fs/d_path.c

   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
  11 static int prepend(char **buffer, int *buflen, const char *str, int namelen)
  12 {
  13         *buflen -= namelen;
  14         if (*buflen < 0)
  15                 return -ENAMETOOLONG;
  16         *buffer -= namelen;
  17         memcpy(*buffer, str, namelen);
  18         return 0;
  19 }
  20
  21 /**
  22  * prepend_name - prepend a pathname in front of current buffer pointer
  23  * @buffer: buffer pointer
  24  * @buflen: allocated length of the buffer
  25  * @name:   name string and length qstr structure
  26  *
  27  * With RCU path tracing, it may race with d_move(). Use READ_ONCE() to
  28  * make sure that either the old or the new name pointer and length are
  29  * fetched. However, there may be mismatch between length and pointer.
  30  * The length cannot be trusted, we need to copy it byte-by-byte until
  31  * the length is reached or a null byte is found. It also prepends "/" at
  32  * the beginning of the name. The sequence number check at the caller will
  33  * retry it again when a d_move() does happen. So any garbage in the buffer
  34  * due to mismatched pointer and length will be discarded.
  35  *
  36  * Load acquire is needed to make sure that we see that terminating NUL.
  37  */
  38 static int prepend_name(char **buffer, int *buflen, const struct qstr *name)
  39 {
  40         const char *dname = smp_load_acquire(&name->name); /* ^^^ */
  41         u32 dlen = READ_ONCE(name->len);
  42         char *p;
  43
  44         *buflen -= dlen + 1;
  45         if (*buflen < 0)
  46                 return -ENAMETOOLONG;
  47         p = *buffer -= dlen + 1;
  48         *p++ = '/';
  49         while (dlen--) {
  50                 char c = *dname++;
  51                 if (!c)
  52                         break;
  53                 *p++ = c;
  54         }
  55         return 0;
  56 }
  57
  58 /**
  59  * prepend_path - Prepend path string to a buffer
  60  * @path: the dentry/vfsmount to report
  61  * @root: root vfsmnt/dentry
  62  * @buffer: pointer to the end of the buffer
  63  * @buflen: pointer to buffer length
  64  *
  65  * The function will first try to write out the pathname without taking any
  66  * lock other than the RCU read lock to make sure that dentries won't go away.
  67  * It only checks the sequence number of the global rename_lock as any change
  68  * in the dentry's d_seq will be preceded by changes in the rename_lock
  69  * sequence number. If the sequence number had been changed, it will restart
  70  * the whole pathname back-tracing sequence again by taking the rename_lock.
  71  * In this case, there is no need to take the RCU read lock as the recursive
  72  * parent pointer references will keep the dentry chain alive as long as no
  73  * rename operation is performed.
  74  */
  75 static int prepend_path(const struct path *path,
  76                         const struct path *root,
  77                         char **buffer, int *buflen)
  78 {
  79         struct dentry *dentry;
  80         struct vfsmount *vfsmnt;
  81         struct mount *mnt;
  82         int error = 0;
  83         unsigned seq, m_seq = 0;
  84         char *bptr;
  85         int blen;
  86
  87         rcu_read_lock();
  88 restart_mnt:
  89         read_seqbegin_or_lock(&mount_lock, &m_seq);
  90         seq = 0;
  91         rcu_read_lock();
  92 restart:
  93         bptr = *buffer;
  94         blen = *buflen;
  95         error = 0;
  96         dentry = path->dentry;
  97         vfsmnt = path->mnt;
  98         mnt = real_mount(vfsmnt);
  99         read_seqbegin_or_lock(&rename_lock, &seq);
 100         while (dentry != root->dentry || vfsmnt != root->mnt) {
 101                 struct dentry * parent;
 102
 103                 if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
 104                         struct mount *parent = READ_ONCE(mnt->mnt_parent);
 105                         /* Escaped? */
 106                         if (dentry != vfsmnt->mnt_root) {
 107                                 bptr = *buffer;
 108                                 blen = *buflen;
 109                                 error = 3;
 110                                 break;
 111                         }
 112                         /* Global root? */
 113                         if (mnt != parent) {
 114                                 dentry = READ_ONCE(mnt->mnt_mountpoint);
 115                                 mnt = parent;
 116                                 vfsmnt = &mnt->mnt;
 117                                 continue;
 118                         }
 119                         if (!error)
 120                                 error = is_mounted(vfsmnt) ? 1 : 2;
 121                         break;
 122                 }
 123                 parent = dentry->d_parent;
 124                 prefetch(parent);
 125                 error = prepend_name(&bptr, &blen, &dentry->d_name);
 126                 if (error)
 127                         break;
 128
 129                 dentry = parent;
 130         }
 131         if (!(seq & 1))
 132                 rcu_read_unlock();
 133         if (need_seqretry(&rename_lock, seq)) {
 134                 seq = 1;
 135                 goto restart;
 136         }
 137         done_seqretry(&rename_lock, seq);
 138
 139         if (!(m_seq & 1))
 140                 rcu_read_unlock();
 141         if (need_seqretry(&mount_lock, m_seq)) {
 142                 m_seq = 1;
 143                 goto restart_mnt;
 144         }
 145         done_seqretry(&mount_lock, m_seq);
 146
 147         if (error >= 0 && bptr == *buffer) {
 148                 if (--blen < 0)
 149                         error = -ENAMETOOLONG;
 150                 else
 151                         *--bptr = '/';
 152         }
 153         *buffer = bptr;
 154         *buflen = blen;
 155         return error;
 156 }
 157
 158 /**
 159  * __d_path - return the path of a dentry
 160  * @path: the dentry/vfsmount to report
 161  * @root: root vfsmnt/dentry
 162  * @buf: buffer to return value in
 163  * @buflen: buffer length
 164  *
 165  * Convert a dentry into an ASCII path name.
 166  *
 167  * Returns a pointer into the buffer or an error code if the
 168  * path was too long.
 169  *
 170  * "buflen" should be positive.
 171  *
 172  * If the path is not reachable from the supplied root, return %NULL.
 173  */
 174 char *__d_path(const struct path *path,
 175                const struct path *root,
 176                char *buf, int buflen)
 177 {
 178         char *res = buf + buflen;
 179         int error;
 180
 181         prepend(&res, &buflen, "\0", 1);
 182         error = prepend_path(path, root, &res, &buflen);
 183
 184         if (error < 0)
 185                 return ERR_PTR(error);
 186         if (error > 0)
 187                 return NULL;
 188         return res;
 189 }
 190
 191 char *d_absolute_path(const struct path *path,
 192                char *buf, int buflen)
 193 {
 194         struct path root = {};
 195         char *res = buf + buflen;
 196         int error;
 197
 198         prepend(&res, &buflen, "\0", 1);
 199         error = prepend_path(path, &root, &res, &buflen);
 200
 201         if (error > 1)
 202                 error = -EINVAL;
 203         if (error < 0)
 204                 return ERR_PTR(error);
 205         return res;
 206 }
 207
 208 /*
 209  * same as __d_path but appends "(deleted)" for unlinked files.
 210  */
 211 static int path_with_deleted(const struct path *path,
 212                              const struct path *root,
 213                              char **buf, int *buflen)
 214 {
 215         prepend(buf, buflen, "\0", 1);
 216         if (d_unlinked(path->dentry)) {
 217                 int error = prepend(buf, buflen, " (deleted)", 10);
 218                 if (error)
 219                         return error;
 220         }
 221
 222         return prepend_path(path, root, buf, buflen);
 223 }
 224
 225 static int prepend_unreachable(char **buffer, int *buflen)
 226 {
 227         return prepend(buffer, buflen, "(unreachable)", 13);
 228 }
 229
 230 static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
 231 {
 232         unsigned seq;
 233
 234         do {
 235                 seq = read_seqcount_begin(&fs->seq);
 236                 *root = fs->root;
 237         } while (read_seqcount_retry(&fs->seq, seq));
 238 }
 239
 240 /**
 241  * d_path - return the path of a dentry
 242  * @path: path to report
 243  * @buf: buffer to return value in
 244  * @buflen: buffer length
 245  *
 246  * Convert a dentry into an ASCII path name. If the entry has been deleted
 247  * the string " (deleted)" is appended. Note that this is ambiguous.
 248  *
 249  * Returns a pointer into the buffer or an error code if the path was
 250  * too long. Note: Callers should use the returned pointer, not the passed
 251  * in buffer, to use the name! The implementation often starts at an offset
 252  * into the buffer, and may leave 0 bytes at the start.
 253  *
 254  * "buflen" should be positive.
 255  */
 256 char *d_path(const struct path *path, char *buf, int buflen)
 257 {
 258         char *res = buf + buflen;
 259         struct path root;
 260         int error;
 261
 262         /*
 263          * We have various synthetic filesystems that never get mounted.  On
 264          * these filesystems dentries are never used for lookup purposes, and
 265          * thus don't need to be hashed.  They also don't need a name until a
 266          * user wants to identify the object in /proc/pid/fd/.  The little hack
 267          * below allows us to generate a name for these objects on demand:
 268          *
 269          * Some pseudo inodes are mountable.  When they are mounted
 270          * path->dentry == path->mnt->mnt_root.  In that case don't call d_dname
 271          * and instead have d_path return the mounted path.
 272          */
 273         if (path->dentry->d_op && path->dentry->d_op->d_dname &&
 274             (!IS_ROOT(path->dentry) || path->dentry != path->mnt->mnt_root))
 275                 return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
 276
 277         rcu_read_lock();
 278         get_fs_root_rcu(current->fs, &root);
 279         error = path_with_deleted(path, &root, &res, &buflen);
 280         rcu_read_unlock();
 281
 282         if (error < 0)
 283                 res = ERR_PTR(error);
 284         return res;
 285 }
 286 EXPORT_SYMBOL(d_path);
 287
 288 /*
 289  * Helper function for dentry_operations.d_dname() members
 290  */
 291 char *dynamic_dname(struct dentry *dentry, char *buffer, int buflen,
 292                         const char *fmt, ...)
 293 {
 294         va_list args;
 295         char temp[64];
 296         int sz;
 297
 298         va_start(args, fmt);
 299         sz = vsnprintf(temp, sizeof(temp), fmt, args) + 1;
 300         va_end(args);
 301
 302         if (sz > sizeof(temp) || sz > buflen)
 303                 return ERR_PTR(-ENAMETOOLONG);
 304
 305         buffer += buflen - sz;
 306         return memcpy(buffer, temp, sz);
 307 }
 308
 309 char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
 310 {
 311         char *end = buffer + buflen;
 312         /* these dentries are never renamed, so d_lock is not needed */
 313         if (prepend(&end, &buflen, " (deleted)", 11) ||
 314             prepend(&end, &buflen, dentry->d_name.name, dentry->d_name.len) ||
 315             prepend(&end, &buflen, "/", 1))
 316                 end = ERR_PTR(-ENAMETOOLONG);
 317         return end;
 318 }
 319 EXPORT_SYMBOL(simple_dname);
 320
 321 /*
 322  * Write full pathname from the root of the filesystem into the buffer.
 323  */
 324 static char *__dentry_path(struct dentry *d, char *buf, int buflen)
 325 {
 326         struct dentry *dentry;
 327         char *end, *retval;
 328         int len, seq = 0;
 329         int error = 0;
 330
 331         if (buflen < 2)
 332                 goto Elong;
 333
 334         rcu_read_lock();
 335 restart:
 336         dentry = d;
 337         end = buf + buflen;
 338         len = buflen;
 339         prepend(&end, &len, "\0", 1);
 340         /* Get '/' right */
 341         retval = end-1;
 342         *retval = '/';
 343         read_seqbegin_or_lock(&rename_lock, &seq);
 344         while (!IS_ROOT(dentry)) {
 345                 struct dentry *parent = dentry->d_parent;
 346
 347                 prefetch(parent);
 348                 error = prepend_name(&end, &len, &dentry->d_name);
 349                 if (error)
 350                         break;
 351
 352                 retval = end;
 353                 dentry = parent;
 354         }
 355         if (!(seq & 1))
 356                 rcu_read_unlock();
 357         if (need_seqretry(&rename_lock, seq)) {
 358                 seq = 1;
 359                 goto restart;
 360         }
 361         done_seqretry(&rename_lock, seq);
 362         if (error)
 363                 goto Elong;
 364         return retval;
 365 Elong:
 366         return ERR_PTR(-ENAMETOOLONG);
 367 }
 368
 369 char *dentry_path_raw(struct dentry *dentry, char *buf, int buflen)
 370 {
 371         return __dentry_path(dentry, buf, buflen);
 372 }
 373 EXPORT_SYMBOL(dentry_path_raw);
 374
 375 char *dentry_path(struct dentry *dentry, char *buf, int buflen)
 376 {
 377         char *p = NULL;
 378         char *retval;
 379
 380         if (d_unlinked(dentry)) {
 381                 p = buf + buflen;
 382                 if (prepend(&p, &buflen, "//deleted", 10) != 0)
 383                         goto Elong;
 384                 buflen++;
 385         }
 386         retval = __dentry_path(dentry, buf, buflen);
 387         if (!IS_ERR(retval) && p)
 388                 *p = '/';       /* restore '/' overriden with '\0' */
 389         return retval;
 390 Elong:
 391         return ERR_PTR(-ENAMETOOLONG);
 392 }
 393
 394 static void get_fs_root_and_pwd_rcu(struct fs_struct *fs, struct path *root,
 395                                     struct path *pwd)
 396 {
 397         unsigned seq;
 398
 399         do {
 400                 seq = read_seqcount_begin(&fs->seq);
 401                 *root = fs->root;
 402                 *pwd = fs->pwd;
 403         } while (read_seqcount_retry(&fs->seq, seq));
 404 }
 405
 406 /*
 407  * NOTE! The user-level library version returns a
 408  * character pointer. The kernel system call just
 409  * returns the length of the buffer filled (which
 410  * includes the ending '\0' character), or a negative
 411  * error value. So libc would do something like
 412  *
 413  *      char *getcwd(char * buf, size_t size)
 414  *      {
 415  *              int retval;
 416  *
 417  *              retval = sys_getcwd(buf, size);
 418  *              if (retval >= 0)
 419  *                      return buf;
 420  *              errno = -retval;
 421  *              return NULL;
 422  *      }
 423  */
 424 SYSCALL_DEFINE2(getcwd, char __user *, buf, unsigned long, size)
 425 {
 426         int error;
 427         struct path pwd, root;
 428         char *page = __getname();
 429
 430         if (!page)
 431                 return -ENOMEM;
 432
 433         rcu_read_lock();
 434         get_fs_root_and_pwd_rcu(current->fs, &root, &pwd);
 435
 436         error = -ENOENT;
 437         if (!d_unlinked(pwd.dentry)) {
 438                 unsigned long len;
 439                 char *cwd = page + PATH_MAX;
 440                 int buflen = PATH_MAX;
 441
 442                 prepend(&cwd, &buflen, "\0", 1);
 443                 error = prepend_path(&pwd, &root, &cwd, &buflen);
 444                 rcu_read_unlock();
 445
 446                 if (error < 0)
 447                         goto out;
 448
 449                 /* Unreachable from current root */
 450                 if (error > 0) {
 451                         error = prepend_unreachable(&cwd, &buflen);
 452                         if (error)
 453                                 goto out;
 454                 }
 455
 456                 error = -ERANGE;
 457                 len = PATH_MAX + page - cwd;
 458                 if (len <= size) {
 459                         error = len;
 460                         if (copy_to_user(buf, cwd, len))
 461                                 error = -EFAULT;
 462                 }
 463         } else {
 464                 rcu_read_unlock();
 465         }
 466
 467 out:
 468         __putname(page);
 469         return error;
 470 }