sysdeps/generic/check_fds.c

   1 /* Copyright (C) 2000, 2002, 2003 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library 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 GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    02111-1307 USA.  */
  18
  19 #include <errno.h>
  20 #include <fcntl.h>
  21 #include <paths.h>
  22 #include <unistd.h>
  23 #include <sys/stat.h>
  24 #include <sys/sysmacros.h>
  25
  26 /* Try to get a machine dependent instruction which will make the
  27    program crash.  This is used in case everything else fails.  */
  28 #include <abort-instr.h>
  29 #ifndef ABORT_INSTRUCTION
  30 /* No such instruction is available.  */
  31 # define ABORT_INSTRUCTION
  32 #endif
  33
  34 #include <device-nrs.h>
  35 #include <not-cancel.h>
  36
  37
  38 /* Should other OSes (e.g., Hurd) have different versions which can
  39    be written in a better way?  */
  40 static void
  41 check_one_fd (int fd, int mode)
  42 {
  43   /* Note that fcntl() with this parameter is not a cancellation point.  */
  44   if (__builtin_expect (__libc_fcntl (fd, F_GETFD), 0) == -1
  45       && errno == EBADF)
  46     {
  47       struct stat64 st;
  48
  49       /* Something is wrong with this descriptor, it's probably not
  50          opened.  Open /dev/null so that the SUID program we are
  51          about to start does not accidently use this descriptor.  */
  52       int nullfd = open_not_cancel (_PATH_DEVNULL, mode, 0);
  53       /* We are very paranoid here.  With all means we try to ensure
  54          that we are actually opening the /dev/null device and nothing
  55          else.
  56
  57          Note that the following code assumes that STDIN_FILENO,
  58          STDOUT_FILENO, STDERR_FILENO are the three lowest file
  59          decsriptor numbers, in this order.  */
  60       if (__builtin_expect (nullfd != fd, 0)
  61           || __builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0) != 0
  62           || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
  63 #if defined DEV_NULL_MAJOR && defined DEV_NULL_MINOR
  64           || st.st_rdev != makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR)
  65 #endif
  66           )
  67         /* We cannot even give an error message here since it would
  68            run into the same problems.  */
  69         while (1)
  70           /* Try for ever and ever.  */
  71           ABORT_INSTRUCTION;
  72     }
  73 }
  74
  75
  76 void
  77 __libc_check_standard_fds (void)
  78 {
  79   /* This is really paranoid but some people actually are.  If /dev/null
  80      should happen to be a symlink to somewhere else and not the device
  81      commonly known as "/dev/null" we bail out.  We can detect this with
  82      the O_NOFOLLOW flag for open() but only on some system.  */
  83 #ifndef O_NOFOLLOW
  84 # define O_NOFOLLOW     0
  85 #endif
  86   /* Check all three standard file descriptors.  */
  87   check_one_fd (STDIN_FILENO, O_RDONLY | O_NOFOLLOW);
  88   check_one_fd (STDOUT_FILENO, O_RDWR | O_NOFOLLOW);
  89   check_one_fd (STDERR_FILENO, O_RDWR | O_NOFOLLOW);
  90 }