1 /* Invoke freopen, but avoid some glitches.
2 
3    Copyright (C) 2009-2018 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
protect_fd(int fd)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 *
freopen_safer(char const * name,char const * mode,FILE * f)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 }
112