1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef _LINUX_CLEANUP_H
3#define _LINUX_CLEANUP_H
4
5#include <linux/compiler.h>
6
7/**
8 * DOC: scope-based cleanup helpers
9 *
10 * The "goto error" pattern is notorious for introducing subtle resource
11 * leaks. It is tedious and error prone to add new resource acquisition
12 * constraints into code paths that already have several unwind
13 * conditions. The "cleanup" helpers enable the compiler to help with
14 * this tedium and can aid in maintaining LIFO (last in first out)
15 * unwind ordering to avoid unintentional leaks.
16 *
17 * As drivers make up the majority of the kernel code base, here is an
18 * example of using these helpers to clean up PCI drivers. The target of
19 * the cleanups are occasions where a goto is used to unwind a device
20 * reference (pci_dev_put()), or unlock the device (pci_dev_unlock())
21 * before returning.
22 *
23 * The DEFINE_FREE() macro can arrange for PCI device references to be
24 * dropped when the associated variable goes out of scope::
25 *
26 * DEFINE_FREE(pci_dev_put, struct pci_dev *, if (_T) pci_dev_put(_T))
27 * ...
28 * struct pci_dev *dev __free(pci_dev_put) =
29 * pci_get_slot(parent, PCI_DEVFN(0, 0));
30 *
31 * The above will automatically call pci_dev_put() if @dev is non-NULL
32 * when @dev goes out of scope (automatic variable scope). If a function
33 * wants to invoke pci_dev_put() on error, but return @dev (i.e. without
34 * freeing it) on success, it can do::
35 *
36 * return no_free_ptr(dev);
37 *
38 * ...or::
39 *
40 * return_ptr(dev);
41 *
42 * The DEFINE_GUARD() macro can arrange for the PCI device lock to be
43 * dropped when the scope where guard() is invoked ends::
44 *
45 * DEFINE_GUARD(pci_dev, struct pci_dev *, pci_dev_lock(_T), pci_dev_unlock(_T))
46 * ...
47 * guard(pci_dev)(dev);
48 *
49 * The lifetime of the lock obtained by the guard() helper follows the
50 * scope of automatic variable declaration. Take the following example::
51 *
52 * func(...)
53 * {
54 * if (...) {
55 * ...
56 * guard(pci_dev)(dev); // pci_dev_lock() invoked here
57 * ...
58 * } // <- implied pci_dev_unlock() triggered here
59 * }
60 *
61 * Observe the lock is held for the remainder of the "if ()" block not
62 * the remainder of "func()".
63 *
64 * Now, when a function uses both __free() and guard(), or multiple
65 * instances of __free(), the LIFO order of variable definition order
66 * matters. GCC documentation says:
67 *
68 * "When multiple variables in the same scope have cleanup attributes,
69 * at exit from the scope their associated cleanup functions are run in
70 * reverse order of definition (last defined, first cleanup)."
71 *
72 * When the unwind order matters it requires that variables be defined
73 * mid-function scope rather than at the top of the file. Take the
74 * following example and notice the bug highlighted by "!!"::
75 *
76 * LIST_HEAD(list);
77 * DEFINE_MUTEX(lock);
78 *
79 * struct object {
80 * struct list_head node;
81 * };
82 *
83 * static struct object *alloc_add(void)
84 * {
85 * struct object *obj;
86 *
87 * lockdep_assert_held(&lock);
88 * obj = kzalloc(sizeof(*obj), GFP_KERNEL);
89 * if (obj) {
90 * LIST_HEAD_INIT(&obj->node);
91 * list_add(obj->node, &list):
92 * }
93 * return obj;
94 * }
95 *
96 * static void remove_free(struct object *obj)
97 * {
98 * lockdep_assert_held(&lock);
99 * list_del(&obj->node);
100 * kfree(obj);
101 * }
102 *
103 * DEFINE_FREE(remove_free, struct object *, if (_T) remove_free(_T))
104 * static int init(void)
105 * {
106 * struct object *obj __free(remove_free) = NULL;
107 * int err;
108 *
109 * guard(mutex)(&lock);
110 * obj = alloc_add();
111 *
112 * if (!obj)
113 * return -ENOMEM;
114 *
115 * err = other_init(obj);
116 * if (err)
117 * return err; // remove_free() called without the lock!!
118 *
119 * no_free_ptr(obj);
120 * return 0;
121 * }
122 *
123 * That bug is fixed by changing init() to call guard() and define +
124 * initialize @obj in this order::
125 *
126 * guard(mutex)(&lock);
127 * struct object *obj __free(remove_free) = alloc_add();
128 *
129 * Given that the "__free(...) = NULL" pattern for variables defined at
130 * the top of the function poses this potential interdependency problem
131 * the recommendation is to always define and assign variables in one
132 * statement and not group variable definitions at the top of the
133 * function when __free() is used.
134 *
135 * Lastly, given that the benefit of cleanup helpers is removal of
136 * "goto", and that the "goto" statement can jump between scopes, the
137 * expectation is that usage of "goto" and cleanup helpers is never
138 * mixed in the same function. I.e. for a given routine, convert all
139 * resources that need a "goto" cleanup to scope-based cleanup, or
140 * convert none of them.
141 */
142
143/*
144 * DEFINE_FREE(name, type, free):
145 * simple helper macro that defines the required wrapper for a __free()
146 * based cleanup function. @free is an expression using '_T' to access the
147 * variable. @free should typically include a NULL test before calling a
148 * function, see the example below.
149 *
150 * __free(name):
151 * variable attribute to add a scoped based cleanup to the variable.
152 *
153 * no_free_ptr(var):
154 * like a non-atomic xchg(var, NULL), such that the cleanup function will
155 * be inhibited -- provided it sanely deals with a NULL value.
156 *
157 * NOTE: this has __must_check semantics so that it is harder to accidentally
158 * leak the resource.
159 *
160 * return_ptr(p):
161 * returns p while inhibiting the __free().
162 *
163 * Ex.
164 *
165 * DEFINE_FREE(kfree, void *, if (_T) kfree(_T))
166 *
167 * void *alloc_obj(...)
168 * {
169 * struct obj *p __free(kfree) = kmalloc(...);
170 * if (!p)
171 * return NULL;
172 *
173 * if (!init_obj(p))
174 * return NULL;
175 *
176 * return_ptr(p);
177 * }
178 *
179 * NOTE: the DEFINE_FREE()'s @free expression includes a NULL test even though
180 * kfree() is fine to be called with a NULL value. This is on purpose. This way
181 * the compiler sees the end of our alloc_obj() function as:
182 *
183 * tmp = p;
184 * p = NULL;
185 * if (p)
186 * kfree(p);
187 * return tmp;
188 *
189 * And through the magic of value-propagation and dead-code-elimination, it
190 * eliminates the actual cleanup call and compiles into:
191 *
192 * return p;
193 *
194 * Without the NULL test it turns into a mess and the compiler can't help us.
195 */
196
197#define DEFINE_FREE(_name, _type, _free) \
198 static inline void __free_##_name(void *p) { _type _T = *(_type *)p; _free; }
199
200#define __free(_name) __cleanup(__free_##_name)
201
202#define __get_and_null(p, nullvalue) \
203 ({ \
204 __auto_type __ptr = &(p); \
205 __auto_type __val = *__ptr; \
206 *__ptr = nullvalue; \
207 __val; \
208 })
209
210static inline __must_check
211const volatile void * __must_check_fn(const volatile void *val)
212{ return val; }
213
214#define no_free_ptr(p) \
215 ((typeof(p)) __must_check_fn((__force const volatile void *)__get_and_null(p, NULL)))
216
217#define return_ptr(p) return no_free_ptr(p)
218
219/*
220 * Only for situations where an allocation is handed in to another function
221 * and consumed by that function on success.
222 *
223 * struct foo *f __free(kfree) = kzalloc(sizeof(*f), GFP_KERNEL);
224 *
225 * setup(f);
226 * if (some_condition)
227 * return -EINVAL;
228 * ....
229 * ret = bar(f);
230 * if (!ret)
231 * retain_and_null_ptr(f);
232 * return ret;
233 *
234 * After retain_and_null_ptr(f) the variable f is NULL and cannot be
235 * dereferenced anymore.
236 */
237#define retain_and_null_ptr(p) ((void)__get_and_null(p, NULL))
238
239/*
240 * DEFINE_CLASS(name, type, exit, init, init_args...):
241 * helper to define the destructor and constructor for a type.
242 * @exit is an expression using '_T' -- similar to FREE above.
243 * @init is an expression in @init_args resulting in @type
244 *
245 * EXTEND_CLASS(name, ext, init, init_args...):
246 * extends class @name to @name@ext with the new constructor
247 *
248 * CLASS(name, var)(args...):
249 * declare the variable @var as an instance of the named class
250 *
251 * Ex.
252 *
253 * DEFINE_CLASS(fdget, struct fd, fdput(_T), fdget(fd), int fd)
254 *
255 * CLASS(fdget, f)(fd);
256 * if (fd_empty(f))
257 * return -EBADF;
258 *
259 * // use 'f' without concern
260 */
261
262#define DEFINE_CLASS(_name, _type, _exit, _init, _init_args...) \
263typedef _type class_##_name##_t; \
264static inline void class_##_name##_destructor(_type *p) \
265{ _type _T = *p; _exit; } \
266static inline _type class_##_name##_constructor(_init_args) \
267{ _type t = _init; return t; }
268
269#define EXTEND_CLASS(_name, ext, _init, _init_args...) \
270typedef class_##_name##_t class_##_name##ext##_t; \
271static inline void class_##_name##ext##_destructor(class_##_name##_t *p)\
272{ class_##_name##_destructor(p); } \
273static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
274{ class_##_name##_t t = _init; return t; }
275
276#define CLASS(_name, var) \
277 class_##_name##_t var __cleanup(class_##_name##_destructor) = \
278 class_##_name##_constructor
279
280
281/*
282 * DEFINE_GUARD(name, type, lock, unlock):
283 * trivial wrapper around DEFINE_CLASS() above specifically
284 * for locks.
285 *
286 * DEFINE_GUARD_COND(name, ext, condlock)
287 * wrapper around EXTEND_CLASS above to add conditional lock
288 * variants to a base class, eg. mutex_trylock() or
289 * mutex_lock_interruptible().
290 *
291 * guard(name):
292 * an anonymous instance of the (guard) class, not recommended for
293 * conditional locks.
294 *
295 * scoped_guard (name, args...) { }:
296 * similar to CLASS(name, scope)(args), except the variable (with the
297 * explicit name 'scope') is declard in a for-loop such that its scope is
298 * bound to the next (compound) statement.
299 *
300 * for conditional locks the loop body is skipped when the lock is not
301 * acquired.
302 *
303 * scoped_cond_guard (name, fail, args...) { }:
304 * similar to scoped_guard(), except it does fail when the lock
305 * acquire fails.
306 *
307 * Only for conditional locks.
308 */
309
310#define __DEFINE_CLASS_IS_CONDITIONAL(_name, _is_cond) \
311static __maybe_unused const bool class_##_name##_is_conditional = _is_cond
312
313#define __DEFINE_GUARD_LOCK_PTR(_name, _exp) \
314 static inline void * class_##_name##_lock_ptr(class_##_name##_t *_T) \
315 { return (void *)(__force unsigned long)*(_exp); }
316
317#define DEFINE_CLASS_IS_GUARD(_name) \
318 __DEFINE_CLASS_IS_CONDITIONAL(_name, false); \
319 __DEFINE_GUARD_LOCK_PTR(_name, _T)
320
321#define DEFINE_CLASS_IS_COND_GUARD(_name) \
322 __DEFINE_CLASS_IS_CONDITIONAL(_name, true); \
323 __DEFINE_GUARD_LOCK_PTR(_name, _T)
324
325#define DEFINE_GUARD(_name, _type, _lock, _unlock) \
326 DEFINE_CLASS(_name, _type, if (_T) { _unlock; }, ({ _lock; _T; }), _type _T); \
327 DEFINE_CLASS_IS_GUARD(_name)
328
329#define DEFINE_GUARD_COND(_name, _ext, _condlock) \
330 __DEFINE_CLASS_IS_CONDITIONAL(_name##_ext, true); \
331 EXTEND_CLASS(_name, _ext, \
332 ({ void *_t = _T; if (_T && !(_condlock)) _t = NULL; _t; }), \
333 class_##_name##_t _T) \
334 static inline void * class_##_name##_ext##_lock_ptr(class_##_name##_t *_T) \
335 { return class_##_name##_lock_ptr(_T); }
336
337#define guard(_name) \
338 CLASS(_name, __UNIQUE_ID(guard))
339
340#define __guard_ptr(_name) class_##_name##_lock_ptr
341#define __is_cond_ptr(_name) class_##_name##_is_conditional
342
343/*
344 * Helper macro for scoped_guard().
345 *
346 * Note that the "!__is_cond_ptr(_name)" part of the condition ensures that
347 * compiler would be sure that for the unconditional locks the body of the
348 * loop (caller-provided code glued to the else clause) could not be skipped.
349 * It is needed because the other part - "__guard_ptr(_name)(&scope)" - is too
350 * hard to deduce (even if could be proven true for unconditional locks).
351 */
352#define __scoped_guard(_name, _label, args...) \
353 for (CLASS(_name, scope)(args); \
354 __guard_ptr(_name)(&scope) || !__is_cond_ptr(_name); \
355 ({ goto _label; })) \
356 if (0) { \
357_label: \
358 break; \
359 } else
360
361#define scoped_guard(_name, args...) \
362 __scoped_guard(_name, __UNIQUE_ID(label), args)
363
364#define __scoped_cond_guard(_name, _fail, _label, args...) \
365 for (CLASS(_name, scope)(args); true; ({ goto _label; })) \
366 if (!__guard_ptr(_name)(&scope)) { \
367 BUILD_BUG_ON(!__is_cond_ptr(_name)); \
368 _fail; \
369_label: \
370 break; \
371 } else
372
373#define scoped_cond_guard(_name, _fail, args...) \
374 __scoped_cond_guard(_name, _fail, __UNIQUE_ID(label), args)
375
376/*
377 * Additional helper macros for generating lock guards with types, either for
378 * locks that don't have a native type (eg. RCU, preempt) or those that need a
379 * 'fat' pointer (eg. spin_lock_irqsave).
380 *
381 * DEFINE_LOCK_GUARD_0(name, lock, unlock, ...)
382 * DEFINE_LOCK_GUARD_1(name, type, lock, unlock, ...)
383 * DEFINE_LOCK_GUARD_1_COND(name, ext, condlock)
384 *
385 * will result in the following type:
386 *
387 * typedef struct {
388 * type *lock; // 'type := void' for the _0 variant
389 * __VA_ARGS__;
390 * } class_##name##_t;
391 *
392 * As above, both _lock and _unlock are statements, except this time '_T' will
393 * be a pointer to the above struct.
394 */
395
396#define __DEFINE_UNLOCK_GUARD(_name, _type, _unlock, ...) \
397typedef struct { \
398 _type *lock; \
399 __VA_ARGS__; \
400} class_##_name##_t; \
401 \
402static inline void class_##_name##_destructor(class_##_name##_t *_T) \
403{ \
404 if (_T->lock) { _unlock; } \
405} \
406 \
407__DEFINE_GUARD_LOCK_PTR(_name, &_T->lock)
408
409#define __DEFINE_LOCK_GUARD_1(_name, _type, _lock) \
410static inline class_##_name##_t class_##_name##_constructor(_type *l) \
411{ \
412 class_##_name##_t _t = { .lock = l }, *_T = &_t; \
413 _lock; \
414 return _t; \
415}
416
417#define __DEFINE_LOCK_GUARD_0(_name, _lock) \
418static inline class_##_name##_t class_##_name##_constructor(void) \
419{ \
420 class_##_name##_t _t = { .lock = (void*)1 }, \
421 *_T __maybe_unused = &_t; \
422 _lock; \
423 return _t; \
424}
425
426#define DEFINE_LOCK_GUARD_1(_name, _type, _lock, _unlock, ...) \
427__DEFINE_CLASS_IS_CONDITIONAL(_name, false); \
428__DEFINE_UNLOCK_GUARD(_name, _type, _unlock, __VA_ARGS__) \
429__DEFINE_LOCK_GUARD_1(_name, _type, _lock)
430
431#define DEFINE_LOCK_GUARD_0(_name, _lock, _unlock, ...) \
432__DEFINE_CLASS_IS_CONDITIONAL(_name, false); \
433__DEFINE_UNLOCK_GUARD(_name, void, _unlock, __VA_ARGS__) \
434__DEFINE_LOCK_GUARD_0(_name, _lock)
435
436#define DEFINE_LOCK_GUARD_1_COND(_name, _ext, _condlock) \
437 __DEFINE_CLASS_IS_CONDITIONAL(_name##_ext, true); \
438 EXTEND_CLASS(_name, _ext, \
439 ({ class_##_name##_t _t = { .lock = l }, *_T = &_t;\
440 if (_T->lock && !(_condlock)) _T->lock = NULL; \
441 _t; }), \
442 typeof_member(class_##_name##_t, lock) l) \
443 static inline void * class_##_name##_ext##_lock_ptr(class_##_name##_t *_T) \
444 { return class_##_name##_lock_ptr(_T); }
445
446
447#endif /* _LINUX_CLEANUP_H */
448

source code of linux/include/linux/cleanup.h

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