/* SPDX-License-Identifier: GPL-2.0 OR MIT */ #ifndef __LINUX_OVERFLOW_H #define __LINUX_OVERFLOW_H #include #include #include /* * 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); \ })) /** * 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 */