/* 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 */
