/* Copyright (C) 2000-2018 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library 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

   Lesser General Public License for more details.

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

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#include <errno.h>

#include <fcntl.h>

#include <paths.h>

#include <unistd.h>

#include <sys/stat.h>

#include <sys/sysmacros.h>

/* Try to get a machine dependent instruction which will make the

   program crash.  This is used in case everything else fails.  */

#include <abort-instr.h>

#ifndef ABORT_INSTRUCTION

/* No such instruction is available.  */

# define ABORT_INSTRUCTION

#endif

#include <device-nrs.h>

#include <not-cancel.h>

/* Should other OSes (e.g., Hurd) have different versions which can

   be written in a better way?  */

static void

check_one_fd (int fd, int mode)

{

  if (__builtin_expect (__fcntl64_nocancel (fd, F_GETFD), 0) == -1

      && errno == EBADF)

    {

      const char *name;

      dev_t dev;

      /* For writable descriptors we use /dev/full.  */

      if ((mode & O_ACCMODE) == O_WRONLY)

	{

	  name = _PATH_DEV "full";

	  dev = __gnu_dev_makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);

	}

      else

	{

	  name = _PATH_DEVNULL;

	  dev = __gnu_dev_makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);

	}

      /* Something is wrong with this descriptor, it's probably not

	 opened.  Open /dev/null so that the SUID program we are

	 about to start does not accidentally use this descriptor.  */

      int nullfd = __open_nocancel (name, mode, 0);

      /* We are very paranoid here.  With all means we try to ensure

	 that we are actually opening the /dev/null device and nothing

	 else.

	 Note that the following code assumes that STDIN_FILENO,

	 STDOUT_FILENO, STDERR_FILENO are the three lowest file

	 decsriptor numbers, in this order.  */

      struct stat64 st;

      if (__builtin_expect (nullfd != fd, 0)

	  || __builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0) != 0

	  || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0

	  || st.st_rdev != dev)

	/* We cannot even give an error message here since it would

	   run into the same problems.  */

	while (1)

	  /* Try for ever and ever.  */

	  ABORT_INSTRUCTION;

    }

}

void

__libc_check_standard_fds (void)

{

  /* Check all three standard file descriptors.  The O_NOFOLLOW flag

     is really paranoid but some people actually are.  If /dev/null

     should happen to be a symlink to somewhere else and not the

     device commonly known as "/dev/null" we bail out.  */

  check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);

  check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);

  check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);

}