lib/fdopendir.c

   1 /* provide a replacement fdopendir function
   2    Copyright (C) 2004-2015 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 <http://www.gnu.org/licenses/>.  */
  16
  17 /* written by Jim Meyering */
  18
  19 #include <config.h>
  20
  21 #include <dirent.h>
  22
  23 #include <stdlib.h>
  24 #include <unistd.h>
  25
  26 #if !HAVE_FDOPENDIR
  27
  28 # include "openat.h"
  29 # include "openat-priv.h"
  30 # include "save-cwd.h"
  31
  32 # if GNULIB_DIRENT_SAFER
  33 #  include "dirent--.h"
  34 # endif
  35
  36 # ifndef REPLACE_FCHDIR
  37 #  define REPLACE_FCHDIR 0
  38 # endif
  39
  40 static DIR *fdopendir_with_dup (int, int, struct saved_cwd const *);
  41 static DIR *fd_clone_opendir (int, struct saved_cwd const *);
  42
  43 /* Replacement for POSIX fdopendir.
  44
  45    First, try to simulate it via opendir ("/proc/self/fd/...").  Failing
  46    that, simulate it by using fchdir metadata, or by doing
  47    save_cwd/fchdir/opendir(".")/restore_cwd.
  48    If either the save_cwd or the restore_cwd fails (relatively unlikely),
  49    then give a diagnostic and exit nonzero.
  50
  51    If successful, the resulting stream is based on FD in
  52    implementations where streams are based on file descriptors and in
  53    applications where no other thread or signal handler allocates or
  54    frees file descriptors.  In other cases, consult dirfd on the result
  55    to find out whether FD is still being used.
  56
  57    Otherwise, this function works just like POSIX fdopendir.
  58
  59    W A R N I N G:
  60
  61    Unlike other fd-related functions, this one places constraints on FD.
  62    If this function returns successfully, FD is under control of the
  63    dirent.h system, and the caller should not close or modify the state of
  64    FD other than by the dirent.h functions.  */
  65 DIR *
  66 fdopendir (int fd)
  67 {
  68   DIR *dir = fdopendir_with_dup (fd, -1, NULL);
  69
  70   if (! REPLACE_FCHDIR && ! dir)
  71     {
  72       int saved_errno = errno;
  73       if (EXPECTED_ERRNO (saved_errno))
  74         {
  75           struct saved_cwd cwd;
  76           if (save_cwd (&cwd) != 0)
  77             openat_save_fail (errno);
  78           dir = fdopendir_with_dup (fd, -1, &cwd);
  79           saved_errno = errno;
  80           free_cwd (&cwd);
  81           errno = saved_errno;
  82         }
  83     }
  84
  85   return dir;
  86 }
  87
  88 /* Like fdopendir, except that if OLDER_DUPFD is not -1, it is known
  89    to be a dup of FD which is less than FD - 1 and which will be
  90    closed by the caller and not otherwise used by the caller.  This
  91    function makes sure that FD is closed and all file descriptors less
  92    than FD are open, and then calls fd_clone_opendir on a dup of FD.
  93    That way, barring race conditions, fd_clone_opendir returns a
  94    stream whose file descriptor is FD.
  95
  96    If REPLACE_FCHDIR or CWD is null, use opendir ("/proc/self/fd/...",
  97    falling back on fchdir metadata.  Otherwise, CWD is a saved version
  98    of the working directory; use fchdir/opendir(".")/restore_cwd(CWD).  */
  99 static DIR *
 100 fdopendir_with_dup (int fd, int older_dupfd, struct saved_cwd const *cwd)
 101 {
 102   int dupfd = dup (fd);
 103   if (dupfd < 0 && errno == EMFILE)
 104     dupfd = older_dupfd;
 105   if (dupfd < 0)
 106     return NULL;
 107   else
 108     {
 109       DIR *dir;
 110       int saved_errno;
 111       if (dupfd < fd - 1 && dupfd != older_dupfd)
 112         {
 113           dir = fdopendir_with_dup (fd, dupfd, cwd);
 114           saved_errno = errno;
 115         }
 116       else
 117         {
 118           close (fd);
 119           dir = fd_clone_opendir (dupfd, cwd);
 120           saved_errno = errno;
 121           if (! dir)
 122             {
 123               int fd1 = dup (dupfd);
 124               if (fd1 != fd)
 125                 openat_save_fail (fd1 < 0 ? errno : EBADF);
 126             }
 127         }
 128
 129       if (dupfd != older_dupfd)
 130         close (dupfd);
 131       errno = saved_errno;
 132       return dir;
 133     }
 134 }
 135
 136 /* Like fdopendir, except the result controls a clone of FD.  It is
 137    the caller's responsibility both to close FD and (if the result is
 138    not null) to closedir the result.  */
 139 static DIR *
 140 fd_clone_opendir (int fd, struct saved_cwd const *cwd)
 141 {
 142   if (REPLACE_FCHDIR || ! cwd)
 143     {
 144       DIR *dir = NULL;
 145       int saved_errno = EOPNOTSUPP;
 146       char buf[OPENAT_BUFFER_SIZE];
 147       char *proc_file = openat_proc_name (buf, fd, ".");
 148       if (proc_file)
 149         {
 150           dir = opendir (proc_file);
 151           saved_errno = errno;
 152           if (proc_file != buf)
 153             free (proc_file);
 154         }
 155 # if REPLACE_FCHDIR
 156       if (! dir && EXPECTED_ERRNO (saved_errno))
 157         {
 158           char const *name = _gl_directory_name (fd);
 159           DIR *dp = name ? opendir (name) : NULL;
 160
 161           /* The caller has done an elaborate dance to arrange for opendir to
 162              consume just the right file descriptor.  If dirfd returns -1,
 163              though, we're on a system like mingw where opendir does not
 164              consume a file descriptor.  Consume it via 'dup' instead.  */
 165           if (dp && dirfd (dp) < 0)
 166             dup (fd);
 167
 168           return dp;
 169         }
 170 # endif
 171       errno = saved_errno;
 172       return dir;
 173     }
 174   else
 175     {
 176       if (fchdir (fd) != 0)
 177         return NULL;
 178       else
 179         {
 180           DIR *dir = opendir (".");
 181           int saved_errno = errno;
 182           if (restore_cwd (cwd) != 0)
 183             openat_restore_fail (errno);
 184           errno = saved_errno;
 185           return dir;
 186         }
 187     }
 188 }
 189
 190 #else /* HAVE_FDOPENDIR */
 191
 192 # include <errno.h>
 193 # include <sys/stat.h>
 194
 195 # undef fdopendir
 196
 197 /* Like fdopendir, but work around GNU/Hurd bug by validating FD.  */
 198
 199 DIR *
 200 rpl_fdopendir (int fd)
 201 {
 202   struct stat st;
 203   if (fstat (fd, &st))
 204     return NULL;
 205   if (!S_ISDIR (st.st_mode))
 206     {
 207       errno = ENOTDIR;
 208       return NULL;
 209     }
 210   return fdopendir (fd);
 211 }
 212
 213 #endif /* HAVE_FDOPENDIR */