projects/linux·blob master · refs

450 lines · 11.5 KiB

1/* SPDX-License-Identifier: GPL-2.0 */
2#include <linux/syscalls.h>
3#include <linux/export.h>
4#include <linux/uaccess.h>
5#include <linux/fs_struct.h>
6#include <linux/fs.h>
7#include <linux/slab.h>
8#include <linux/prefetch.h>
9#include "mount.h"
10#include "internal.h"
11
12struct prepend_buffer {
13 char *buf;
14 int len;
15};
16#define DECLARE_BUFFER(__name, __buf, __len) \
17 struct prepend_buffer __name = {.buf = __buf + __len, .len = __len}
18
19static char *extract_string(struct prepend_buffer *p)
20{
21 if (likely(p->len >= 0))
22 return p->buf;
23 return ERR_PTR(-ENAMETOOLONG);
24}
25
26static bool prepend_char(struct prepend_buffer *p, unsigned char c)
27{
28 if (likely(p->len > 0)) {
29 p->len--;
30 *--p->buf = c;
31 return true;
32 }
33 p->len = -1;
34 return false;
35}
36
37/*
38 * The source of the prepend data can be an optimistic load
39 * of a dentry name and length. And because we don't hold any
40 * locks, the length and the pointer to the name may not be
41 * in sync if a concurrent rename happens, and the kernel
42 * copy might fault as a result.
43 *
44 * The end result will correct itself when we check the
45 * rename sequence count, but we need to be able to handle
46 * the fault gracefully.
47 */
48static bool prepend_copy(void *dst, const void *src, int len)
49{
50 if (unlikely(copy_from_kernel_nofault(dst, src, len))) {
51 memset(dst, 'x', len);
52 return false;
53 }
54 return true;
55}
56
57static bool prepend(struct prepend_buffer *p, const char *str, int namelen)
58{
59 // Already overflowed?
60 if (p->len < 0)
61 return false;
62
63 // Will overflow?
64 if (p->len < namelen) {
65 // Fill as much as possible from the end of the name
66 str += namelen - p->len;
67 p->buf -= p->len;
68 prepend_copy(p->buf, str, p->len);
69 p->len = -1;
70 return false;
71 }
72
73 // Fits fully
74 p->len -= namelen;
75 p->buf -= namelen;
76 return prepend_copy(p->buf, str, namelen);
77}
78
79/**
80 * prepend_name - prepend a pathname in front of current buffer pointer
81 * @p: prepend buffer which contains buffer pointer and allocated length
82 * @name: name string and length qstr structure
83 *
84 * With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
85 * make sure that either the old or the new name pointer and length are
86 * fetched. However, there may be mismatch between length and pointer.
87 * But since the length cannot be trusted, we need to copy the name very
88 * carefully when doing the prepend_copy(). It also prepends "/" at
89 * the beginning of the name. The sequence number check at the caller will
90 * retry it again when a d_move() does happen. So any garbage in the buffer
91 * due to mismatched pointer and length will be discarded.
92 *
93 * Load acquire is needed to make sure that we see the new name data even
94 * if we might get the length wrong.
95 */
96static bool prepend_name(struct prepend_buffer *p, const struct qstr *name)
97{
98 const char *dname = smp_load_acquire(&name->name); /* ^^^ */
99 u32 dlen = READ_ONCE(name->len);
100
101 return prepend(p, dname, dlen) && prepend_char(p, '/');
102}
103
104static int __prepend_path(const struct dentry *dentry, const struct mount *mnt,
105 const struct path *root, struct prepend_buffer *p)
106{
107 while (dentry != root->dentry || &mnt->mnt != root->mnt) {
108 const struct dentry *parent = READ_ONCE(dentry->d_parent);
109
110 if (dentry == mnt->mnt.mnt_root) {
111 struct mount *m = READ_ONCE(mnt->mnt_parent);
112 struct mnt_namespace *mnt_ns;
113
114 if (likely(mnt != m)) {
115 dentry = READ_ONCE(mnt->mnt_mountpoint);
116 mnt = m;
117 continue;
118 }
119 /* Global root */
120 mnt_ns = READ_ONCE(mnt->mnt_ns);
121 /* open-coded is_mounted() to use local mnt_ns */
122 if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
123 return 1; // absolute root
124 else
125 return 2; // detached or not attached yet
126 }
127
128 if (unlikely(dentry == parent))
129 /* Escaped? */
130 return 3;
131
132 prefetch(parent);
133 if (!prepend_name(p, &dentry->d_name))
134 break;
135 dentry = parent;
136 }
137 return 0;
138}
139
140/**
141 * prepend_path - Prepend path string to a buffer
142 * @path: the dentry/vfsmount to report
143 * @root: root vfsmnt/dentry
144 * @p: prepend buffer which contains buffer pointer and allocated length
145 *
146 * The function will first try to write out the pathname without taking any
147 * lock other than the RCU read lock to make sure that dentries won't go away.
148 * It only checks the sequence number of the global rename_lock as any change
149 * in the dentry's d_seq will be preceded by changes in the rename_lock
150 * sequence number. If the sequence number had been changed, it will restart
151 * the whole pathname back-tracing sequence again by taking the rename_lock.
152 * In this case, there is no need to take the RCU read lock as the recursive
153 * parent pointer references will keep the dentry chain alive as long as no
154 * rename operation is performed.
155 */
156static int prepend_path(const struct path *path,
157 const struct path *root,
158 struct prepend_buffer *p)
159{
160 unsigned seq, m_seq = 0;
161 struct prepend_buffer b;
162 int error;
163
164 rcu_read_lock();
165restart_mnt:
166 read_seqbegin_or_lock(&mount_lock, &m_seq);
167 seq = 0;
168 rcu_read_lock();
169restart:
170 b = *p;
171 read_seqbegin_or_lock(&rename_lock, &seq);
172 error = __prepend_path(path->dentry, real_mount(path->mnt), root, &b);
173 if (!(seq & 1))
174 rcu_read_unlock();
175 if (need_seqretry(&rename_lock, seq)) {
176 seq = 1;
177 goto restart;
178 }
179 done_seqretry(&rename_lock, seq);
180
181 if (!(m_seq & 1))
182 rcu_read_unlock();
183 if (need_seqretry(&mount_lock, m_seq)) {
184 m_seq = 1;
185 goto restart_mnt;
186 }
187 done_seqretry(&mount_lock, m_seq);
188
189 if (unlikely(error == 3))
190 b = *p;
191
192 if (b.len == p->len)
193 prepend_char(&b, '/');
194
195 *p = b;
196 return error;
197}
198
199/**
200 * __d_path - return the path of a dentry
201 * @path: the dentry/vfsmount to report
202 * @root: root vfsmnt/dentry
203 * @buf: buffer to return value in
204 * @buflen: buffer length
205 *
206 * Convert a dentry into an ASCII path name.
207 *
208 * Returns a pointer into the buffer or an error code if the
209 * path was too long.
210 *
211 * "buflen" should be positive.
212 *
213 * If the path is not reachable from the supplied root, return %NULL.
214 */
215char *__d_path(const struct path *path,
216 const struct path *root,
217 char *buf, int buflen)
218{
219 DECLARE_BUFFER(b, buf, buflen);
220
221 prepend_char(&b, 0);
222 if (unlikely(prepend_path(path, root, &b) > 0))
223 return NULL;
224 return extract_string(&b);
225}
226
227char *d_absolute_path(const struct path *path,
228 char *buf, int buflen)
229{
230 struct path root = {};
231 DECLARE_BUFFER(b, buf, buflen);
232
233 prepend_char(&b, 0);
234 if (unlikely(prepend_path(path, &root, &b) > 1))
235 return ERR_PTR(-EINVAL);
236 return extract_string(&b);
237}
238
239static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
240{
241 unsigned seq;
242
243 do {
244 seq = read_seqbegin(&fs->seq);
245 *root = fs->root;
246 } while (read_seqretry(&fs->seq, seq));
247}
248
249/**
250 * d_path - return the path of a dentry
251 * @path: path to report
252 * @buf: buffer to return value in
253 * @buflen: buffer length
254 *
255 * Convert a dentry into an ASCII path name. If the entry has been deleted
256 * the string " (deleted)" is appended. Note that this is ambiguous.
257 *
258 * Returns a pointer into the buffer or an error code if the path was
259 * too long. Note: Callers should use the returned pointer, not the passed
260 * in buffer, to use the name! The implementation often starts at an offset
261 * into the buffer, and may leave 0 bytes at the start.
262 *
263 * "buflen" should be positive.
264 */
265char *d_path(const struct path *path, char *buf, int buflen)
266{
267 DECLARE_BUFFER(b, buf, buflen);
268 struct path root;
269
270 /*
271 * We have various synthetic filesystems that never get mounted. On
272 * these filesystems dentries are never used for lookup purposes, and
273 * thus don't need to be hashed. They also don't need a name until a
274 * user wants to identify the object in /proc/pid/fd/. The little hack
275 * below allows us to generate a name for these objects on demand:
276 *
277 * Some pseudo inodes are mountable. When they are mounted
278 * path->dentry == path->mnt->mnt_root. In that case don't call d_dname
279 * and instead have d_path return the mounted path.
280 */
281 if (path->dentry->d_op && path->dentry->d_op->d_dname &&
282 (!IS_ROOT(path->dentry) || path->dentry != path->mnt->mnt_root ||
283 failfs_mnt(path->mnt)))
284 return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
285
286 rcu_read_lock();
287 get_fs_root_rcu(current->fs, &root);
288 if (unlikely(d_unlinked(path->dentry)))
289 prepend(&b, " (deleted)", 11);
290 else
291 prepend_char(&b, 0);
292 prepend_path(path, &root, &b);
293 rcu_read_unlock();
294
295 return extract_string(&b);
296}
297EXPORT_SYMBOL(d_path);
298
299/*
300 * Helper function for dentry_operations.d_dname() members
301 */
302char *dynamic_dname(char *buffer, int buflen, const char *fmt, ...)
303{
304 va_list args;
305 char *start;
306 int sz;
307
308 va_start(args, fmt);
309 sz = vsnprintf(buffer, buflen, fmt, args) + 1;
310 va_end(args);
311
312 if (sz > NAME_MAX || sz > buflen)
313 return ERR_PTR(-ENAMETOOLONG);
314
315 /* Move the formatted d_name to the end of the buffer. */
316 start = buffer + (buflen - sz);
317 return memmove(start, buffer, sz);
318}
319
320char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
321{
322 DECLARE_BUFFER(b, buffer, buflen);
323 /* these dentries are never renamed, so d_lock is not needed */
324 prepend(&b, " (deleted)", 11);
325 prepend(&b, dentry->d_name.name, dentry->d_name.len);
326 prepend_char(&b, '/');
327 return extract_string(&b);
328}
329
330/*
331 * Write full pathname from the root of the filesystem into the buffer.
332 */
333static char *__dentry_path(const struct dentry *d, struct prepend_buffer *p)
334{
335 const struct dentry *dentry;
336 struct prepend_buffer b;
337 int seq = 0;
338
339 rcu_read_lock();
340restart:
341 dentry = d;
342 b = *p;
343 read_seqbegin_or_lock(&rename_lock, &seq);
344 while (!IS_ROOT(dentry)) {
345 const struct dentry *parent = dentry->d_parent;
346
347 prefetch(parent);
348 if (!prepend_name(&b, &dentry->d_name))
349 break;
350 dentry = parent;
351 }
352 if (!(seq & 1))
353 rcu_read_unlock();
354 if (need_seqretry(&rename_lock, seq)) {
355 seq = 1;
356 goto restart;
357 }
358 done_seqretry(&rename_lock, seq);
359 if (b.len == p->len)
360 prepend_char(&b, '/');
361 return extract_string(&b);
362}
363
364char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
365{
366 DECLARE_BUFFER(b, buf, buflen);
367
368 prepend_char(&b, 0);
369 return __dentry_path(dentry, &b);
370}
371EXPORT_SYMBOL(dentry_path_raw);
372
373char *dentry_path(const struct dentry *dentry, char *buf, int buflen)
374{
375 DECLARE_BUFFER(b, buf, buflen);
376
377 if (unlikely(d_unlinked(dentry)))
378 prepend(&b, "//deleted", 10);
379 else
380 prepend_char(&b, 0);
381 return __dentry_path(dentry, &b);
382}
383
384static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
385 struct path *pwd)
386{
387 unsigned seq;
388
389 do {
390 seq = read_seqbegin(&fs->seq);
391 *root = fs->root;
392 *pwd = fs->pwd;
393 } while (read_seqretry(&fs->seq, seq));
394}
395
396/*
397 * NOTE! The user-level library version returns a
398 * character pointer. The kernel system call just
399 * returns the length of the buffer filled (which
400 * includes the ending '\0' character), or a negative
401 * error value. So libc would do something like
402 *
403 * char *getcwd(char * buf, size_t size)
404 * {
405 * int retval;
406 *
407 * retval = sys_getcwd(buf, size);
408 * if (retval >= 0)
409 * return buf;
410 * errno = -retval;
411 * return NULL;
412 * }
413 */
414SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
415{
416 int error;
417 struct path pwd, root;
418 char *page = __getname();
419
420 if (!page)
421 return -ENOMEM;
422
423 rcu_read_lock();
424 get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);
425
426 if (unlikely(d_unlinked(pwd.dentry))) {
427 rcu_read_unlock();
428 error = -ENOENT;
429 } else {
430 unsigned len;
431 DECLARE_BUFFER(b, page, PATH_MAX);
432
433 prepend_char(&b, 0);
434 if (unlikely(prepend_path(&pwd, &root, &b) > 0))
435 prepend(&b, "(unreachable)", 13);
436 rcu_read_unlock();
437
438 len = PATH_MAX - b.len;
439 if (unlikely(len > PATH_MAX))
440 error = -ENAMETOOLONG;
441 else if (unlikely(len > size))
442 error = -ERANGE;
443 else if (copy_to_user(buf, b.buf, len))
444 error = -EFAULT;
445 else
446 error = len;
447 }
448 __putname(page);
449 return error;
450}