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
50/* return true if the notify queue is empty, false otherwise */
51bool fsnotify_notify_queue_is_empty(struct fsnotify_group *group)
52{
53	assert_spin_locked(&group->notification_lock);
54	return list_empty(&group->notification_list) ? true : false;
55}
56
57void fsnotify_destroy_event(struct fsnotify_group *group,
58			    struct fsnotify_event *event)
59{
60	/* Overflow events are per-group and we don't want to free them */
61	if (!event || event == group->overflow_event)
62		return;
63	/*
64	 * If the event is still queued, we have a problem... Do an unreliable
65	 * lockless check first to avoid locking in the common case. The
66	 * locking may be necessary for permission events which got removed
67	 * from the list by a different CPU than the one freeing the event.
68	 */
69	if (!list_empty(&event->list)) {
70		spin_lock(&group->notification_lock);
71		WARN_ON(!list_empty(&event->list));
72		spin_unlock(&group->notification_lock);
73	}
74	group->ops->free_event(event);
75}
76
77/*
78 * Add an event to the group notification queue.  The group can later pull this
79 * event off the queue to deal with.  The function returns 0 if the event was
80 * added to the queue, 1 if the event was merged with some other queued event,
81 * 2 if the event was not queued - either the queue of events has overflown
82 * or the group is shutting down.
83 */
84int fsnotify_add_event(struct fsnotify_group *group,
85		       struct fsnotify_event *event,
86		       int (*merge)(struct list_head *,
87				    struct fsnotify_event *))
88{
89	int ret = 0;
90	struct list_head *list = &group->notification_list;
91
92	pr_debug("%s: group=%p event=%p\n", __func__, group, event);
93
94	spin_lock(&group->notification_lock);
95
96	if (group->shutdown) {
97		spin_unlock(&group->notification_lock);
98		return 2;
99	}
100
101	if (event == group->overflow_event ||
102	    group->q_len >= group->max_events) {
103		ret = 2;
104		/* Queue overflow event only if it isn't already queued */
105		if (!list_empty(&group->overflow_event->list)) {
106			spin_unlock(&group->notification_lock);
107			return ret;
108		}
109		event = group->overflow_event;
110		goto queue;
111	}
112
113	if (!list_empty(list) && merge) {
114		ret = merge(list, event);
115		if (ret) {
116			spin_unlock(&group->notification_lock);
117			return ret;
118		}
119	}
120
121queue:
122	group->q_len++;
123	list_add_tail(&event->list, list);
124	spin_unlock(&group->notification_lock);
125
126	wake_up(&group->notification_waitq);
127	kill_fasync(&group->fsn_fa, SIGIO, POLL_IN);
128	return ret;
129}
130
131void fsnotify_remove_queued_event(struct fsnotify_group *group,
132				  struct fsnotify_event *event)
133{
134	assert_spin_locked(&group->notification_lock);
135	/*
136	 * We need to init list head for the case of overflow event so that
137	 * check in fsnotify_add_event() works
138	 */
139	list_del_init(&event->list);
140	group->q_len--;
141}
142
143/*
144 * Remove and return the first event from the notification list.  It is the
145 * responsibility of the caller to destroy the obtained event
146 */
147struct fsnotify_event *fsnotify_remove_first_event(struct fsnotify_group *group)
148{
149	struct fsnotify_event *event;
150
151	assert_spin_locked(&group->notification_lock);
152
153	pr_debug("%s: group=%p\n", __func__, group);
154
155	event = list_first_entry(&group->notification_list,
156				 struct fsnotify_event, list);
157	fsnotify_remove_queued_event(group, event);
158	return event;
159}
160
161/*
162 * This will not remove the event, that must be done with
163 * fsnotify_remove_first_event()
164 */
165struct fsnotify_event *fsnotify_peek_first_event(struct fsnotify_group *group)
166{
167	assert_spin_locked(&group->notification_lock);
168
169	return list_first_entry(&group->notification_list,
170				struct fsnotify_event, list);
171}
172
173/*
174 * Called when a group is being torn down to clean up any outstanding
175 * event notifications.
176 */
177void fsnotify_flush_notify(struct fsnotify_group *group)
178{
179	struct fsnotify_event *event;
180
181	spin_lock(&group->notification_lock);
182	while (!fsnotify_notify_queue_is_empty(group)) {
183		event = fsnotify_remove_first_event(group);
184		spin_unlock(&group->notification_lock);
185		fsnotify_destroy_event(group, event);
186		spin_lock(&group->notification_lock);
187	}
188	spin_unlock(&group->notification_lock);
189}
190