dnotify.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * Directory notifications for Linux.
  4 *
  5 * Copyright (C) 2000,2001,2002 Stephen Rothwell
  6 *
  7 * Copyright (C) 2009 Eric Paris <Red Hat Inc>
  8 * dnotify was largly rewritten to use the new fsnotify infrastructure
  9 */
 10#include <linux/fs.h>
 11#include <linux/module.h>
 12#include <linux/sched.h>
 13#include <linux/sched/signal.h>
 14#include <linux/dnotify.h>
 15#include <linux/init.h>
 16#include <linux/security.h>
 17#include <linux/spinlock.h>
 18#include <linux/slab.h>
 19#include <linux/fsnotify_backend.h>
 20
 21static int dir_notify_enable __read_mostly = 1;
 22#ifdef CONFIG_SYSCTL
 23static const struct ctl_table dnotify_sysctls[] = {
 24	{
 25		.procname	= "dir-notify-enable",
 26		.data		= &dir_notify_enable,
 27		.maxlen		= sizeof(int),
 28		.mode		= 0644,
 29		.proc_handler	= proc_dointvec,
 30	},
 31};
 32static void __init dnotify_sysctl_init(void)
 33{
 34	register_sysctl_init("fs", dnotify_sysctls);
 35}
 36#else
 37#define dnotify_sysctl_init() do { } while (0)
 38#endif
 39
 40static struct kmem_cache *dnotify_struct_cache __ro_after_init;
 41static struct kmem_cache *dnotify_mark_cache __ro_after_init;
 42static struct fsnotify_group *dnotify_group __ro_after_init;
 43
 44/*
 45 * dnotify will attach one of these to each inode (i_fsnotify_marks) which
 46 * is being watched by dnotify.  If multiple userspace applications are watching
 47 * the same directory with dnotify their information is chained in dn
 48 */
 49struct dnotify_mark {
 50	struct fsnotify_mark fsn_mark;
 51	struct dnotify_struct *dn;
 52};
 53
 54/*
 55 * When a process starts or stops watching an inode the set of events which
 56 * dnotify cares about for that inode may change.  This function runs the
 57 * list of everything receiving dnotify events about this directory and calculates
 58 * the set of all those events.  After it updates what dnotify is interested in
 59 * it calls the fsnotify function so it can update the set of all events relevant
 60 * to this inode.
 61 */
 62static void dnotify_recalc_inode_mask(struct fsnotify_mark *fsn_mark)
 63{
 64	__u32 new_mask = 0;
 65	struct dnotify_struct *dn;
 66	struct dnotify_mark *dn_mark  = container_of(fsn_mark,
 67						     struct dnotify_mark,
 68						     fsn_mark);
 69
 70	assert_spin_locked(&fsn_mark->lock);
 71
 72	for (dn = dn_mark->dn; dn != NULL; dn = dn->dn_next)
 73		new_mask |= (dn->dn_mask & ~FS_DN_MULTISHOT);
 74	if (fsn_mark->mask == new_mask)
 75		return;
 76	fsn_mark->mask = new_mask;
 77
 78	fsnotify_recalc_mask(fsn_mark->connector);
 79}
 80
 81/*
 82 * Mains fsnotify call where events are delivered to dnotify.
 83 * Find the dnotify mark on the relevant inode, run the list of dnotify structs
 84 * on that mark and determine which of them has expressed interest in receiving
 85 * events of this type.  When found send the correct process and signal and
 86 * destroy the dnotify struct if it was not registered to receive multiple
 87 * events.
 88 */
 89static int dnotify_handle_event(struct fsnotify_mark *inode_mark, u32 mask,
 90				struct inode *inode, struct inode *dir,
 91				const struct qstr *name, u32 cookie)
 92{
 93	struct dnotify_mark *dn_mark;
 94	struct dnotify_struct *dn;
 95	struct dnotify_struct **prev;
 96	struct fown_struct *fown;
 97	__u32 test_mask = mask & ~FS_EVENT_ON_CHILD;
 98
 99	/* not a dir, dnotify doesn't care */
100	if (!dir && !(mask & FS_ISDIR))
101		return 0;
102
103	dn_mark = container_of(inode_mark, struct dnotify_mark, fsn_mark);
104
105	spin_lock(&inode_mark->lock);
106	prev = &dn_mark->dn;
107	while ((dn = *prev) != NULL) {
108		if ((dn->dn_mask & test_mask) == 0) {
109			prev = &dn->dn_next;
110			continue;
111		}
112		fown = file_f_owner(dn->dn_filp);
113		send_sigio(fown, dn->dn_fd, POLL_MSG);
114		if (dn->dn_mask & FS_DN_MULTISHOT)
115			prev = &dn->dn_next;
116		else {
117			*prev = dn->dn_next;
118			kmem_cache_free(dnotify_struct_cache, dn);
119			dnotify_recalc_inode_mask(inode_mark);
120		}
121	}
122
123	spin_unlock(&inode_mark->lock);
124
125	return 0;
126}
127
128static void dnotify_free_mark(struct fsnotify_mark *fsn_mark)
129{
130	struct dnotify_mark *dn_mark = container_of(fsn_mark,
131						    struct dnotify_mark,
132						    fsn_mark);
133
134	BUG_ON(dn_mark->dn);
135
136	kmem_cache_free(dnotify_mark_cache, dn_mark);
137}
138
139static const struct fsnotify_ops dnotify_fsnotify_ops = {
140	.handle_inode_event = dnotify_handle_event,
141	.free_mark = dnotify_free_mark,
142};
143
144/*
145 * Called every time a file is closed.  Looks first for a dnotify mark on the
146 * inode.  If one is found run all of the ->dn structures attached to that
147 * mark for one relevant to this process closing the file and remove that
148 * dnotify_struct.  If that was the last dnotify_struct also remove the
149 * fsnotify_mark.
150 */
151void dnotify_flush(struct file *filp, fl_owner_t id)
152{
153	struct fsnotify_mark *fsn_mark;
154	struct dnotify_mark *dn_mark;
155	struct dnotify_struct *dn;
156	struct dnotify_struct **prev;
157	struct inode *inode;
158	bool free = false;
159
160	inode = file_inode(filp);
161	if (!S_ISDIR(inode->i_mode))
162		return;
163
164	fsn_mark = fsnotify_find_inode_mark(inode, dnotify_group);
165	if (!fsn_mark)
166		return;
167	dn_mark = container_of(fsn_mark, struct dnotify_mark, fsn_mark);
168
169	fsnotify_group_lock(dnotify_group);
170
171	spin_lock(&fsn_mark->lock);
172	prev = &dn_mark->dn;
173	while ((dn = *prev) != NULL) {
174		if ((dn->dn_owner == id) && (dn->dn_filp == filp)) {
175			*prev = dn->dn_next;
176			kmem_cache_free(dnotify_struct_cache, dn);
177			dnotify_recalc_inode_mask(fsn_mark);
178			break;
179		}
180		prev = &dn->dn_next;
181	}
182
183	spin_unlock(&fsn_mark->lock);
184
185	/* nothing else could have found us thanks to the dnotify_groups
186	   mark_mutex */
187	if (dn_mark->dn == NULL) {
188		fsnotify_detach_mark(fsn_mark);
189		free = true;
190	}
191
192	fsnotify_group_unlock(dnotify_group);
193
194	if (free)
195		fsnotify_free_mark(fsn_mark);
196	fsnotify_put_mark(fsn_mark);
197}
198
199/* this conversion is done only at watch creation */
200static __u32 convert_arg(unsigned int arg)
201{
202	__u32 new_mask = FS_EVENT_ON_CHILD;
203
204	if (arg & DN_MULTISHOT)
205		new_mask |= FS_DN_MULTISHOT;
206	if (arg & DN_DELETE)
207		new_mask |= (FS_DELETE | FS_MOVED_FROM);
208	if (arg & DN_MODIFY)
209		new_mask |= FS_MODIFY;
210	if (arg & DN_ACCESS)
211		new_mask |= FS_ACCESS;
212	if (arg & DN_ATTRIB)
213		new_mask |= FS_ATTRIB;
214	if (arg & DN_RENAME)
215		new_mask |= FS_RENAME;
216	if (arg & DN_CREATE)
217		new_mask |= (FS_CREATE | FS_MOVED_TO);
218
219	return new_mask;
220}
221
222/*
223 * If multiple processes watch the same inode with dnotify there is only one
224 * dnotify mark in inode->i_fsnotify_marks but we chain a dnotify_struct
225 * onto that mark.  This function either attaches the new dnotify_struct onto
226 * that list, or it |= the mask onto an existing dnofiy_struct.
227 */
228static int attach_dn(struct dnotify_struct *dn, struct dnotify_mark *dn_mark,
229		     fl_owner_t id, int fd, struct file *filp, __u32 mask)
230{
231	struct dnotify_struct *odn;
232
233	odn = dn_mark->dn;
234	while (odn != NULL) {
235		/* adding more events to existing dnofiy_struct? */
236		if ((odn->dn_owner == id) && (odn->dn_filp == filp)) {
237			odn->dn_fd = fd;
238			odn->dn_mask |= mask;
239			return -EEXIST;
240		}
241		odn = odn->dn_next;
242	}
243
244	dn->dn_mask = mask;
245	dn->dn_fd = fd;
246	dn->dn_filp = filp;
247	dn->dn_owner = id;
248	dn->dn_next = dn_mark->dn;
249	dn_mark->dn = dn;
250
251	return 0;
252}
253
254/*
255 * When a process calls fcntl to attach a dnotify watch to a directory it ends
256 * up here.  Allocate both a mark for fsnotify to add and a dnotify_struct to be
257 * attached to the fsnotify_mark.
258 */
259int fcntl_dirnotify(int fd, struct file *filp, unsigned int arg)
260{
261	struct dnotify_mark *new_dn_mark, *dn_mark;
262	struct fsnotify_mark *new_fsn_mark, *fsn_mark;
263	struct dnotify_struct *dn;
264	struct inode *inode;
265	fl_owner_t id = current->files;
266	struct file *f = NULL;
267	int destroy = 0, error = 0;
268	__u32 mask;
269
270	/* we use these to tell if we need to kfree */
271	new_fsn_mark = NULL;
272	dn = NULL;
273
274	if (!dir_notify_enable) {
275		error = -EINVAL;
276		goto out_err;
277	}
278
279	/* a 0 mask means we are explicitly removing the watch */
280	if ((arg & ~DN_MULTISHOT) == 0) {
281		dnotify_flush(filp, id);
282		error = 0;
283		goto out_err;
284	}
285
286	/* dnotify only works on directories */
287	inode = file_inode(filp);
288	if (!S_ISDIR(inode->i_mode)) {
289		error = -ENOTDIR;
290		goto out_err;
291	}
292
293	/*
294	 * convert the userspace DN_* "arg" to the internal FS_*
295	 * defined in fsnotify
296	 */
297	mask = convert_arg(arg);
298
299	error = security_path_notify(&filp->f_path, mask,
300			FSNOTIFY_OBJ_TYPE_INODE);
301	if (error)
302		goto out_err;
303
304	/* expect most fcntl to add new rather than augment old */
305	dn = kmem_cache_alloc(dnotify_struct_cache, GFP_KERNEL);
306	if (!dn) {
307		error = -ENOMEM;
308		goto out_err;
309	}
310
311	error = file_f_owner_allocate(filp);
312	if (error)
313		goto out_err;
314
315	/* new fsnotify mark, we expect most fcntl calls to add a new mark */
316	new_dn_mark = kmem_cache_alloc(dnotify_mark_cache, GFP_KERNEL);
317	if (!new_dn_mark) {
318		error = -ENOMEM;
319		goto out_err;
320	}
321
322	/* set up the new_fsn_mark and new_dn_mark */
323	new_fsn_mark = &new_dn_mark->fsn_mark;
324	fsnotify_init_mark(new_fsn_mark, dnotify_group);
325	new_fsn_mark->mask = mask;
326	new_dn_mark->dn = NULL;
327
328	/* this is needed to prevent the fcntl/close race described below */
329	fsnotify_group_lock(dnotify_group);
330
331	/* add the new_fsn_mark or find an old one. */
332	fsn_mark = fsnotify_find_inode_mark(inode, dnotify_group);
333	if (fsn_mark) {
334		dn_mark = container_of(fsn_mark, struct dnotify_mark, fsn_mark);
335		spin_lock(&fsn_mark->lock);
336	} else {
337		error = fsnotify_add_inode_mark_locked(new_fsn_mark, inode, 0);
338		if (error) {
339			fsnotify_group_unlock(dnotify_group);
340			goto out_err;
341		}
342		spin_lock(&new_fsn_mark->lock);
343		fsn_mark = new_fsn_mark;
344		dn_mark = new_dn_mark;
345		/* we used new_fsn_mark, so don't free it */
346		new_fsn_mark = NULL;
347	}
348
349	f = fget_raw(fd);
350
351	/* if (f != filp) means that we lost a race and another task/thread
352	 * actually closed the fd we are still playing with before we grabbed
353	 * the dnotify_groups mark_mutex and fsn_mark->lock.  Since closing the
354	 * fd is the only time we clean up the marks we need to get our mark
355	 * off the list. */
356	if (f != filp) {
357		/* if we added ourselves, shoot ourselves, it's possible that
358		 * the flush actually did shoot this fsn_mark.  That's fine too
359		 * since multiple calls to destroy_mark is perfectly safe, if
360		 * we found a dn_mark already attached to the inode, just sod
361		 * off silently as the flush at close time dealt with it.
362		 */
363		if (dn_mark == new_dn_mark)
364			destroy = 1;
365		error = 0;
366		goto out;
367	}
368
369	__f_setown(filp, task_pid(current), PIDTYPE_TGID, 0);
370
371	error = attach_dn(dn, dn_mark, id, fd, filp, mask);
372	/* !error means that we attached the dn to the dn_mark, so don't free it */
373	if (!error)
374		dn = NULL;
375	/* -EEXIST means that we didn't add this new dn and used an old one.
376	 * that isn't an error (and the unused dn should be freed) */
377	else if (error == -EEXIST)
378		error = 0;
379
380	dnotify_recalc_inode_mask(fsn_mark);
381out:
382	spin_unlock(&fsn_mark->lock);
383
384	if (destroy)
385		fsnotify_detach_mark(fsn_mark);
386	fsnotify_group_unlock(dnotify_group);
387	if (destroy)
388		fsnotify_free_mark(fsn_mark);
389	fsnotify_put_mark(fsn_mark);
390out_err:
391	if (new_fsn_mark)
392		fsnotify_put_mark(new_fsn_mark);
393	if (dn)
394		kmem_cache_free(dnotify_struct_cache, dn);
395	if (f)
396		fput(f);
397	return error;
398}
399
400static int __init dnotify_init(void)
401{
402	dnotify_struct_cache = KMEM_CACHE(dnotify_struct,
403					  SLAB_PANIC|SLAB_ACCOUNT);
404	dnotify_mark_cache = KMEM_CACHE(dnotify_mark, SLAB_PANIC|SLAB_ACCOUNT);
405
406	dnotify_group = fsnotify_alloc_group(&dnotify_fsnotify_ops, 0);
407	if (IS_ERR(dnotify_group))
408		panic("unable to allocate fsnotify group for dnotify\n");
409	dnotify_sysctl_init();
410	return 0;
411}
412
413module_init(dnotify_init)