composition with the size_add() and size_mul() helpers, neither
226 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
227 * The lvalue must be size_t to avoid implicit type conversion.
228 */
size_sub(size_t minuend,size_t subtrahend)229 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
230 {
231 size_t bytes;
232
233 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
234 check_sub_overflow(minuend, subtrahend, &bytes))
235 return SIZE_MAX;
236
237 return bytes;
238 }
239
240 /**
241 * array_size() - Calculate size of 2-dimensional array.
242 * @a: dimension one
243 * @b: dimension two
244 *
245 * Calculates size of 2-dimensional array: @a * @b.
246 *
247 * Returns: number of bytes needed to represent the array or SIZE_MAX on
248 * overflow.
249 */
250 #define array_size(a, b) size_mul(a, b)
251
252 /**
253 * array3_size() - Calculate size of 3-dimensional array.
254 * @a: dimension one
255 * @b: dimension two
256 * @c: dimension three
257 *
258 * Calculates size of 3-dimensional array: @a * @b * @c.
259 *
260 * Returns: number of bytes needed to represent the array or SIZE_MAX on
261 * overflow.
262 */
263 #define array3_size(a, b, c) size_mul(size_mul(a, b), c)
264
265 /**
266 * flex_array_size() - Calculate size of a flexible array member
267 * within an enclosing structure.
268 * @p: Pointer to the structure.
269 * @member: Name of the flexible array member.
270 * @count: Number of elements in the array.
271 *
272 * Calculates size of a flexible array of @count number of @member
273 * elements, at the end of structure @p.
274 *
275 * Return: number of bytes needed or SIZE_MAX on overflow.
276 */
277 #define flex_array_size(p, member, count) \
278 __builtin_choose_expr(__is_constexpr(count), \
279 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \
280 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
281
282 /**
283 * struct_size() - Calculate size of structure with trailing flexible array.
284 * @p: Pointer to the structure.
285 * @member: Name of the array member.
286 * @count: Number of elements in the array.
287 *
288 * Calculates size of memory needed for structure of @p followed by an
289 * array of @count number of @member elements.
290 *
291 * Return: number of bytes needed or SIZE_MAX on overflow.
292 */
293 #define struct_size(p, member, count) \
294 __builtin_choose_expr(__is_constexpr(count), \
295 sizeof(*(p)) + flex_array_size(p, member, count), \
296 size_add(sizeof(*(p)), flex_array_size(p, member, count)))
297
298 /**
299 * struct_size_t() - Calculate size of structure with trailing flexible array
300 * @type: structure type name.
301 * @member: Name of the array member.
302 * @count: Number of elements in the array.
303 *
304 * Calculates size of memory needed for structure @type followed by an
305 * array of @count number of @member elements. Prefer using struct_size()
306 * when possible instead, to keep calculations associated with a specific
307 * instance variable of type @type.
308 *
309 * Return: number of bytes needed or SIZE_MAX on overflow.
310 */
311 #define struct_size_t(type, member, count) \
312 struct_size((type *)NULL, member, count)
313
314 #endif /* __LINUX_OVERFLOW_H */
315