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
38 static 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 */
fsnotify_get_cookie(void)44 u32 fsnotify_get_cookie(void)
45 {
46 return atomic_inc_return(&fsnotify_sync_cookie);
47 }
48 EXPORT_SYMBOL_GPL(fsnotify_get_cookie);
49
fsnotify_destroy_event(struct fsnotify_group * group,struct fsnotify_event * event)50 void 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 */
fsnotify_insert_event(struct fsnotify_group * group,struct fsnotify_event * event,int (* merge)(struct fsnotify_group *,struct fsnotify_event *),void (* insert)(struct fsnotify_group *,struct fsnotify_event *))81 int 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
120 queue:
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
fsnotify_remove_queued_event(struct fsnotify_group * group,struct fsnotify_event * event)132 void 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 */
fsnotify_peek_first_event(struct fsnotify_group * group)148 struct 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 */
fsnotify_remove_first_event(struct fsnotify_group * group)163 struct 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 */
fsnotify_flush_notify(struct fsnotify_group * group)181 void 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