313 lines
10 KiB
C
313 lines
10 KiB
C
/* SPDX-License-Identifier: GPL-2.0 OR MIT */
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#ifndef __LINUX_OVERFLOW_H
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#define __LINUX_OVERFLOW_H
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#include <linux/compiler.h>
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#include <linux/limits.h>
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#include <linux/const.h>
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/*
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* We need to compute the minimum and maximum values representable in a given
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* type. These macros may also be useful elsewhere. It would seem more obvious
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* to do something like:
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*
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* #define type_min(T) (T)(is_signed_type(T) ? (T)1 << (8*sizeof(T)-1) : 0)
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* #define type_max(T) (T)(is_signed_type(T) ? ((T)1 << (8*sizeof(T)-1)) - 1 : ~(T)0)
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*
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* Unfortunately, the middle expressions, strictly speaking, have
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* undefined behaviour, and at least some versions of gcc warn about
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* the type_max expression (but not if -fsanitize=undefined is in
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* effect; in that case, the warning is deferred to runtime...).
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*
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* The slightly excessive casting in type_min is to make sure the
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* macros also produce sensible values for the exotic type _Bool. [The
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* overflow checkers only almost work for _Bool, but that's
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* a-feature-not-a-bug, since people shouldn't be doing arithmetic on
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* _Bools. Besides, the gcc builtins don't allow _Bool* as third
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* argument.]
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*
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* Idea stolen from
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* https://mail-index.netbsd.org/tech-misc/2007/02/05/0000.html -
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* credit to Christian Biere.
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*/
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#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 - is_signed_type(type)))
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#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
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#define type_min(T) ((T)((T)-type_max(T)-(T)1))
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/*
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* Avoids triggering -Wtype-limits compilation warning,
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* while using unsigned data types to check a < 0.
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*/
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#define is_non_negative(a) ((a) > 0 || (a) == 0)
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#define is_negative(a) (!(is_non_negative(a)))
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/*
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* Allows for effectively applying __must_check to a macro so we can have
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* both the type-agnostic benefits of the macros while also being able to
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* enforce that the return value is, in fact, checked.
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*/
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static inline bool __must_check __must_check_overflow(bool overflow)
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{
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return unlikely(overflow);
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}
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/**
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* check_add_overflow() - Calculate addition with overflow checking
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* @a: first addend
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* @b: second addend
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* @d: pointer to store sum
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*
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* Returns 0 on success.
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*
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* *@d holds the results of the attempted addition, but is not considered
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* "safe for use" on a non-zero return value, which indicates that the
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* sum has overflowed or been truncated.
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*/
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#define check_add_overflow(a, b, d) \
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__must_check_overflow(__builtin_add_overflow(a, b, d))
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/**
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* check_sub_overflow() - Calculate subtraction with overflow checking
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* @a: minuend; value to subtract from
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* @b: subtrahend; value to subtract from @a
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* @d: pointer to store difference
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*
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* Returns 0 on success.
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*
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* *@d holds the results of the attempted subtraction, but is not considered
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* "safe for use" on a non-zero return value, which indicates that the
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* difference has underflowed or been truncated.
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*/
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#define check_sub_overflow(a, b, d) \
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__must_check_overflow(__builtin_sub_overflow(a, b, d))
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/**
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* check_mul_overflow() - Calculate multiplication with overflow checking
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* @a: first factor
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* @b: second factor
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* @d: pointer to store product
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*
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* Returns 0 on success.
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*
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* *@d holds the results of the attempted multiplication, but is not
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* considered "safe for use" on a non-zero return value, which indicates
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* that the product has overflowed or been truncated.
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*/
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#define check_mul_overflow(a, b, d) \
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__must_check_overflow(__builtin_mul_overflow(a, b, d))
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/**
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* check_shl_overflow() - Calculate a left-shifted value and check overflow
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* @a: Value to be shifted
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* @s: How many bits left to shift
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* @d: Pointer to where to store the result
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*
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* Computes *@d = (@a << @s)
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*
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* Returns true if '*@d' cannot hold the result or when '@a << @s' doesn't
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* make sense. Example conditions:
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*
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* - '@a << @s' causes bits to be lost when stored in *@d.
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* - '@s' is garbage (e.g. negative) or so large that the result of
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* '@a << @s' is guaranteed to be 0.
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* - '@a' is negative.
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* - '@a << @s' sets the sign bit, if any, in '*@d'.
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*
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* '*@d' will hold the results of the attempted shift, but is not
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* considered "safe for use" if true is returned.
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*/
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#define check_shl_overflow(a, s, d) __must_check_overflow(({ \
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typeof(a) _a = a; \
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typeof(s) _s = s; \
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typeof(d) _d = d; \
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u64 _a_full = _a; \
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unsigned int _to_shift = \
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is_non_negative(_s) && _s < 8 * sizeof(*d) ? _s : 0; \
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*_d = (_a_full << _to_shift); \
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(_to_shift != _s || is_negative(*_d) || is_negative(_a) || \
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(*_d >> _to_shift) != _a); \
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}))
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#define __overflows_type_constexpr(x, T) ( \
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is_unsigned_type(typeof(x)) ? \
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(x) > type_max(typeof(T)) : \
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is_unsigned_type(typeof(T)) ? \
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(x) < 0 || (x) > type_max(typeof(T)) : \
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(x) < type_min(typeof(T)) || (x) > type_max(typeof(T)))
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#define __overflows_type(x, T) ({ \
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typeof(T) v = 0; \
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check_add_overflow((x), v, &v); \
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})
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/**
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* overflows_type - helper for checking the overflows between value, variables,
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* or data type
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*
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* @n: source constant value or variable to be checked
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* @T: destination variable or data type proposed to store @x
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*
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* Compares the @x expression for whether or not it can safely fit in
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* the storage of the type in @T. @x and @T can have different types.
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* If @x is a constant expression, this will also resolve to a constant
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* expression.
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*
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* Returns: true if overflow can occur, false otherwise.
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*/
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#define overflows_type(n, T) \
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__builtin_choose_expr(__is_constexpr(n), \
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__overflows_type_constexpr(n, T), \
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__overflows_type(n, T))
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/**
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* castable_to_type - like __same_type(), but also allows for casted literals
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*
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* @n: variable or constant value
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* @T: variable or data type
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*
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* Unlike the __same_type() macro, this allows a constant value as the
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* first argument. If this value would not overflow into an assignment
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* of the second argument's type, it returns true. Otherwise, this falls
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* back to __same_type().
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*/
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#define castable_to_type(n, T) \
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__builtin_choose_expr(__is_constexpr(n), \
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!__overflows_type_constexpr(n, T), \
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__same_type(n, T))
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/**
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* size_mul() - Calculate size_t multiplication with saturation at SIZE_MAX
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* @factor1: first factor
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* @factor2: second factor
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*
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* Returns: calculate @factor1 * @factor2, both promoted to size_t,
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* with any overflow causing the return value to be SIZE_MAX. The
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* lvalue must be size_t to avoid implicit type conversion.
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*/
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static inline size_t __must_check size_mul(size_t factor1, size_t factor2)
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{
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size_t bytes;
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if (check_mul_overflow(factor1, factor2, &bytes))
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return SIZE_MAX;
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return bytes;
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}
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/**
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* size_add() - Calculate size_t addition with saturation at SIZE_MAX
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* @addend1: first addend
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* @addend2: second addend
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*
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* Returns: calculate @addend1 + @addend2, both promoted to size_t,
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* with any overflow causing the return value to be SIZE_MAX. The
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* lvalue must be size_t to avoid implicit type conversion.
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*/
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static inline size_t __must_check size_add(size_t addend1, size_t addend2)
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{
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size_t bytes;
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if (check_add_overflow(addend1, addend2, &bytes))
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return SIZE_MAX;
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return bytes;
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}
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/**
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* size_sub() - Calculate size_t subtraction with saturation at SIZE_MAX
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* @minuend: value to subtract from
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* @subtrahend: value to subtract from @minuend
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*
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* Returns: calculate @minuend - @subtrahend, both promoted to size_t,
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* with any overflow causing the return value to be SIZE_MAX. For
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* composition with the size_add() and size_mul() helpers, neither
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* argument may be SIZE_MAX (or the result with be forced to SIZE_MAX).
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* The lvalue must be size_t to avoid implicit type conversion.
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*/
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static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend)
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{
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size_t bytes;
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if (minuend == SIZE_MAX || subtrahend == SIZE_MAX ||
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check_sub_overflow(minuend, subtrahend, &bytes))
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return SIZE_MAX;
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return bytes;
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}
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/**
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* array_size() - Calculate size of 2-dimensional array.
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* @a: dimension one
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* @b: dimension two
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*
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* Calculates size of 2-dimensional array: @a * @b.
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*
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* Returns: number of bytes needed to represent the array or SIZE_MAX on
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* overflow.
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*/
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#define array_size(a, b) size_mul(a, b)
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/**
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* array3_size() - Calculate size of 3-dimensional array.
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* @a: dimension one
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* @b: dimension two
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* @c: dimension three
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*
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* Calculates size of 3-dimensional array: @a * @b * @c.
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*
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* Returns: number of bytes needed to represent the array or SIZE_MAX on
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* overflow.
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*/
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#define array3_size(a, b, c) size_mul(size_mul(a, b), c)
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/**
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* flex_array_size() - Calculate size of a flexible array member
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* within an enclosing structure.
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* @p: Pointer to the structure.
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* @member: Name of the flexible array member.
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* @count: Number of elements in the array.
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*
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* Calculates size of a flexible array of @count number of @member
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* elements, at the end of structure @p.
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*
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* Return: number of bytes needed or SIZE_MAX on overflow.
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*/
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#define flex_array_size(p, member, count) \
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__builtin_choose_expr(__is_constexpr(count), \
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(count) * sizeof(*(p)->member) + __must_be_array((p)->member), \
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size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member)))
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/**
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* struct_size() - Calculate size of structure with trailing flexible array.
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* @p: Pointer to the structure.
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* @member: Name of the array member.
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* @count: Number of elements in the array.
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*
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* Calculates size of memory needed for structure of @p followed by an
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* array of @count number of @member elements.
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*
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* Return: number of bytes needed or SIZE_MAX on overflow.
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*/
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#define struct_size(p, member, count) \
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__builtin_choose_expr(__is_constexpr(count), \
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sizeof(*(p)) + flex_array_size(p, member, count), \
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size_add(sizeof(*(p)), flex_array_size(p, member, count)))
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/**
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* struct_size_t() - Calculate size of structure with trailing flexible array
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* @type: structure type name.
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* @member: Name of the array member.
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* @count: Number of elements in the array.
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*
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* Calculates size of memory needed for structure @type followed by an
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* array of @count number of @member elements. Prefer using struct_size()
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* when possible instead, to keep calculations associated with a specific
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* instance variable of type @type.
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*
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* Return: number of bytes needed or SIZE_MAX on overflow.
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*/
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#define struct_size_t(type, member, count) \
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struct_size((type *)NULL, member, count)
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#endif /* __LINUX_OVERFLOW_H */
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