include/linux/overflow.h

Source file repositories/reference/linux-study-clean/include/linux/overflow.h

File Facts

System
Linux kernel
Corpus path
include/linux/overflow.h
Extension
.h
Size
20051 bytes
Lines
592
Domain
Core OS
Bucket
Core Kernel Interface
Inferred role
Core OS: implementation source
Status
source implementation candidate

Why This File Exists

Core operating-system implementation surface: boot, tasks, memory, VFS, syscall-facing interfaces, synchronization, credentials, and isolation.

Dependency Surface

Detected Declarations

Annotated Snippet

#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_max(t)	__type_max(typeof(t))
#define __type_min(T) ((T)((T)-type_max(T)-(T)1))
#define type_min(t)	__type_min(typeof(t))


/*
 * 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 __always_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 true on wrap-around, false otherwise.
 *
 * *@d holds the results of the attempted addition, regardless of whether
 * wrap-around occurred.
 */
#define check_add_overflow(a, b, d)	\
	__must_check_overflow(__builtin_add_overflow(a, b, d))

/**
 * wrapping_add() - Intentionally perform a wrapping addition
 * @type: type for result of calculation
 * @a: first addend
 * @b: second addend
 *
 * Return the potentially wrapped-around addition without
 * tripping any wrap-around sanitizers that may be enabled.
 */
#define wrapping_add(type, a, b)				\
	({							\
		type __val;					\
		__builtin_add_overflow(a, b, &__val);		\
		__val;						\
	})

/**
 * wrapping_assign_add() - Intentionally perform a wrapping increment assignment
 * @var: variable to be incremented
 * @offset: amount to add
 *
 * Increments @var by @offset with wrap-around. Returns the resulting
 * value of @var. Will not trip any wrap-around sanitizers.
 *
 * Returns the new value of @var.
 */
#define wrapping_assign_add(var, offset)				\
	({								\

Annotation

Implementation Notes