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/* Invoke freopen, but avoid some glitches.
Copyright (C) 2009-2020 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Eric Blake. */
#include <config.h>
#include "stdio-safer.h"
#include <errno.h>
#include <fcntl.h>
#include <stdbool.h>
#include <unistd.h>
#ifndef FALLTHROUGH
# if __GNUC__ < 7
# define FALLTHROUGH ((void) 0)
# else
# define FALLTHROUGH __attribute__ ((__fallthrough__))
# endif
#endif
/* Guarantee that FD is open; all smaller FDs must already be open.
Return true if successful. */
static bool
protect_fd (int fd)
{
int value = open ("/dev/null", O_RDONLY);
if (value != fd)
{
if (0 <= value)
{
close (value);
errno = EBADF; /* Unexpected; this is as good as anything else. */
}
return false;
}
return true;
}
/* Like freopen, but guarantee that reopening stdin, stdout, or stderr
preserves the invariant that STDxxx_FILENO==fileno(stdxxx), and
that no other stream will interfere with the standard streams.
This is necessary because most freopen implementations will change
the associated fd of a stream to the lowest available slot. */
FILE *
freopen_safer (char const *name, char const *mode, FILE *f)
{
/* Unfortunately, we cannot use the fopen_safer approach of using
fdopen (dup_safer (fileno (freopen (cmd, mode, f)))), because we
need to return f itself. The implementation of freopen(NULL,m,f)
is system-dependent, so the best we can do is guarantee that all
lower-valued standard fds are open prior to the freopen call,
even though this puts more pressure on open fds. */
bool protect_in = false;
bool protect_out = false;
bool protect_err = false;
int saved_errno;
switch (fileno (f))
{
default: /* -1 or not a standard stream. */
if (dup2 (STDERR_FILENO, STDERR_FILENO) != STDERR_FILENO)
protect_err = true;
FALLTHROUGH;
case STDERR_FILENO:
if (dup2 (STDOUT_FILENO, STDOUT_FILENO) != STDOUT_FILENO)
protect_out = true;
FALLTHROUGH;
case STDOUT_FILENO:
if (dup2 (STDIN_FILENO, STDIN_FILENO) != STDIN_FILENO)
protect_in = true;
FALLTHROUGH;
case STDIN_FILENO:
/* Nothing left to protect. */
break;
}
if (protect_in && !protect_fd (STDIN_FILENO))
f = NULL;
else if (protect_out && !protect_fd (STDOUT_FILENO))
f = NULL;
else if (protect_err && !protect_fd (STDERR_FILENO))
f = NULL;
else
f = freopen (name, mode, f);
saved_errno = errno;
if (protect_err)
close (STDERR_FILENO);
if (protect_out)
close (STDOUT_FILENO);
if (protect_in)
close (STDIN_FILENO);
if (!f)
errno = saved_errno;
return f;
}
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