lib/freopen-safer.c

   1 /* Invoke freopen, but avoid some glitches.
   2
   3    Copyright (C) 2009-2019 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 <https://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 #ifndef FALLTHROUGH
  30 # if __GNUC__ < 7
  31 #  define FALLTHROUGH ((void) 0)
  32 # else
  33 #  define FALLTHROUGH __attribute__ ((__fallthrough__))
  34 # endif
  35 #endif
  36
  37 /* Guarantee that FD is open; all smaller FDs must already be open.
  38    Return true if successful.  */
  39 static bool
  40 protect_fd (int fd)
  41 {
  42   int value = open ("/dev/null", O_RDONLY);
  43   if (value != fd)
  44     {
  45       if (0 <= value)
  46         {
  47           close (value);
  48           errno = EBADF; /* Unexpected; this is as good as anything else.  */
  49         }
  50       return false;
  51     }
  52   return true;
  53 }
  54
  55 /* Like freopen, but guarantee that reopening stdin, stdout, or stderr
  56    preserves the invariant that STDxxx_FILENO==fileno(stdxxx), and
  57    that no other stream will interfere with the standard streams.
  58    This is necessary because most freopen implementations will change
  59    the associated fd of a stream to the lowest available slot.  */
  60
  61 FILE *
  62 freopen_safer (char const *name, char const *mode, FILE *f)
  63 {
  64   /* Unfortunately, we cannot use the fopen_safer approach of using
  65      fdopen (dup_safer (fileno (freopen (cmd, mode, f)))), because we
  66      need to return f itself.  The implementation of freopen(NULL,m,f)
  67      is system-dependent, so the best we can do is guarantee that all
  68      lower-valued standard fds are open prior to the freopen call,
  69      even though this puts more pressure on open fds.  */
  70   bool protect_in = false;
  71   bool protect_out = false;
  72   bool protect_err = false;
  73   int saved_errno;
  74
  75   switch (fileno (f))
  76     {
  77     default: /* -1 or not a standard stream.  */
  78       if (dup2 (STDERR_FILENO, STDERR_FILENO) != STDERR_FILENO)
  79         protect_err = true;
  80       FALLTHROUGH;
  81     case STDERR_FILENO:
  82       if (dup2 (STDOUT_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
  83         protect_out = true;
  84       FALLTHROUGH;
  85     case STDOUT_FILENO:
  86       if (dup2 (STDIN_FILENO, STDIN_FILENO) != STDIN_FILENO)
  87         protect_in = true;
  88       FALLTHROUGH;
  89     case STDIN_FILENO:
  90       /* Nothing left to protect.  */
  91       break;
  92     }
  93   if (protect_in && !protect_fd (STDIN_FILENO))
  94     f = NULL;
  95   else if (protect_out && !protect_fd (STDOUT_FILENO))
  96     f = NULL;
  97   else if (protect_err && !protect_fd (STDERR_FILENO))
  98     f = NULL;
  99   else
 100     f = freopen (name, mode, f);
 101   saved_errno = errno;
 102   if (protect_err)
 103     close (STDERR_FILENO);
 104   if (protect_out)
 105     close (STDOUT_FILENO);
 106   if (protect_in)
 107     close (STDIN_FILENO);
 108   if (!f)
 109     errno = saved_errno;
 110   return f;
 111 }