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