1 files changed, 205 insertions, 0 deletions
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
new file mode 100644
index 000000000000..c8890ec358a7
--- /dev/null
+++ b/include/linux/overflow.h
@@ -0,0 +1,205 @@
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+#ifndef __LINUX_OVERFLOW_H
+#define __LINUX_OVERFLOW_H
+#include <linux/compiler.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))
+#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) ({          \
+        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) ({          \
+        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) ({          \
+        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)                                     \
+        __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)                                     \
+        __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)                                     \
+        __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 */
+#endif /* __LINUX_OVERFLOW_H */

diff --git a/include/linux/overflow.h b/include/linux/overflow.h new file mode 100644 index 000000000000..c8890ec358a7 --- /dev/null +++ b/include/linux/overflow.h
@@ -0,0 +1,205 @@
	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
	7	/*
	8	* In the fallback code below, we need to compute the minimum and
	9	* maximum values representable in a given type. These macros may also
	10	* be useful elsewhere, so we provide them outside the
	11	* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW block.
	12	*
	13	* It would seem more obvious to do something like
	14	*
	15	* #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
	16	* #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
	17	*
	18	* Unfortunately, the middle expressions, strictly speaking, have
	19	* undefined behaviour, and at least some versions of gcc warn about
	20	* the type_max expression (but not if -fsanitize=undefined is in
	21	* effect; in that case, the warning is deferred to runtime...).
	22	*
	23	* The slightly excessive casting in type_min is to make sure the
	24	* macros also produce sensible values for the exotic type _Bool. [The
	25	* overflow checkers only almost work for _Bool, but that's
	26	* a-feature-not-a-bug, since people shouldn't be doing arithmetic on
	27	* _Bools. Besides, the gcc builtins don't allow _Bool* as third
	28	* argument.]
	29	*
	30	* Idea stolen from
	31	* https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
	32	* credit to Christian Biere.
	33	*/
	34	#define is_signed_type(type) (((type)(-1)) < (type)1)
	35	#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
	36	#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
	37	#define type_min(T) ((T)((T)-type_max(T)-(T)1))
	38
	39
	40	#ifdef COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW
	41	/*
	42	* For simplicity and code hygiene, the fallback code below insists on
	43	* a, b and *d having the same type (similar to the min() and max()
	44	* macros), whereas gcc's type-generic overflow checkers accept
	45	* different types. Hence we don't just make check_add_overflow an
	46	* alias for __builtin_add_overflow, but add type checks similar to
	47	* below.
	48	*/
	49	#define check_add_overflow(a, b, d) ({ \
	50	typeof(a) __a = (a); \
	51	typeof(b) __b = (b); \
	52	typeof(d) __d = (d); \
	53	(void) (&__a == &__b); \
	54	(void) (&__a == __d); \
	55	__builtin_add_overflow(__a, __b, __d); \
	56	})
	57
	58	#define check_sub_overflow(a, b, d) ({ \
	59	typeof(a) __a = (a); \
	60	typeof(b) __b = (b); \
	61	typeof(d) __d = (d); \
	62	(void) (&__a == &__b); \
	63	(void) (&__a == __d); \
	64	__builtin_sub_overflow(__a, __b, __d); \
	65	})
	66
	67	#define check_mul_overflow(a, b, d) ({ \
	68	typeof(a) __a = (a); \
	69	typeof(b) __b = (b); \
	70	typeof(d) __d = (d); \
	71	(void) (&__a == &__b); \
	72	(void) (&__a == __d); \
	73	__builtin_mul_overflow(__a, __b, __d); \
	74	})
	75
	76	#else
	77
	78
	79	/* Checking for unsigned overflow is relatively easy without causing UB. */
	80	#define __unsigned_add_overflow(a, b, d) ({ \
	81	typeof(a) __a = (a); \
	82	typeof(b) __b = (b); \
	83	typeof(d) __d = (d); \
	84	(void) (&__a == &__b); \
	85	(void) (&__a == __d); \
	86	*__d = __a + __b; \
	87	*__d < __a; \
	88	})
	89	#define __unsigned_sub_overflow(a, b, d) ({ \
	90	typeof(a) __a = (a); \
	91	typeof(b) __b = (b); \
	92	typeof(d) __d = (d); \
	93	(void) (&__a == &__b); \
	94	(void) (&__a == __d); \
	95	*__d = __a - __b; \
	96	__a < __b; \
	97	})
	98	/*
	99	* If one of a or b is a compile-time constant, this avoids a division.
	100	*/
	101	#define __unsigned_mul_overflow(a, b, d) ({ \
	102	typeof(a) __a = (a); \
	103	typeof(b) __b = (b); \
	104	typeof(d) __d = (d); \
	105	(void) (&__a == &__b); \
	106	(void) (&__a == __d); \
	107	__d = __a __b; \
	108	__builtin_constant_p(__b) ? \
	109	__b > 0 && __a > type_max(typeof(__a)) / __b : \
	110	__a > 0 && __b > type_max(typeof(__b)) / __a; \
	111	})
	112
	113	/*
	114	* For signed types, detecting overflow is much harder, especially if
	115	* we want to avoid UB. But the interface of these macros is such that
	116	* we must provide a result in *d, and in fact we must produce the
	117	* result promised by gcc's builtins, which is simply the possibly
	118	* wrapped-around value. Fortunately, we can just formally do the
	119	* operations in the widest relevant unsigned type (u64) and then
	120	* truncate the result - gcc is smart enough to generate the same code
	121	* with and without the (u64) casts.
	122	*/
	123
	124	/*
	125	* Adding two signed integers can overflow only if they have the same
	126	* sign, and overflow has happened iff the result has the opposite
	127	* sign.
	128	*/
	129	#define __signed_add_overflow(a, b, d) ({ \
	130	typeof(a) __a = (a); \
	131	typeof(b) __b = (b); \
	132	typeof(d) __d = (d); \
	133	(void) (&__a == &__b); \
	134	(void) (&__a == __d); \
	135	*__d = (u64)__a + (u64)__b; \
	136	(((~(__a ^ __b)) & (*__d ^ __a)) \
	137	& type_min(typeof(__a))) != 0; \
	138	})
	139
	140	/*
	141	* Subtraction is similar, except that overflow can now happen only
	142	* when the signs are opposite. In this case, overflow has happened if
	143	* the result has the opposite sign of a.
	144	*/
	145	#define __signed_sub_overflow(a, b, d) ({ \
	146	typeof(a) __a = (a); \
	147	typeof(b) __b = (b); \
	148	typeof(d) __d = (d); \
	149	(void) (&__a == &__b); \
	150	(void) (&__a == __d); \
	151	*__d = (u64)__a - (u64)__b; \
	152	((((__a ^ __b)) & (*__d ^ __a)) \
	153	& type_min(typeof(__a))) != 0; \
	154	})
	155
	156	/*
	157	* Signed multiplication is rather hard. gcc always follows C99, so
	158	* division is truncated towards 0. This means that we can write the
	159	* overflow check like this:
	160	*
	161	* (a > 0 && (b > MAX/a \|\| b < MIN/a)) \|\|
	162	* (a < -1 && (b > MIN/a \|\| b < MAX/a) \|\|
	163	* (a == -1 && b == MIN)
	164	*
	165	* The redundant casts of -1 are to silence an annoying -Wtype-limits
	166	* (included in -Wextra) warning: When the type is u8 or u16, the
	167	* __b_c_e in check_mul_overflow obviously selects
	168	* __unsigned_mul_overflow, but unfortunately gcc still parses this
	169	* code and warns about the limited range of __b.
	170	*/
	171
	172	#define __signed_mul_overflow(a, b, d) ({ \
	173	typeof(a) __a = (a); \
	174	typeof(b) __b = (b); \
	175	typeof(d) __d = (d); \
	176	typeof(a) __tmax = type_max(typeof(a)); \
	177	typeof(a) __tmin = type_min(typeof(a)); \
	178	(void) (&__a == &__b); \
	179	(void) (&__a == __d); \
	180	__d = (u64)__a (u64)__b; \
	181	(__b > 0 && (__a > __tmax/__b \|\| __a < __tmin/__b)) \|\| \
	182	(__b < (typeof(__b))-1 && (__a > __tmin/__b \|\| __a < __tmax/__b)) \|\| \
	183	(__b == (typeof(__b))-1 && __a == __tmin); \
	184	})
	185
	186
	187	#define check_add_overflow(a, b, d) \
	188	__builtin_choose_expr(is_signed_type(typeof(a)), \
	189	__signed_add_overflow(a, b, d), \
	190	__unsigned_add_overflow(a, b, d))
	191
	192	#define check_sub_overflow(a, b, d) \
	193	__builtin_choose_expr(is_signed_type(typeof(a)), \
	194	__signed_sub_overflow(a, b, d), \
	195	__unsigned_sub_overflow(a, b, d))
	196
	197	#define check_mul_overflow(a, b, d) \
	198	__builtin_choose_expr(is_signed_type(typeof(a)), \
	199	__signed_mul_overflow(a, b, d), \
	200	__unsigned_mul_overflow(a, b, d))
	201
	202
	203	#endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
	204
	205	#endif /* __LINUX_OVERFLOW_H */