contrib/diffutils/lib/freopen-safer.c

   1 /* Invoke freopen, but avoid some glitches.
   2
   3    Copyright (C) 2009-2013 Free Software Foundation, Inc.
   4
   5    This program is free software: you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 3 of the License, or
   8    (at your option) any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License
  16    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Eric Blake.  */
  19
  20 #include <config.h>
  21
  22 #include "stdio-safer.h"
  23
  24 #include <errno.h>
  25 #include <fcntl.h>
  26 #include <stdbool.h>
  27 #include <unistd.h>
  28
  29 /* Guarantee that FD is open; all smaller FDs must already be open.
  30    Return true if successful.  */
  31 static bool
  32 protect_fd (int fd)
  33 {
  34   int value = open ("/dev/null", O_RDONLY);
  35   if (value != fd)
  36     {
  37       if (0 <= value)
  38         {
  39           close (value);
  40           errno = EBADF; /* Unexpected; this is as good as anything else.  */
  41         }
  42       return false;
  43     }
  44   return true;
  45 }
  46
  47 /* Like freopen, but guarantee that reopening stdin, stdout, or stderr
  48    preserves the invariant that STDxxx_FILENO==fileno(stdxxx), and
  49    that no other stream will interfere with the standard streams.
  50    This is necessary because most freopen implementations will change
  51    the associated fd of a stream to the lowest available slot.  */
  52
  53 FILE *
  54 freopen_safer (char const *name, char const *mode, FILE *f)
  55 {
  56   /* Unfortunately, we cannot use the fopen_safer approach of using
  57      fdopen (dup_safer (fileno (freopen (cmd, mode, f)))), because we
  58      need to return f itself.  The implementation of freopen(NULL,m,f)
  59      is system-dependent, so the best we can do is guarantee that all
  60      lower-valued standard fds are open prior to the freopen call,
  61      even though this puts more pressure on open fds.  */
  62   bool protect_in = false;
  63   bool protect_out = false;
  64   bool protect_err = false;
  65   int saved_errno;
  66
  67   switch (fileno (f))
  68     {
  69     default: /* -1 or not a standard stream.  */
  70       if (dup2 (STDERR_FILENO, STDERR_FILENO) != STDERR_FILENO)
  71         protect_err = true;
  72       /* fall through */
  73     case STDERR_FILENO:
  74       if (dup2 (STDOUT_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
  75         protect_out = true;
  76       /* fall through */
  77     case STDOUT_FILENO:
  78       if (dup2 (STDIN_FILENO, STDIN_FILENO) != STDIN_FILENO)
  79         protect_in = true;
  80       /* fall through */
  81     case STDIN_FILENO:
  82       /* Nothing left to protect.  */
  83       break;
  84     }
  85   if (protect_in && !protect_fd (STDIN_FILENO))
  86     f = NULL;
  87   else if (protect_out && !protect_fd (STDOUT_FILENO))
  88     f = NULL;
  89   else if (protect_err && !protect_fd (STDERR_FILENO))
  90     f = NULL;
  91   else
  92     f = freopen (name, mode, f);
  93   saved_errno = errno;
  94   if (protect_err)
  95     close (STDERR_FILENO);
  96   if (protect_out)
  97     close (STDOUT_FILENO);
  98   if (protect_in)
  99     close (STDIN_FILENO);
 100   if (!f)
 101     errno = saved_errno;
 102   return f;
 103 }