1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 *  Copyright (C) 2008 Red Hat, Inc., Eric Paris <eparis@redhat.com>
4 */
5
6/*
7 * Basic idea behind the notification queue: An fsnotify group (like inotify)
8 * sends the userspace notification about events asynchronously some time after
9 * the event happened.  When inotify gets an event it will need to add that
10 * event to the group notify queue.  Since a single event might need to be on
11 * multiple group's notification queues we can't add the event directly to each
12 * queue and instead add a small "event_holder" to each queue.  This event_holder
13 * has a pointer back to the original event.  Since the majority of events are
14 * going to end up on one, and only one, notification queue we embed one
15 * event_holder into each event.  This means we have a single allocation instead
16 * of always needing two.  If the embedded event_holder is already in use by
17 * another group a new event_holder (from fsnotify_event_holder_cachep) will be
18 * allocated and used.
19 */
20
21#include <linux/fs.h>
22#include <linux/init.h>
23#include <linux/kernel.h>
24#include <linux/list.h>
25#include <linux/module.h>
26#include <linux/mount.h>
27#include <linux/mutex.h>
28#include <linux/namei.h>
29#include <linux/path.h>
30#include <linux/slab.h>
31#include <linux/spinlock.h>
32
33#include <linux/atomic.h>
34
35#include <linux/fsnotify_backend.h>
36#include "fsnotify.h"
37
38static atomic_t fsnotify_sync_cookie = ATOMIC_INIT(0);
39
40/**
41 * fsnotify_get_cookie - return a unique cookie for use in synchronizing events.
42 * Called from fsnotify_move, which is inlined into filesystem modules.
43 */
44u32 fsnotify_get_cookie(void)
45{
46	return atomic_inc_return(&fsnotify_sync_cookie);
47}
48EXPORT_SYMBOL_GPL(fsnotify_get_cookie);
49
50void fsnotify_destroy_event(struct fsnotify_group *group,
51			    struct fsnotify_event *event)
52{
53	/* Overflow events are per-group and we don't want to free them */
54	if (!event || event == group->overflow_event)
55		return;
56	/*
57	 * If the event is still queued, we have a problem... Do an unreliable
58	 * lockless check first to avoid locking in the common case. The
59	 * locking may be necessary for permission events which got removed
60	 * from the list by a different CPU than the one freeing the event.
61	 */
62	if (!list_empty(&event->list)) {
63		spin_lock(&group->notification_lock);
64		WARN_ON(!list_empty(&event->list));
65		spin_unlock(&group->notification_lock);
66	}
67	group->ops->free_event(group, event);
68}
69
70/*
71 * Try to add an event to the notification queue.
72 * The group can later pull this event off the queue to deal with.
73 * The group can use the @merge hook to merge the event with a queued event.
74 * The group can use the @insert hook to insert the event into hash table.
75 * The function returns:
76 * 0 if the event was added to a queue
77 * 1 if the event was merged with some other queued event
78 * 2 if the event was not queued - either the queue of events has overflown
79 *   or the group is shutting down.
80 */
81int fsnotify_insert_event(struct fsnotify_group *group,
82			  struct fsnotify_event *event,
83			  int (*merge)(struct fsnotify_group *,
84				       struct fsnotify_event *),
85			  void (*insert)(struct fsnotify_group *,
86					 struct fsnotify_event *))
87{
88	int ret = 0;
89	struct list_head *list = &group->notification_list;
90
91	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
92
93	spin_lock(&group->notification_lock);
94
95	if (group->shutdown) {
96		spin_unlock(&group->notification_lock);
97		return 2;
98	}
99
100	if (event == group->overflow_event ||
101	    group->q_len >= group->max_events) {
102		ret = 2;
103		/* Queue overflow event only if it isn't already queued */
104		if (!list_empty(&group->overflow_event->list)) {
105			spin_unlock(&group->notification_lock);
106			return ret;
107		}
108		event = group->overflow_event;
109		goto queue;
110	}
111
112	if (!list_empty(list) && merge) {
113		ret = merge(group, event);
114		if (ret) {
115			spin_unlock(&group->notification_lock);
116			return ret;
117		}
118	}
119
120queue:
121	group->q_len++;
122	list_add_tail(&event->list, list);
123	if (insert)
124		insert(group, event);
125	spin_unlock(&group->notification_lock);
126
127	wake_up(&group->notification_waitq);
128	kill_fasync(&group->fsn_fa, SIGIO, POLL_IN);
129	return ret;
130}
131
132void fsnotify_remove_queued_event(struct fsnotify_group *group,
133				  struct fsnotify_event *event)
134{
135	assert_spin_locked(&group->notification_lock);
136	/*
137	 * We need to init list head for the case of overflow event so that
138	 * check in fsnotify_add_event() works
139	 */
140	list_del_init(&event->list);
141	group->q_len--;
142}
143
144/*
145 * Return the first event on the notification list without removing it.
146 * Returns NULL if the list is empty.
147 */
148struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group)
149{
150	assert_spin_locked(&group->notification_lock);
151
152	if (fsnotify_notify_queue_is_empty(group))
153		return NULL;
154
155	return list_first_entry(&group->notification_list,
156				struct fsnotify_event, list);
157}
158
159/*
160 * Remove and return the first event from the notification list.  It is the
161 * responsibility of the caller to destroy the obtained event
162 */
163struct fsnotify_event *fsnotify_remove_first_event(struct fsnotify_group *group)
164{
165	struct fsnotify_event *event = fsnotify_peek_first_event(group);
166
167	if (!event)
168		return NULL;
169
170	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
171
172	fsnotify_remove_queued_event(group, event);
173
174	return event;
175}
176
177/*
178 * Called when a group is being torn down to clean up any outstanding
179 * event notifications.
180 */
181void fsnotify_flush_notify(struct fsnotify_group *group)
182{
183	struct fsnotify_event *event;
184
185	spin_lock(&group->notification_lock);
186	while (!fsnotify_notify_queue_is_empty(group)) {
187		event = fsnotify_remove_first_event(group);
188		spin_unlock(&group->notification_lock);
189		fsnotify_destroy_event(group, event);
190		spin_lock(&group->notification_lock);
191	}
192	spin_unlock(&group->notification_lock);
193}
194