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 */
__must_check_overflow(bool overflow)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 /**
132 * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
133 * @factor1: first factor
134 * @factor2: second factor
135 *
136 * Returns: calculate @factor1 * @factor2, both promoted to size_t,
137 * with any overflow causing the return value to be SIZE_MAX. The
138 * lvalue must be size_t to avoid implicit type conversion.
139 */
size_mul(size_t factor1,size_t factor2)140 static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
141 {
142 size_t bytes;
143
144 if (check_mul_overflow(factor1, factor2, &bytes))
145 return SIZE_MAX;
146
147 return bytes;
148 }
149
150 /**
151 * size_add() - Calculate size_t addition with saturation at SIZE_MAX
152 * @addend1: first addend
153 * @addend2: second addend
154 *
155 * Returns: calculate @addend1 + @addend2, both promoted to size_t,
156 * with any overflow causing the return value to be SIZE_MAX. The
157 * lvalue must be size_t to avoid implicit type conversion.
158 */
size_add(size_t addend1,size_t addend2)159 static inline size_t __must_check size_add(size_t addend1, size_t addend2)
160 {
161 size_t bytes;
162
163 if (check_add_overflow(addend1, addend2, &bytes))
164 return SIZE_MAX;
165
166 return bytes;
167 }
168
169 /**
170 * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
171 * @minuend: value to subtract from
172 * @subtrahend: value to subtract from @minuend
173 *
174 * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
175 * with any overflow causing the return value to be SIZE_MAX. For
176 * composition with the size_add() and size_mul() helpers, neither
177 * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
178 * The lvalue must be size_t to avoid implicit type conversion.
179 */
size_sub(size_t minuend,size_t subtrahend)180 static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
181 {
182 size_t bytes;
183
184 if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
185 check_sub_overflow(minuend, subtrahend, &bytes))
186 return SIZE_MAX;
187
188 return bytes;
189 }
190
191 /**
192 * array_size() - Calculate size of 2-dimensional array.
193 * @a: dimension one
194 * @b: dimension two
195 *
196 * Calculates size of 2-dimensional array: @a * @b.
197 *
198 * Returns: number of bytes needed to represent the array or SIZE_MAX on
199 * overflow.
200 */
201 #define array_size(a, b) size_mul(a, b)
202
203 /**
204 * array3_size() - Calculate size of 3-dimensional array.
205 * @a: dimension one
206 * @b: dimension two
207 * @c: dimension three
208 *
209 * Calculates size of 3-dimensional array: @a * @b * @c.
210 *
211 * Returns: number of bytes needed to represent the array or SIZE_MAX on
212 * overflow.
213 */
214 #define array3_size(a, b, c) size_mul(size_mul(a, b), c)
215
216 /**
217 * flex_array_size() - Calculate size of a flexible array member
218 * within an enclosing structure.
219 * @p: Pointer to the structure.
220 * @member: Name of the flexible array member.
221 * @count: Number of elements in the array.
222 *
223 * Calculates size of a flexible array of @count number of @member
224 * elements, at the end of structure @p.
225 *
226 * Return: number of bytes needed or SIZE_MAX on overflow.
227 */
228 #define flex_array_size(p, member, count) \
229 __builtin_choose_expr(__is_constexpr(count), \
230 (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \
231 size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
232
233 /**
234 * struct_size() - Calculate size of structure with trailing flexible array.
235 * @p: Pointer to the structure.
236 * @member: Name of the array member.
237 * @count: Number of elements in the array.
238 *
239 * Calculates size of memory needed for structure @p followed by an
240 * array of @count number of @member elements.
241 *
242 * Return: number of bytes needed or SIZE_MAX on overflow.
243 */
244 #define struct_size(p, member, count) \
245 __builtin_choose_expr(__is_constexpr(count), \
246 sizeof(*(p)) + flex_array_size(p, member, count), \
247 size_add(sizeof(*(p)), flex_array_size(p, member, count)))
248
249 #endif /* __LINUX_OVERFLOW_H */
250