--- /dev/null
+/* 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 __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 false 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 */
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