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- /* SPDX-License-Identifier: GPL-2.0 OR MIT */
- #ifndef __LINUX_OVERFLOW_H
- #define __LINUX_OVERFLOW_H
- #include <linux/compiler.h>
- #include <linux/limits.h>
- #include <linux/const.h>
- /*
- * We need to compute the minimum and maximum values representable in a given
- * type. These macros may also be useful elsewhere. 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);
- }
- /**
- * check_add_overflow() - Calculate addition with overflow checking
- * @a: first addend
- * @b: second addend
- * @d: pointer to store sum
- *
- * Returns 0 on success.
- *
- * *@d holds the results of the attempted addition, but is not considered
- * "safe for use" on a non-zero return value, which indicates that the
- * sum has overflowed or been truncated.
- */
- #define check_add_overflow(a, b, d) \
- __must_check_overflow(__builtin_add_overflow(a, b, d))
- /**
- * check_sub_overflow() - Calculate subtraction with overflow checking
- * @a: minuend; value to subtract from
- * @b: subtrahend; value to subtract from @a
- * @d: pointer to store difference
- *
- * Returns 0 on success.
- *
- * *@d holds the results of the attempted subtraction, but is not considered
- * "safe for use" on a non-zero return value, which indicates that the
- * difference has underflowed or been truncated.
- */
- #define check_sub_overflow(a, b, d) \
- __must_check_overflow(__builtin_sub_overflow(a, b, d))
- /**
- * check_mul_overflow() - Calculate multiplication with overflow checking
- * @a: first factor
- * @b: second factor
- * @d: pointer to store product
- *
- * Returns 0 on success.
- *
- * *@d holds the results of the attempted multiplication, but is not
- * considered "safe for use" on a non-zero return value, which indicates
- * that the product has overflowed or been truncated.
- */
- #define check_mul_overflow(a, b, d) \
- __must_check_overflow(__builtin_mul_overflow(a, b, d))
- /**
- * 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); \
- }))
- #define __overflows_type_constexpr(x, T) ( \
- is_unsigned_type(typeof(x)) ? \
- (x) > type_max(typeof(T)) : \
- is_unsigned_type(typeof(T)) ? \
- (x) < 0 || (x) > type_max(typeof(T)) : \
- (x) < type_min(typeof(T)) || (x) > type_max(typeof(T)))
- #define __overflows_type(x, T) ({ \
- typeof(T) v = 0; \
- check_add_overflow((x), v, &v); \
- })
- /**
- * overflows_type - helper for checking the overflows between value, variables,
- * or data type
- *
- * @n: source constant value or variable to be checked
- * @T: destination variable or data type proposed to store @x
- *
- * Compares the @x expression for whether or not it can safely fit in
- * the storage of the type in @T. @x and @T can have different types.
- * If @x is a constant expression, this will also resolve to a constant
- * expression.
- *
- * Returns: true if overflow can occur, false otherwise.
- */
- #define overflows_type(n, T) \
- __builtin_choose_expr(__is_constexpr(n), \
- __overflows_type_constexpr(n, T), \
- __overflows_type(n, T))
- /**
- * castable_to_type - like __same_type(), but also allows for casted literals
- *
- * @n: variable or constant value
- * @T: variable or data type
- *
- * Unlike the __same_type() macro, this allows a constant value as the
- * first argument. If this value would not overflow into an assignment
- * of the second argument's type, it returns true. Otherwise, this falls
- * back to __same_type().
- */
- #define castable_to_type(n, T) \
- __builtin_choose_expr(__is_constexpr(n), \
- !__overflows_type_constexpr(n, T), \
- __same_type(n, T))
- /**
- * size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
- * @factor1: first factor
- * @factor2: second factor
- *
- * Returns: calculate @factor1 * @factor2, both promoted to size_t,
- * with any overflow causing the return value to be SIZE_MAX. The
- * lvalue must be size_t to avoid implicit type conversion.
- */
- static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
- {
- size_t bytes;
- if (check_mul_overflow(factor1, factor2, &bytes))
- return SIZE_MAX;
- return bytes;
- }
- /**
- * size_add() - Calculate size_t addition with saturation at SIZE_MAX
- * @addend1: first addend
- * @addend2: second addend
- *
- * Returns: calculate @addend1 + @addend2, both promoted to size_t,
- * with any overflow causing the return value to be SIZE_MAX. The
- * lvalue must be size_t to avoid implicit type conversion.
- */
- static inline size_t __must_check size_add(size_t addend1, size_t addend2)
- {
- size_t bytes;
- if (check_add_overflow(addend1, addend2, &bytes))
- return SIZE_MAX;
- return bytes;
- }
- /**
- * size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
- * @minuend: value to subtract from
- * @subtrahend: value to subtract from @minuend
- *
- * Returns: calculate @minuend - @subtrahend, both promoted to size_t,
- * with any overflow causing the return value to be SIZE_MAX. For
- * composition with the size_add() and size_mul() helpers, neither
- * argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
- * The lvalue must be size_t to avoid implicit type conversion.
- */
- static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
- {
- size_t bytes;
- if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
- check_sub_overflow(minuend, subtrahend, &bytes))
- return SIZE_MAX;
- return bytes;
- }
- /**
- * 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.
- */
- #define array_size(a, b) size_mul(a, b)
- /**
- * 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.
- */
- #define array3_size(a, b, c) size_mul(size_mul(a, b), c)
- /**
- * 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) \
- __builtin_choose_expr(__is_constexpr(count), \
- (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \
- size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
- /**
- * struct_size() - Calculate size of structure with trailing flexible 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) \
- __builtin_choose_expr(__is_constexpr(count), \
- sizeof(*(p)) + flex_array_size(p, member, count), \
- size_add(sizeof(*(p)), flex_array_size(p, member, count)))
- #endif /* __LINUX_OVERFLOW_H */
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