projects/linux·blob master · refs

413 lines · 9.5 KiB

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#include <linux/rculist.h>
21
22/*
23 * Read-mostly filesystem drivers list.
24 *
25 * Readers walk under rcu_read_lock(); writers take file_systems_lock
26 * and publish via _rcu hlist primitives. unregister_filesystem()
27 * synchronize_rcu()s after unlock so the embedded file_system_type
28 * can't go away under a reader. To keep using a filesystem after
29 * the RCU section ends, take a module reference via try_module_get().
30 */
31static HLIST_HEAD(file_systems);
32static DEFINE_SPINLOCK(file_systems_lock);
33
34#ifdef CONFIG_PROC_FS
35/*
36 * Cache a stringified version of the filesystem list.
37 *
38 * The fs list gets queried a lot by userspace because of libselinux, including
39 * rather surprising programs (would you guess *sed* is on the list?). In order
40 * to reduce the overhead we cache the resulting string, which normally hangs
41 * around below 512 bytes in size.
42 *
43 * As the list almost never changes, its creation is not particularly optimized
44 * to keep things simple.
45 *
46 * We sort it out on read in order to not introduce a failure point for fs
47 * registration (in principle we may be unable to alloc memory for the list).
48 */
49struct file_systems_string {
50 struct rcu_head rcu;
51 unsigned long gen;
52 size_t len;
53 char string[];
54};
55
56static unsigned long file_systems_gen;
57static struct file_systems_string __read_mostly __rcu *file_systems_string;
58
59static void invalidate_filesystems_string(void);
60#else
61static inline void invalidate_filesystems_string(void) { }
62#endif
63
64/* WARNING: This can be used only if we _already_ own a reference */
65struct file_system_type *get_filesystem(struct file_system_type *fs)
66{
67 __module_get(fs->owner);
68 return fs;
69}
70
71void put_filesystem(struct file_system_type *fs)
72{
73 module_put(fs->owner);
74}
75
76static struct file_system_type *find_filesystem(const char *name, unsigned len)
77{
78 struct file_system_type *fs;
79
80 hlist_for_each_entry_rcu(fs, &file_systems, list,
81 lockdep_is_held(&file_systems_lock))
82 if (strncmp(fs->name, name, len) == 0 && !fs->name[len])
83 return fs;
84 return NULL;
85}
86
87/**
88 * register_filesystem - register a new filesystem
89 * @fs: the file system structure
90 *
91 * Adds the file system passed to the list of file systems the kernel
92 * is aware of for mount and other syscalls. Returns 0 on success,
93 * or a negative errno code on an error.
94 *
95 * The &struct file_system_type that is passed is linked into the kernel
96 * structures and must not be freed until the file system has been
97 * unregistered.
98 */
99int register_filesystem(struct file_system_type *fs)
100{
101 if (fs->parameters &&
102 !fs_validate_description(fs->name, fs->parameters))
103 return -EINVAL;
104
105 BUG_ON(strchr(fs->name, '.'));
106 if (!hlist_unhashed_lockless(&fs->list))
107 return -EBUSY;
108
109 guard(spinlock)(&file_systems_lock);
110 if (find_filesystem(fs->name, strlen(fs->name)))
111 return -EBUSY;
112 hlist_add_tail_rcu(&fs->list, &file_systems);
113 invalidate_filesystems_string();
114 return 0;
115}
116EXPORT_SYMBOL(register_filesystem);
117
118/**
119 * unregister_filesystem - unregister a file system
120 * @fs: filesystem to unregister
121 *
122 * Remove a file system that was previously successfully registered
123 * with the kernel. An error is returned if the file system is not found.
124 * Zero is returned on a success.
125 *
126 * Once this function has returned the &struct file_system_type structure
127 * may be freed or reused.
128 */
129int unregister_filesystem(struct file_system_type *fs)
130{
131 scoped_guard(spinlock, &file_systems_lock) {
132 if (hlist_unhashed(&fs->list))
133 return -EINVAL;
134 hlist_del_init_rcu(&fs->list);
135 invalidate_filesystems_string();
136 }
137 synchronize_rcu();
138 return 0;
139}
140EXPORT_SYMBOL(unregister_filesystem);
141
142#ifdef CONFIG_SYSFS_SYSCALL
143static int fs_index(const char __user *__name)
144{
145 struct file_system_type *p;
146 char *name __free(kfree) = strndup_user(__name, PATH_MAX);
147 int index = 0;
148
149 if (IS_ERR(name))
150 return PTR_ERR(name);
151
152 guard(rcu)();
153 hlist_for_each_entry_rcu(p, &file_systems, list) {
154 if (strcmp(p->name, name) == 0)
155 return index;
156 index++;
157 }
158 return -EINVAL;
159}
160
161static int fs_name(unsigned int index, char __user *buf)
162{
163 struct file_system_type *p, *found = NULL;
164 int len, res;
165
166 scoped_guard(rcu) {
167 hlist_for_each_entry_rcu(p, &file_systems, list) {
168 if (index--)
169 continue;
170 if (try_module_get(p->owner))
171 found = p;
172 break;
173 }
174 }
175 if (!found)
176 return -EINVAL;
177
178 /* OK, we got the reference, so we can safely block */
179 len = strlen(found->name) + 1;
180 res = copy_to_user(buf, found->name, len) ? -EFAULT : 0;
181 put_filesystem(found);
182 return res;
183}
184
185static int fs_maxindex(void)
186{
187 struct file_system_type *p;
188 int index = 0;
189
190 guard(rcu)();
191 hlist_for_each_entry_rcu(p, &file_systems, list)
192 index++;
193 return index;
194}
195
196/*
197 * Whee.. Weird sysv syscall.
198 */
199SYSCALL_DEFINE3(sysfs, int, option, unsigned long, arg1, unsigned long, arg2)
200{
201 int retval = -EINVAL;
202
203 switch (option) {
204 case 1:
205 retval = fs_index((const char __user *) arg1);
206 break;
207
208 case 2:
209 retval = fs_name(arg1, (char __user *) arg2);
210 break;
211
212 case 3:
213 retval = fs_maxindex();
214 break;
215 }
216 return retval;
217}
218#endif
219
220int __init list_bdev_fs_names(char *buf, size_t size)
221{
222 struct file_system_type *p;
223 size_t len;
224 int count = 0;
225
226 guard(rcu)();
227 hlist_for_each_entry_rcu(p, &file_systems, list) {
228 if (!(p->fs_flags & FS_REQUIRES_DEV))
229 continue;
230 len = strlen(p->name) + 1;
231 if (len > size) {
232 pr_warn("%s: truncating file system list\n", __func__);
233 break;
234 }
235 memcpy(buf, p->name, len);
236 buf += len;
237 size -= len;
238 count++;
239 }
240 return count;
241}
242
243#ifdef CONFIG_PROC_FS
244static void invalidate_filesystems_string(void)
245{
246 struct file_systems_string *old;
247
248 lockdep_assert_held_write(&file_systems_lock);
249 file_systems_gen++;
250 old = rcu_replace_pointer(file_systems_string, NULL,
251 lockdep_is_held(&file_systems_lock));
252 if (old)
253 kfree_rcu(old, rcu);
254}
255
256static __cold noinline int regen_filesystems_string(void)
257{
258 struct file_system_type *p;
259 struct file_systems_string *old, *new;
260 size_t newlen, usedlen;
261 unsigned long gen;
262
263retry:
264 newlen = 0;
265
266 /* pre-calc space for each fs */
267 spin_lock(&file_systems_lock);
268 gen = file_systems_gen;
269 hlist_for_each_entry_rcu(p, &file_systems, list) {
270 if (!(p->fs_flags & FS_REQUIRES_DEV))
271 newlen += strlen("nodev");
272 newlen += strlen("\t") + strlen(p->name) + strlen("\n");
273 }
274 spin_unlock(&file_systems_lock);
275
276 new = kmalloc(offsetof(struct file_systems_string, string) + newlen + 1,
277 GFP_KERNEL);
278 if (!new)
279 return -ENOMEM;
280
281 new->gen = gen;
282 new->len = newlen;
283 new->string[newlen] = '\0';
284
285 spin_lock(&file_systems_lock);
286 old = file_systems_string;
287
288 /*
289 * Did someone beat us to it?
290 */
291 if (old && old->gen == file_systems_gen) {
292 spin_unlock(&file_systems_lock);
293 kfree(new);
294 return 0;
295 }
296
297 /*
298 * Did the list change in the meantime?
299 */
300 if (gen != file_systems_gen) {
301 spin_unlock(&file_systems_lock);
302 kfree(new);
303 goto retry;
304 }
305
306 /*
307 * Populate the string.
308 *
309 * We know we have just enough space because we calculated the right
310 * size the previous time we had the lock and confirmed the list has
311 * not changed after reacquiring it.
312 */
313 usedlen = 0;
314 hlist_for_each_entry_rcu(p, &file_systems, list) {
315 usedlen += sprintf(&new->string[usedlen], "%s\t%s\n",
316 (p->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
317 p->name);
318 }
319
320 if (WARN_ON_ONCE(new->len != strlen(new->string))) {
321 /*
322 * Should never happen of course, keep this in case someone changes string
323 * generation above and messes it up.
324 */
325 spin_unlock(&file_systems_lock);
326 kfree(new);
327 return -EINVAL;
328 }
329
330 rcu_assign_pointer(file_systems_string, new);
331 spin_unlock(&file_systems_lock);
332 if (old)
333 kfree_rcu(old, rcu);
334 return 0;
335}
336
337static __cold noinline int filesystems_proc_show_fallback(struct seq_file *m, void *v)
338{
339 struct file_system_type *p;
340
341 guard(rcu)();
342 hlist_for_each_entry_rcu(p, &file_systems, list) {
343 seq_printf(m, "%s\t%s\n",
344 (p->fs_flags & FS_REQUIRES_DEV) ? "" : "nodev",
345 p->name);
346 }
347 return 0;
348}
349
350static int filesystems_proc_show(struct seq_file *m, void *v)
351{
352 struct file_systems_string *fss;
353
354 for (;;) {
355 scoped_guard(rcu) {
356 fss = rcu_dereference(file_systems_string);
357 if (likely(fss)) {
358 seq_write(m, fss->string, fss->len);
359 return 0;
360 }
361 }
362
363 int err = regen_filesystems_string();
364 if (unlikely(err))
365 return filesystems_proc_show_fallback(m, v);
366 }
367}
368
369static int __init proc_filesystems_init(void)
370{
371 struct proc_dir_entry *pde;
372
373 pde = proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
374 if (!pde)
375 return -ENOMEM;
376 proc_make_permanent(pde);
377 return 0;
378}
379module_init(proc_filesystems_init);
380#endif
381
382static struct file_system_type *__get_fs_type(const char *name, int len)
383{
384 struct file_system_type *fs;
385
386 guard(rcu)();
387 fs = find_filesystem(name, len);
388 if (fs && !try_module_get(fs->owner))
389 fs = NULL;
390 return fs;
391}
392
393struct file_system_type *get_fs_type(const char *name)
394{
395 struct file_system_type *fs;
396 const char *dot = strchr(name, '.');
397 int len = dot ? dot - name : strlen(name);
398
399 fs = __get_fs_type(name, len);
400 if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
401 fs = __get_fs_type(name, len);
402 if (!fs)
403 pr_warn_once("request_module fs-%.*s succeeded, but still no fs?\n",
404 len, name);
405 }
406
407 if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
408 put_filesystem(fs);
409 fs = NULL;
410 }
411 return fs;
412}
413EXPORT_SYMBOL(get_fs_type);