/* provide a replacement fdopendir function

   Copyright (C) 2004-2015 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify

   it under the terms of the GNU General Public License as published by

   the Free Software Foundation; either version 3 of the License, or

   (at your option) any later version.

   This program is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License

   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */

/* written by Jim Meyering */

#include <config.h>

#include <dirent.h>

#include <stdlib.h>

#include <unistd.h>

#if !HAVE_FDOPENDIR

# include "openat.h"

# include "openat-priv.h"

# include "save-cwd.h"

# if GNULIB_DIRENT_SAFER

#  include "dirent--.h"

# endif

# ifndef REPLACE_FCHDIR

#  define REPLACE_FCHDIR 0

# endif

static DIR *fdopendir_with_dup (int, int, struct saved_cwd const *);

static DIR *fd_clone_opendir (int, struct saved_cwd const *);

/* Replacement for POSIX fdopendir.

   First, try to simulate it via opendir ("/proc/self/fd/...").  Failing

   that, simulate it by using fchdir metadata, or by doing

   save_cwd/fchdir/opendir(".")/restore_cwd.

   If either the save_cwd or the restore_cwd fails (relatively unlikely),

   then give a diagnostic and exit nonzero.

   If successful, the resulting stream is based on FD in

   implementations where streams are based on file descriptors and in

   applications where no other thread or signal handler allocates or

   frees file descriptors.  In other cases, consult dirfd on the result

   to find out whether FD is still being used.

   Otherwise, this function works just like POSIX fdopendir.

   W A R N I N G:

   Unlike other fd-related functions, this one places constraints on FD.

   If this function returns successfully, FD is under control of the

   dirent.h system, and the caller should not close or modify the state of

   FD other than by the dirent.h functions.  */

DIR *

fdopendir (int fd)

{

  DIR *dir = fdopendir_with_dup (fd, -1, NULL);

  if (! REPLACE_FCHDIR && ! dir)

    {

      int saved_errno = errno;

      if (EXPECTED_ERRNO (saved_errno))

        {

          struct saved_cwd cwd;

          if (save_cwd (&cwd) != 0)

            openat_save_fail (errno);

          dir = fdopendir_with_dup (fd, -1, &cwd;;

          saved_errno = errno;

          free_cwd (&cwd;;

          errno = saved_errno;

        }

    }

  return dir;

}

/* Like fdopendir, except that if OLDER_DUPFD is not -1, it is known

   to be a dup of FD which is less than FD - 1 and which will be

   closed by the caller and not otherwise used by the caller.  This

   function makes sure that FD is closed and all file descriptors less

   than FD are open, and then calls fd_clone_opendir on a dup of FD.

   That way, barring race conditions, fd_clone_opendir returns a

   stream whose file descriptor is FD.

   If REPLACE_FCHDIR or CWD is null, use opendir ("/proc/self/fd/...",

   falling back on fchdir metadata.  Otherwise, CWD is a saved version

   of the working directory; use fchdir/opendir(".")/restore_cwd(CWD).  */

static DIR *

fdopendir_with_dup (int fd, int older_dupfd, struct saved_cwd const *cwd)

{

  int dupfd = dup (fd);

  if (dupfd < 0 && errno == EMFILE)

    dupfd = older_dupfd;

  if (dupfd < 0)

    return NULL;

  else

    {

      DIR *dir;

      int saved_errno;

      if (dupfd < fd - 1 && dupfd != older_dupfd)

        {

          dir = fdopendir_with_dup (fd, dupfd, cwd);

          saved_errno = errno;

        }

      else

        {

          close (fd);

          dir = fd_clone_opendir (dupfd, cwd);

          saved_errno = errno;

          if (! dir)

            {

              int fd1 = dup (dupfd);

              if (fd1 != fd)

                openat_save_fail (fd1 < 0 ? errno : EBADF);

            }

        }

      if (dupfd != older_dupfd)

        close (dupfd);

      errno = saved_errno;

      return dir;

    }

}

/* Like fdopendir, except the result controls a clone of FD.  It is

   the caller's responsibility both to close FD and (if the result is

   not null) to closedir the result.  */

static DIR *

fd_clone_opendir (int fd, struct saved_cwd const *cwd)

{

  if (REPLACE_FCHDIR || ! cwd)

    {

      DIR *dir = NULL;

      int saved_errno = EOPNOTSUPP;

      char buf[OPENAT_BUFFER_SIZE];

      char *proc_file = openat_proc_name (buf, fd, ".");

      if (proc_file)

        {

          dir = opendir (proc_file);

          saved_errno = errno;

          if (proc_file != buf)

            free (proc_file);

        }

# if REPLACE_FCHDIR

      if (! dir && EXPECTED_ERRNO (saved_errno))

        {

          char const *name = _gl_directory_name (fd);

          DIR *dp = name ? opendir (name) : NULL;

          /* The caller has done an elaborate dance to arrange for opendir to

             consume just the right file descriptor.  If dirfd returns -1,

             though, we're on a system like mingw where opendir does not

             consume a file descriptor.  Consume it via 'dup' instead.  */

          if (dp && dirfd (dp) < 0)

            dup (fd);

          return dp;

        }

# endif

      errno = saved_errno;

      return dir;

    }

  else

    {

      if (fchdir (fd) != 0)

        return NULL;

      else

        {

          DIR *dir = opendir (".");

          int saved_errno = errno;

          if (restore_cwd (cwd) != 0)

            openat_restore_fail (errno);

          errno = saved_errno;

          return dir;

        }

    }

}

#else /* HAVE_FDOPENDIR */

# include <errno.h>

# include <sys/stat.h>

# undef fdopendir

/* Like fdopendir, but work around GNU/Hurd bug by validating FD.  */

DIR *

rpl_fdopendir (int fd)

{

  struct stat st;

  if (fstat (fd, &st))

    return NULL;

  if (!S_ISDIR (st.st_mode))

    {

      errno = ENOTDIR;

      return NULL;

    }

  return fdopendir (fd);

}

#endif /* HAVE_FDOPENDIR */