lib/chdir-safer.c

   1 /* much like chdir(2), but safer
   2
   3    Copyright (C) 2005-2006, 2008-2017 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* written by Jim Meyering */
  19
  20 #include <config.h>
  21
  22 #include "chdir-safer.h"
  23
  24 #include <stdbool.h>
  25 #include <fcntl.h>
  26 #include <errno.h>
  27 #include <unistd.h>
  28 #include <sys/types.h>
  29 #include <sys/stat.h>
  30 #include "same-inode.h"
  31
  32 #ifndef HAVE_READLINK
  33 # define HAVE_READLINK 0
  34 #endif
  35
  36 /* Like chdir, but fail if DIR is a symbolic link to a directory (or
  37    similar funny business).  This avoids a minor race condition
  38    between when a directory is created or statted and when the process
  39    chdirs into it.
  40
  41    On older systems lacking full support for O_SEARCH, this function
  42    can also fail if DIR is not readable.  */
  43 int
  44 chdir_no_follow (char const *dir)
  45 {
  46   int result = 0;
  47   int saved_errno;
  48   int fd = open (dir,
  49                  O_SEARCH | O_DIRECTORY | O_NOCTTY | O_NOFOLLOW | O_NONBLOCK);
  50   if (fd < 0)
  51     return -1;
  52
  53   /* If open follows symlinks, lstat DIR and fstat FD to ensure that
  54      they are the same file; if they are different files, set errno to
  55      ELOOP (the same value that open uses for symlinks with
  56      O_NOFOLLOW) so the caller can report a failure.
  57      Skip this check if HAVE_READLINK == 0, which should be the case
  58      on any system that lacks symlink support.  */
  59   if (HAVE_READLINK && ! HAVE_WORKING_O_NOFOLLOW)
  60     {
  61       struct stat sb1;
  62       result = lstat (dir, &sb1);
  63       if (result == 0)
  64         {
  65           struct stat sb2;
  66           result = fstat (fd, &sb2);
  67           if (result == 0 && ! SAME_INODE (sb1, sb2))
  68             {
  69               errno = ELOOP;
  70               result = -1;
  71             }
  72         }
  73     }
  74
  75   if (result == 0)
  76     result = fchdir (fd);
  77
  78   saved_errno = errno;
  79   close (fd);
  80   errno = saved_errno;
  81   return result;
  82 }