1// SPDX-License-Identifier: GPL-2.0
2/*
3 *  linux/fs/filesystems.c
4 *
5 *  Copyright (C) 1991, 1992  Linus Torvalds
6 *
7 *  table of configured filesystems
8 */
9
10#include <linux/syscalls.h>
11#include <linux/fs.h>
12#include <linux/proc_fs.h>
13#include <linux/seq_file.h>
14#include <linux/kmod.h>
15#include <linux/init.h>
16#include <linux/module.h>
17#include <linux/slab.h>
18#include <linux/uaccess.h>
19#include <linux/fs_parser.h>
20
21/*
22 * Handling of filesystem drivers list.
23 * Rules:
24 *	Inclusion to/removals from/scanning of list are protected by spinlock.
25 *	During the unload module must call unregister_filesystem().
26 *	We can access the fields of list element if:
27 *		1) spinlock is held or
28 *		2) we hold the reference to the module.
29 *	The latter can be guaranteed by call of try_module_get(); if it
30 *	returned 0 we must skip the element, otherwise we got the reference.
31 *	Once the reference is obtained we can drop the spinlock.
32 */
33
34static struct file_system_type *file_systems;
35static DEFINE_RWLOCK(file_systems_lock);
36
37/* WARNING: This can be used only if we _already_ own a reference */
38struct file_system_type *get_filesystem(struct file_system_type *fs)
39{
40	__module_get(fs->owner);
41	return fs;
42}
43
44void put_filesystem(struct file_system_type *fs)
45{
46	module_put(fs->owner);
47}
48
49static struct file_system_type **find_filesystem(const char *name, unsigned len)
50{
51	struct file_system_type **p;
52	for (p = &file_systems; *p; p = &(*p)->next)
53		if (strncmp((*p)->name, name, len) == 0 &&
54		    !(*p)->name[len])
55			break;
56	return p;
57}
58
59/**
60 *	register_filesystem - register a new filesystem
61 *	@fs: the file system structure
62 *
63 *	Adds the file system passed to the list of file systems the kernel
64 *	is aware of for mount and other syscalls. Returns 0 on success,
65 *	or a negative errno code on an error.
66 *
67 *	The &struct file_system_type that is passed is linked into the kernel
68 *	structures and must not be freed until the file system has been
69 *	unregistered.
70 */
71
72int register_filesystem(struct file_system_type * fs)
73{
74	int res = 0;
75	struct file_system_type ** p;
76
77	if (fs->parameters &&
78	    !fs_validate_description(fs->name, fs->parameters))
79		return -EINVAL;
80
81	BUG_ON(strchr(fs->name, '.'));
82	if (fs->next)
83		return -EBUSY;
84	write_lock(&file_systems_lock);
85	p = find_filesystem(fs->name, strlen(fs->name));
86	if (*p)
87		res = -EBUSY;
88	else
89		*p = fs;
90	write_unlock(&file_systems_lock);
91	return res;
92}
93
94EXPORT_SYMBOL(register_filesystem);
95
96/**
97 *	unregister_filesystem - unregister a file system
98 *	@fs: filesystem to unregister
99 *
100 *	Remove a file system that was previously successfully registered
101 *	with the kernel. An error is returned if the file system is not found.
102 *	Zero is returned on a success.
103 *
104 *	Once this function has returned the &struct file_system_type structure
105 *	may be freed or reused.
106 */
107
108int unregister_filesystem(struct file_system_type * fs)
109{
110	struct file_system_type ** tmp;
111
112	write_lock(&file_systems_lock);
113	tmp = &file_systems;
114	while (*tmp) {
115		if (fs == *tmp) {
116			*tmp = fs->next;
117			fs->next = NULL;
118			write_unlock(&file_systems_lock);
119			synchronize_rcu();
120			return 0;
121		}
122		tmp = &(*tmp)->next;
123	}
124	write_unlock(&file_systems_lock);
125
126	return -EINVAL;
127}
128
129EXPORT_SYMBOL(unregister_filesystem);
130
131#ifdef CONFIG_SYSFS_SYSCALL
132static int fs_index(const char __user * __name)
133{
134	struct file_system_type * tmp;
135	struct filename *name;
136	int err, index;
137
138	name = getname(__name);
139	err = PTR_ERR(name);
140	if (IS_ERR(name))
141		return err;
142
143	err = -EINVAL;
144	read_lock(&file_systems_lock);
145	for (tmp=file_systems, index=0 ; tmp ; tmp=tmp->next, index++) {
146		if (strcmp(tmp->name, name->name) == 0) {
147			err = index;
148			break;
149		}
150	}
151	read_unlock(&file_systems_lock);
152	putname(name);
153	return err;
154}
155
156static int fs_name(unsigned int index, char __user * buf)
157{
158	struct file_system_type * tmp;
159	int len, res;
160
161	read_lock(&file_systems_lock);
162	for (tmp = file_systems; tmp; tmp = tmp->next, index--)
163		if (index <= 0 && try_module_get(tmp->owner))
164			break;
165	read_unlock(&file_systems_lock);
166	if (!tmp)
167		return -EINVAL;
168
169	/* OK, we got the reference, so we can safely block */
170	len = strlen(tmp->name) + 1;
171	res = copy_to_user(buf, tmp->name, len) ? -EFAULT : 0;
172	put_filesystem(tmp);
173	return res;
174}
175
176static int fs_maxindex(void)
177{
178	struct file_system_type * tmp;
179	int index;
180
181	read_lock(&file_systems_lock);
182	for (tmp = file_systems, index = 0 ; tmp ; tmp = tmp->next, index++)
183		;
184	read_unlock(&file_systems_lock);
185	return index;
186}
187
188/*
189 * Whee.. Weird sysv syscall.
190 */
191SYSCALL_DEFINE3(sysfs, int, option, unsigned long, arg1, unsigned long, arg2)
192{
193	int retval = -EINVAL;
194
195	switch (option) {
196		case 1:
197			retval = fs_index((const char __user *) arg1);
198			break;
199
200		case 2:
201			retval = fs_name(arg1, (char __user *) arg2);
202			break;
203
204		case 3:
205			retval = fs_maxindex();
206			break;
207	}
208	return retval;
209}
210#endif
211
212int __init list_bdev_fs_names(char *buf, size_t size)
213{
214	struct file_system_type *p;
215	size_t len;
216	int count = 0;
217
218	read_lock(&file_systems_lock);
219	for (p = file_systems; p; p = p->next) {
220		if (!(p->fs_flags & FS_REQUIRES_DEV))
221			continue;
222		len = strlen(p->name) + 1;
223		if (len > size) {
224			pr_warn("%s: truncating file system list\n", __func__);
225			break;
226		}
227		memcpy(buf, p->name, len);
228		buf += len;
229		size -= len;
230		count++;
231	}
232	read_unlock(&file_systems_lock);
233	return count;
234}
235
236#ifdef CONFIG_PROC_FS
237static int filesystems_proc_show(struct seq_file *m, void *v)
238{
239	struct file_system_type * tmp;
240
241	read_lock(&file_systems_lock);
242	tmp = file_systems;
243	while (tmp) {
244		seq_printf(m, "%s\t%s\n",
245			(tmp->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
246			tmp->name);
247		tmp = tmp->next;
248	}
249	read_unlock(&file_systems_lock);
250	return 0;
251}
252
253static int __init proc_filesystems_init(void)
254{
255	proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
256	return 0;
257}
258module_init(proc_filesystems_init);
259#endif
260
261static struct file_system_type *__get_fs_type(const char *name, int len)
262{
263	struct file_system_type *fs;
264
265	read_lock(&file_systems_lock);
266	fs = *(find_filesystem(name, len));
267	if (fs && !try_module_get(fs->owner))
268		fs = NULL;
269	read_unlock(&file_systems_lock);
270	return fs;
271}
272
273struct file_system_type *get_fs_type(const char *name)
274{
275	struct file_system_type *fs;
276	const char *dot = strchr(name, '.');
277	int len = dot ? dot - name : strlen(name);
278
279	fs = __get_fs_type(name, len);
280	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
281		fs = __get_fs_type(name, len);
282		if (!fs)
283			pr_warn_once("request_module fs-%.*s succeeded, but still no fs?\n",
284				     len, name);
285	}
286
287	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
288		put_filesystem(fs);
289		fs = NULL;
290	}
291	return fs;
292}
293
294EXPORT_SYMBOL(get_fs_type);
295