csu/check_fds.c

   1 /* Copyright (C) 2000, 2002, 2003, 2005 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       const char *name;
  48       dev_t dev;
  49
  50       /* For writable descriptors we use /dev/full.  */
  51       if ((mode & O_ACCMODE) == O_WRONLY)
  52         {
  53           name = _PATH_DEV "full";
  54           dev = makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);
  55         }
  56       else
  57         {
  58           name = _PATH_DEVNULL;
  59           dev = makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);
  60         }
  61
  62       /* Something is wrong with this descriptor, it's probably not
  63          opened.  Open /dev/null so that the SUID program we are
  64          about to start does not accidently use this descriptor.  */
  65       int nullfd = open_not_cancel (name, mode, 0);
  66
  67       /* We are very paranoid here.  With all means we try to ensure
  68          that we are actually opening the /dev/null device and nothing
  69          else.
  70
  71          Note that the following code assumes that STDIN_FILENO,
  72          STDOUT_FILENO, STDERR_FILENO are the three lowest file
  73          decsriptor numbers, in this order.  */
  74       struct stat64 st;
  75       if (__builtin_expect (nullfd != fd, 0)
  76           || __builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0) != 0
  77           || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
  78           || st.st_rdev != dev)
  79         /* We cannot even give an error message here since it would
  80            run into the same problems.  */
  81         while (1)
  82           /* Try for ever and ever.  */
  83           ABORT_INSTRUCTION;
  84     }
  85 }
  86
  87
  88 void
  89 __libc_check_standard_fds (void)
  90 {
  91   /* This is really paranoid but some people actually are.  If /dev/null
  92      should happen to be a symlink to somewhere else and not the device
  93      commonly known as "/dev/null" we bail out.  We can detect this with
  94      the O_NOFOLLOW flag for open() but only on some system.  */
  95 #ifndef O_NOFOLLOW
  96 # define O_NOFOLLOW     0
  97 #endif
  98   /* Check all three standard file descriptors.  */
  99   check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);
 100   check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);
 101   check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);
 102 }