kernel / include / linux / cleanup.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef __LINUX_GUARDS_H
#define __LINUX_GUARDS_H

#include <linux/compiler.h>

/*
 * DEFINE_FREE(name, type, free):
 *	simple helper macro that defines the required wrapper for a __free()
 *	based cleanup function. @free is an expression using '_T' to access the
 *	variable. @free should typically include a NULL test before calling a
 *	function, see the example below.
 *
 * __free(name):
 *	variable attribute to add a scoped based cleanup to the variable.
 *
 * no_free_ptr(var):
 *	like a non-atomic xchg(var, NULL), such that the cleanup function will
 *	be inhibited -- provided it sanely deals with a NULL value.
 *
 *	NOTE: this has __must_check semantics so that it is harder to accidentally
 *	leak the resource.
 *
 * return_ptr(p):
 *	returns p while inhibiting the __free().
 *
 * Ex.
 *
 * DEFINE_FREE(kfree, void *, if (_T) kfree(_T))
 *
 * void *alloc_obj(...)
 * {
 *	struct obj *p __free(kfree) = kmalloc(...);
 *	if (!p)
 *		return NULL;
 *
 *	if (!init_obj(p))
 *		return NULL;
 *
 *	return_ptr(p);
 * }
 *
 * NOTE: the DEFINE_FREE()'s @free expression includes a NULL test even though
 * kfree() is fine to be called with a NULL value. This is on purpose. This way
 * the compiler sees the end of our alloc_obj() function as:
 *
 *	tmp = p;
 *	p = NULL;
 *	if (p)
 *		kfree(p);
 *	return tmp;
 *
 * And through the magic of value-propagation and dead-code-elimination, it
 * eliminates the actual cleanup call and compiles into:
 *
 *	return p;
 *
 * Without the NULL test it turns into a mess and the compiler can't help us.
 */

#define DEFINE_FREE(_name, _type, _free) \
	static inline void __free_##_name(void *p) { _type _T = *(_type *)p; _free; }

#define __free(_name)	__cleanup(__free_##_name)

#define __get_and_null_ptr(p) \
	({ __auto_type __ptr = &(p); \
	   __auto_type __val = *__ptr; \
	   *__ptr = NULL;  __val; })

static inline __must_check
const volatile void * __must_check_fn(const volatile void *val)
{ return val; }

#define no_free_ptr(p) \
	((typeof(p)) __must_check_fn(__get_and_null_ptr(p)))

#define return_ptr(p)	return no_free_ptr(p)


/*
 * DEFINE_CLASS(name, type, exit, init, init_args...):
 *	helper to define the destructor and constructor for a type.
 *	@exit is an expression using '_T' -- similar to FREE above.
 *	@init is an expression in @init_args resulting in @type
 *
 * EXTEND_CLASS(name, ext, init, init_args...):
 *	extends class @name to @name@ext with the new constructor
 *
 * CLASS(name, var)(args...):
 *	declare the variable @var as an instance of the named class
 *
 * Ex.
 *
 * DEFINE_CLASS(fdget, struct fd, fdput(_T), fdget(fd), int fd)
 *
 *	CLASS(fdget, f)(fd);
 *	if (!f.file)
 *		return -EBADF;
 *
 *	// use 'f' without concern
 */

#define DEFINE_CLASS(_name, _type, _exit, _init, _init_args...)		\
typedef _type class_##_name##_t;					\
static inline void class_##_name##_destructor(_type *p)			\
{ _type _T = *p; _exit; }						\
static inline _type class_##_name##_constructor(_init_args)		\
{ _type t = _init; return t; }

#define EXTEND_CLASS(_name, ext, _init, _init_args...)			\
typedef class_##_name##_t class_##_name##ext##_t;			\
static inline void class_##_name##ext##_destructor(class_##_name##_t *p)\
{ class_##_name##_destructor(p); }					\
static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
{ class_##_name##_t t = _init; return t; }

#define CLASS(_name, var)						\
	class_##_name##_t var __cleanup(class_##_name##_destructor) =	\
		class_##_name##_constructor


/*
 * DEFINE_GUARD(name, type, lock, unlock):
 *	trivial wrapper around DEFINE_CLASS() above specifically
 *	for locks.
 *
 * guard(name):
 *	an anonymous instance of the (guard) class
 *
 * scoped_guard (name, args...) { }:
 *	similar to CLASS(name, scope)(args), except the variable (with the
 *	explicit name 'scope') is declard in a for-loop such that its scope is
 *	bound to the next (compound) statement.
 *
 */

#define DEFINE_GUARD(_name, _type, _lock, _unlock) \
	DEFINE_CLASS(_name, _type, _unlock, ({ _lock; _T; }), _type _T)

#define guard(_name) \
	CLASS(_name, __UNIQUE_ID(guard))

#define scoped_guard(_name, args...)					\
	for (CLASS(_name, scope)(args),					\
	     *done = NULL; !done; done = (void *)1)

/*
 * Additional helper macros for generating lock guards with types, either for
 * locks that don't have a native type (eg. RCU, preempt) or those that need a
 * 'fat' pointer (eg. spin_lock_irqsave).
 *
 * DEFINE_LOCK_GUARD_0(name, lock, unlock, ...)
 * DEFINE_LOCK_GUARD_1(name, type, lock, unlock, ...)
 *
 * will result in the following type:
 *
 *   typedef struct {
 *	type *lock;		// 'type := void' for the _0 variant
 *	__VA_ARGS__;
 *   } class_##name##_t;
 *
 * As above, both _lock and _unlock are statements, except this time '_T' will
 * be a pointer to the above struct.
 */

#define __DEFINE_UNLOCK_GUARD(_name, _type, _unlock, ...)		\
typedef struct {							\
	_type *lock;							\
	__VA_ARGS__;							\
} class_##_name##_t;							\
									\
static inline void class_##_name##_destructor(class_##_name##_t *_T)	\
{									\
	if (_T->lock) { _unlock; }					\
}


#define __DEFINE_LOCK_GUARD_1(_name, _type, _lock)			\
static inline class_##_name##_t class_##_name##_constructor(_type *l)	\
{									\
	class_##_name##_t _t = { .lock = l }, *_T = &_t;		\
	_lock;								\
	return _t;							\
}

#define __DEFINE_LOCK_GUARD_0(_name, _lock)				\
static inline class_##_name##_t class_##_name##_constructor(void)	\
{									\
	class_##_name##_t _t = { .lock = (void*)1 },			\
			 *_T __maybe_unused = &_t;			\
	_lock;								\
	return _t;							\
}

#define DEFINE_LOCK_GUARD_1(_name, _type, _lock, _unlock, ...)		\
__DEFINE_UNLOCK_GUARD(_name, _type, _unlock, __VA_ARGS__)		\
__DEFINE_LOCK_GUARD_1(_name, _type, _lock)

#define DEFINE_LOCK_GUARD_0(_name, _lock, _unlock, ...)			\
__DEFINE_UNLOCK_GUARD(_name, void, _unlock, __VA_ARGS__)		\
__DEFINE_LOCK_GUARD_0(_name, _lock)

#endif /* __LINUX_GUARDS_H */