summaryrefslogtreecommitdiffstats
path: root/include/linux/overflow.h
blob: 0f12345c21fb58430ea93884c4ad5f4688ad7d18 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
/* SPDX-License-Identifier: GPL-2.0 OR MIT */
#ifndef __LINUX_OVERFLOW_H
#define __LINUX_OVERFLOW_H

#include <linux/compiler.h>
#include <linux/limits.h>

/*
 * In the fallback code below, we need to compute the minimum and
 * maximum values representable in a given type. These macros may also
 * be useful elsewhere, so we provide them outside the
 * COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
 *
 * It would seem more obvious to do something like
 *
 * #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
 * #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
 *
 * Unfortunately, the middle expressions, strictly speaking, have
 * undefined behaviour, and at least some versions of gcc warn about
 * the type_max expression (but not if -fsanitize=undefined is in
 * effect; in that case, the warning is deferred to runtime...).
 *
 * The slightly excessive casting in type_min is to make sure the
 * macros also produce sensible values for the exotic type _Bool. [The
 * overflow checkers only almost work for _Bool, but that's
 * a-feature-not-a-bug, since people shouldn't be doing arithmetic on
 * _Bools. Besides, the gcc builtins don't allow _Bool* as third
 * argument.]
 *
 * Idea stolen from
 * https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
 * credit to Christian Biere.
 */
#define is_signed_type(type)       (((type)(-1)) < (type)1)
#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
#define type_min(T) ((T)((T)-type_max(T)-(T)1))

/*
 * Avoids triggering -Wtype-limits compilation warning,
 * while using unsigned data types to check a < 0.
 */
#define is_non_negative(a) ((a) > 0 || (a) == 0)
#define is_negative(a) (!(is_non_negative(a)))

/*
 * Allows for effectively applying __must_check to a macro so we can have
 * both the type-agnostic benefits of the macros while also being able to
 * enforce that the return value is, in fact, checked.
 */
static inline bool __must_check __must_check_overflow(bool overflow)
{
	return unlikely(overflow);
}

#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
/*
 * For simplicity and code hygiene, the fallback code below insists on
 * a, b and *d having the same type (similar to the min() and max()
 * macros), whereas gcc's type-generic overflow checkers accept
 * different types. Hence we don't just make check_add_overflow an
 * alias for __builtin_add_overflow, but add type checks similar to
 * below.
 */
#define check_add_overflow(a, b, d) __must_check_overflow(({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	__builtin_add_overflow(__a, __b, __d);	\
}))

#define check_sub_overflow(a, b, d) __must_check_overflow(({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	__builtin_sub_overflow(__a, __b, __d);	\
}))

#define check_mul_overflow(a, b, d) __must_check_overflow(({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	__builtin_mul_overflow(__a, __b, __d);	\
}))

#else


/* Checking for unsigned overflow is relatively easy without causing UB. */
#define __unsigned_add_overflow(a, b, d) ({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	*__d = __a + __b;			\
	*__d < __a;				\
})
#define __unsigned_sub_overflow(a, b, d) ({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	*__d = __a - __b;			\
	__a < __b;				\
})
/*
 * If one of a or b is a compile-time constant, this avoids a division.
 */
#define __unsigned_mul_overflow(a, b, d) ({		\
	typeof(a) __a = (a);				\
	typeof(b) __b = (b);				\
	typeof(d) __d = (d);				\
	(void) (&__a == &__b);				\
	(void) (&__a == __d);				\
	*__d = __a * __b;				\
	__builtin_constant_p(__b) ?			\
	  __b > 0 && __a > type_max(typeof(__a)) / __b : \
	  __a > 0 && __b > type_max(typeof(__b)) / __a;	 \
})

/*
 * For signed types, detecting overflow is much harder, especially if
 * we want to avoid UB. But the interface of these macros is such that
 * we must provide a result in *d, and in fact we must produce the
 * result promised by gcc's builtins, which is simply the possibly
 * wrapped-around value. Fortunately, we can just formally do the
 * operations in the widest relevant unsigned type (u64) and then
 * truncate the result - gcc is smart enough to generate the same code
 * with and without the (u64) casts.
 */

/*
 * Adding two signed integers can overflow only if they have the same
 * sign, and overflow has happened iff the result has the opposite
 * sign.
 */
#define __signed_add_overflow(a, b, d) ({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	*__d = (u64)__a + (u64)__b;		\
	(((~(__a ^ __b)) & (*__d ^ __a))	\
		& type_min(typeof(__a))) != 0;	\
})

/*
 * Subtraction is similar, except that overflow can now happen only
 * when the signs are opposite. In this case, overflow has happened if
 * the result has the opposite sign of a.
 */
#define __signed_sub_overflow(a, b, d) ({	\
	typeof(a) __a = (a);			\
	typeof(b) __b = (b);			\
	typeof(d) __d = (d);			\
	(void) (&__a == &__b);			\
	(void) (&__a == __d);			\
	*__d = (u64)__a - (u64)__b;		\
	((((__a ^ __b)) & (*__d ^ __a))		\
		& type_min(typeof(__a))) != 0;	\
})

/*
 * Signed multiplication is rather hard. gcc always follows C99, so
 * division is truncated towards 0. This means that we can write the
 * overflow check like this:
 *
 * (a > 0 && (b > MAX/a || b < MIN/a)) ||
 * (a < -1 && (b > MIN/a || b < MAX/a) ||
 * (a == -1 && b == MIN)
 *
 * The redundant casts of -1 are to silence an annoying -Wtype-limits
 * (included in -Wextra) warning: When the type is u8 or u16, the
 * __b_c_e in check_mul_overflow obviously selects
 * __unsigned_mul_overflow, but unfortunately gcc still parses this
 * code and warns about the limited range of __b.
 */

#define __signed_mul_overflow(a, b, d) ({				\
	typeof(a) __a = (a);						\
	typeof(b) __b = (b);						\
	typeof(d) __d = (d);						\
	typeof(a) __tmax = type_max(typeof(a));				\
	typeof(a) __tmin = type_min(typeof(a));				\
	(void) (&__a == &__b);						\
	(void) (&__a == __d);						\
	*__d = (u64)__a * (u64)__b;					\
	(__b > 0   && (__a > __tmax/__b || __a < __tmin/__b)) ||	\
	(__b < (typeof(__b))-1  && (__a > __tmin/__b || __a < __tmax/__b)) || \
	(__b == (typeof(__b))-1 && __a == __tmin);			\
})


#define check_add_overflow(a, b, d)	__must_check_overflow(		\
	__builtin_choose_expr(is_signed_type(typeof(a)),		\
			__signed_add_overflow(a, b, d),			\
			__unsigned_add_overflow(a, b, d)))

#define check_sub_overflow(a, b, d)	__must_check_overflow(		\
	__builtin_choose_expr(is_signed_type(typeof(a)),		\
			__signed_sub_overflow(a, b, d),			\
			__unsigned_sub_overflow(a, b, d)))

#define check_mul_overflow(a, b, d)	__must_check_overflow(		\
	__builtin_choose_expr(is_signed_type(typeof(a)),		\
			__signed_mul_overflow(a, b, d),			\
			__unsigned_mul_overflow(a, b, d)))

#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */

/** check_shl_overflow() - Calculate a left-shifted value and check overflow
 *
 * @a: Value to be shifted
 * @s: How many bits left to shift
 * @d: Pointer to where to store the result
 *
 * Computes *@d = (@a << @s)
 *
 * Returns true if '*d' cannot hold the result or when 'a << s' doesn't
 * make sense. Example conditions:
 * - 'a << s' causes bits to be lost when stored in *d.
 * - 's' is garbage (e.g. negative) or so large that the result of
 *   'a << s' is guaranteed to be 0.
 * - 'a' is negative.
 * - 'a << s' sets the sign bit, if any, in '*d'.
 *
 * '*d' will hold the results of the attempted shift, but is not
 * considered "safe for use" if true is returned.
 */
#define check_shl_overflow(a, s, d) __must_check_overflow(({		\
	typeof(a) _a = a;						\
	typeof(s) _s = s;						\
	typeof(d) _d = d;						\
	u64 _a_full = _a;						\
	unsigned int _to_shift =					\
		is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0;	\
	*_d = (_a_full << _to_shift);					\
	(_to_shift != _s || is_negative(*_d) || is_negative(_a) ||	\
	(*_d >> _to_shift) != _a);					\
}))

/**
 * array_size() - Calculate size of 2-dimensional array.
 *
 * @a: dimension one
 * @b: dimension two
 *
 * Calculates size of 2-dimensional array: @a * @b.
 *
 * Returns: number of bytes needed to represent the array or SIZE_MAX on
 * overflow.
 */
static inline __must_check size_t array_size(size_t a, size_t b)
{
	size_t bytes;

	if (check_mul_overflow(a, b, &bytes))
		return SIZE_MAX;

	return bytes;
}

/**
 * array3_size() - Calculate size of 3-dimensional array.
 *
 * @a: dimension one
 * @b: dimension two
 * @c: dimension three
 *
 * Calculates size of 3-dimensional array: @a * @b * @c.
 *
 * Returns: number of bytes needed to represent the array or SIZE_MAX on
 * overflow.
 */
static inline __must_check size_t array3_size(size_t a, size_t b, size_t c)
{
	size_t bytes;

	if (check_mul_overflow(a, b, &bytes))
		return SIZE_MAX;
	if (check_mul_overflow(bytes, c, &bytes))
		return SIZE_MAX;

	return bytes;
}

/*
 * Compute a*b+c, returning SIZE_MAX on overflow. Internal helper for
 * struct_size() below.
 */
static inline __must_check size_t __ab_c_size(size_t a, size_t b, size_t c)
{
	size_t bytes;

	if (check_mul_overflow(a, b, &bytes))
		return SIZE_MAX;
	if (check_add_overflow(bytes, c, &bytes))
		return SIZE_MAX;

	return bytes;
}

/**
 * struct_size() - Calculate size of structure with trailing array.
 * @p: Pointer to the structure.
 * @member: Name of the array member.
 * @count: Number of elements in the array.
 *
 * Calculates size of memory needed for structure @p followed by an
 * array of @count number of @member elements.
 *
 * Return: number of bytes needed or SIZE_MAX on overflow.
 */
#define struct_size(p, member, count)					\
	__ab_c_size(count,						\
		    sizeof(*(p)->member) + __must_be_array((p)->member),\
		    sizeof(*(p)))

/**
 * flex_array_size() - Calculate size of a flexible array member
 *                     within an enclosing structure.
 *
 * @p: Pointer to the structure.
 * @member: Name of the flexible array member.
 * @count: Number of elements in the array.
 *
 * Calculates size of a flexible array of @count number of @member
 * elements, at the end of structure @p.
 *
 * Return: number of bytes needed or SIZE_MAX on overflow.
 */
#define flex_array_size(p, member, count)				\
	array_size(count,						\
		    sizeof(*(p)->member) + __must_be_array((p)->member))

#endif /* __LINUX_OVERFLOW_H */