lib/chdir-long.c

   1 /* provide a chdir function that tries not to fail due to ENAMETOOLONG
   2    Copyright (C) 2004-2020 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12    GNU General Public License for more details.
  13
  14    You should have received a copy of the GNU General Public License
  15    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  16
  17 /* written by Jim Meyering */
  18
  19 #include <config.h>
  20
  21 #include "chdir-long.h"
  22
  23 #include <errno.h>
  24 #include <fcntl.h>
  25 #include <stdlib.h>
  26 #include <stdbool.h>
  27 #include <string.h>
  28 #include <stdio.h>
  29
  30 #include "assure.h"
  31
  32 #ifndef PATH_MAX
  33 # error "compile this file only if your system defines PATH_MAX"
  34 #endif
  35
  36 /* The results of openat() in this file are not leaked to any
  37    single-threaded code that could use stdio.
  38    FIXME - if the kernel ever adds support for multi-thread safety for
  39    avoiding standard fds, then we should use openat_safer.  */
  40
  41 struct cd_buf
  42 {
  43   int fd;
  44 };
  45
  46 static void
  47 cdb_init (struct cd_buf *cdb)
  48 {
  49   cdb->fd = AT_FDCWD;
  50 }
  51
  52 static int
  53 cdb_fchdir (struct cd_buf const *cdb)
  54 {
  55   return fchdir (cdb->fd);
  56 }
  57
  58 static void
  59 cdb_free (struct cd_buf const *cdb)
  60 {
  61   if (0 <= cdb->fd)
  62     {
  63       bool close_fail = close (cdb->fd);
  64       assure (! close_fail);
  65     }
  66 }
  67
  68 /* Given a file descriptor of an open directory (or AT_FDCWD), CDB->fd,
  69    try to open the CDB->fd-relative directory, DIR.  If the open succeeds,
  70    update CDB->fd with the resulting descriptor, close the incoming file
  71    descriptor, and return zero.  Upon failure, return -1 and set errno.  */
  72 static int
  73 cdb_advance_fd (struct cd_buf *cdb, char const *dir)
  74 {
  75   int new_fd = openat (cdb->fd, dir,
  76                        O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NONBLOCK);
  77   if (new_fd < 0)
  78     return -1;
  79
  80   cdb_free (cdb);
  81   cdb->fd = new_fd;
  82
  83   return 0;
  84 }
  85
  86 /* Return a pointer to the first non-slash in S.  */
  87 static char * _GL_ATTRIBUTE_PURE
  88 find_non_slash (char const *s)
  89 {
  90   size_t n_slash = strspn (s, "/");
  91   return (char *) s + n_slash;
  92 }
  93
  94 /* This is a function much like chdir, but without the PATH_MAX limitation
  95    on the length of the directory name.  A significant difference is that
  96    it must be able to modify (albeit only temporarily) the directory
  97    name.  It handles an arbitrarily long directory name by operating
  98    on manageable portions of the name.  On systems without the openat
  99    syscall, this means changing the working directory to more and more
 100    "distant" points along the long directory name and then restoring
 101    the working directory.  If any of those attempts to save or restore
 102    the working directory fails, this function exits nonzero.
 103
 104    Note that this function may still fail with errno == ENAMETOOLONG, but
 105    only if the specified directory name contains a component that is long
 106    enough to provoke such a failure all by itself (e.g. if the component
 107    has length PATH_MAX or greater on systems that define PATH_MAX).  */
 108
 109 int
 110 chdir_long (char *dir)
 111 {
 112   int e = chdir (dir);
 113   if (e == 0 || errno != ENAMETOOLONG)
 114     return e;
 115
 116   {
 117     size_t len = strlen (dir);
 118     char *dir_end = dir + len;
 119     struct cd_buf cdb;
 120     size_t n_leading_slash;
 121
 122     cdb_init (&cdb);
 123
 124     /* If DIR is the empty string, then the chdir above
 125        must have failed and set errno to ENOENT.  */
 126     assure (0 < len);
 127     assure (PATH_MAX <= len);
 128
 129     /* Count leading slashes.  */
 130     n_leading_slash = strspn (dir, "/");
 131
 132     /* Handle any leading slashes as well as any name that matches
 133        the regular expression, m!^//hostname[/]*! .  Handling this
 134        prefix separately usually results in a single additional
 135        cdb_advance_fd call, but it's worthwhile, since it makes the
 136        code in the following loop cleaner.  */
 137     if (n_leading_slash == 2)
 138       {
 139         int err;
 140         /* Find next slash.
 141            We already know that dir[2] is neither a slash nor '\0'.  */
 142         char *slash = memchr (dir + 3, '/', dir_end - (dir + 3));
 143         if (slash == NULL)
 144           {
 145             errno = ENAMETOOLONG;
 146             return -1;
 147           }
 148         *slash = '\0';
 149         err = cdb_advance_fd (&cdb, dir);
 150         *slash = '/';
 151         if (err != 0)
 152           goto Fail;
 153         dir = find_non_slash (slash + 1);
 154       }
 155     else if (n_leading_slash)
 156       {
 157         if (cdb_advance_fd (&cdb, "/") != 0)
 158           goto Fail;
 159         dir += n_leading_slash;
 160       }
 161
 162     assure (*dir != '/');
 163     assure (dir <= dir_end);
 164
 165     while (PATH_MAX <= dir_end - dir)
 166       {
 167         int err;
 168         /* Find a slash that is PATH_MAX or fewer bytes away from dir.
 169            I.e. see if there is a slash that will give us a name of
 170            length PATH_MAX-1 or less.  */
 171         char *slash = memrchr (dir, '/', PATH_MAX);
 172         if (slash == NULL)
 173           {
 174             errno = ENAMETOOLONG;
 175             return -1;
 176           }
 177
 178         *slash = '\0';
 179         assure (slash - dir < PATH_MAX);
 180         err = cdb_advance_fd (&cdb, dir);
 181         *slash = '/';
 182         if (err != 0)
 183           goto Fail;
 184
 185         dir = find_non_slash (slash + 1);
 186       }
 187
 188     if (dir < dir_end)
 189       {
 190         if (cdb_advance_fd (&cdb, dir) != 0)
 191           goto Fail;
 192       }
 193
 194     if (cdb_fchdir (&cdb) != 0)
 195       goto Fail;
 196
 197     cdb_free (&cdb);
 198     return 0;
 199
 200    Fail:
 201     {
 202       int saved_errno = errno;
 203       cdb_free (&cdb);
 204       errno = saved_errno;
 205       return -1;
 206     }
 207   }
 208 }
 209
 210 #if TEST_CHDIR
 211
 212 # include "closeout.h"
 213 # include "error.h"
 214
 215 int
 216 main (int argc, char *argv[])
 217 {
 218   char *line = NULL;
 219   size_t n = 0;
 220   int len;
 221
 222   atexit (close_stdout);
 223
 224   len = getline (&line, &n, stdin);
 225   if (len < 0)
 226     {
 227       int saved_errno = errno;
 228       if (feof (stdin))
 229         exit (0);
 230
 231       error (EXIT_FAILURE, saved_errno,
 232              "reading standard input");
 233     }
 234   else if (len == 0)
 235     exit (0);
 236
 237   if (line[len-1] == '\n')
 238     line[len-1] = '\0';
 239
 240   if (chdir_long (line) != 0)
 241     error (EXIT_FAILURE, errno,
 242            "chdir_long failed: %s", line);
 243
 244   if (argc <= 1)
 245     {
 246       /* Using 'pwd' here makes sense only if it is a robust implementation,
 247          like the one in coreutils after the 2004-04-19 changes.  */
 248       char const *cmd = "pwd";
 249       execlp (cmd, (char *) NULL);
 250       error (EXIT_FAILURE, errno, "%s", cmd);
 251     }
 252
 253   fclose (stdin);
 254   fclose (stderr);
 255
 256   exit (EXIT_SUCCESS);
 257 }
 258 #endif
 259
 260 /*
 261 Local Variables:
 262 compile-command: "gcc -DTEST_CHDIR=1 -g -O -W -Wall chdir-long.c libcoreutils.a"
 263 End:
 264 */