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/* Copyright (C) 2000-2021 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
The GNU C Library 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
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <errno.h>
#include <fcntl.h>
#include <paths.h>
#include <unistd.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>
/* Try to get a machine dependent instruction which will make the
program crash. This is used in case everything else fails. */
#include <abort-instr.h>
#ifndef ABORT_INSTRUCTION
/* No such instruction is available. */
# define ABORT_INSTRUCTION
#endif
#include <device-nrs.h>
#include <not-cancel.h>
/* Should other OSes (e.g., Hurd) have different versions which can
be written in a better way? */
static void
check_one_fd (int fd, int mode)
{
if (__builtin_expect (__fcntl64_nocancel (fd, F_GETFD), 0) == -1
&& errno == EBADF)
{
const char *name;
dev_t dev;
/* For writable descriptors we use /dev/full. */
if ((mode & O_ACCMODE) == O_WRONLY)
{
name = _PATH_DEV "full";
dev = __gnu_dev_makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);
}
else
{
name = _PATH_DEVNULL;
dev = __gnu_dev_makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);
}
/* Something is wrong with this descriptor, it's probably not
opened. Open /dev/null so that the SUID program we are
about to start does not accidentally use this descriptor. */
int nullfd = __open_nocancel (name, mode, 0);
/* We are very paranoid here. With all means we try to ensure
that we are actually opening the /dev/null device and nothing
else.
Note that the following code assumes that STDIN_FILENO,
STDOUT_FILENO, STDERR_FILENO are the three lowest file
decsriptor numbers, in this order. */
struct stat64 st;
if (__builtin_expect (nullfd != fd, 0)
|| __builtin_expect (__fstat64 (fd, &st), 0) != 0
|| __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
|| st.st_rdev != dev)
/* We cannot even give an error message here since it would
run into the same problems. */
while (1)
/* Try for ever and ever. */
ABORT_INSTRUCTION;
}
}
void
__libc_check_standard_fds (void)
{
/* Check all three standard file descriptors. The O_NOFOLLOW flag
is really paranoid but some people actually are. If /dev/null
should happen to be a symlink to somewhere else and not the
device commonly known as "/dev/null" we bail out. */
check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);
check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);
check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);
}
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