1/* Copyright (C) 2000-2021 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18#include <errno.h>
19#include <fcntl.h>
20#include <paths.h>
21#include <unistd.h>
22#include <sys/stat.h>
23#include <sys/sysmacros.h>
24
25/* Try to get a machine dependent instruction which will make the
26 program crash. This is used in case everything else fails. */
27#include <abort-instr.h>
28#ifndef ABORT_INSTRUCTION
29/* No such instruction is available. */
30# define ABORT_INSTRUCTION
31#endif
32
33#include <device-nrs.h>
34#include <not-cancel.h>
35
36
37/* Should other OSes (e.g., Hurd) have different versions which can
38 be written in a better way? */
39static void
40check_one_fd (int fd, int mode)
41{
42 if (__builtin_expect (__fcntl64_nocancel (fd, F_GETFD), 0) == -1
43 && errno == EBADF)
44 {
45 const char *name;
46 dev_t dev;
47
48 /* For writable descriptors we use /dev/full. */
49 if ((mode & O_ACCMODE) == O_WRONLY)
50 {
51 name = _PATH_DEV "full";
52 dev = __gnu_dev_makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);
53 }
54 else
55 {
56 name = _PATH_DEVNULL;
57 dev = __gnu_dev_makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);
58 }
59
60 /* Something is wrong with this descriptor, it's probably not
61 opened. Open /dev/null so that the SUID program we are
62 about to start does not accidentally use this descriptor. */
63 int nullfd = __open_nocancel (name, mode, 0);
64
65 /* We are very paranoid here. With all means we try to ensure
66 that we are actually opening the /dev/null device and nothing
67 else.
68
69 Note that the following code assumes that STDIN_FILENO,
70 STDOUT_FILENO, STDERR_FILENO are the three lowest file
71 decsriptor numbers, in this order. */
72 struct __stat64_t64 st;
73 if (__glibc_unlikely (nullfd != fd)
74 || __glibc_likely (__fstat64_time64 (fd, &st) != 0)
75 || __glibc_unlikely (S_ISCHR (st.st_mode) == 0)
76 || st.st_rdev != dev)
77 /* We cannot even give an error message here since it would
78 run into the same problems. */
79 while (1)
80 /* Try for ever and ever. */
81 ABORT_INSTRUCTION;
82 }
83}
84
85
86void
87__libc_check_standard_fds (void)
88{
89 /* Check all three standard file descriptors. The O_NOFOLLOW flag
90 is really paranoid but some people actually are. If /dev/null
91 should happen to be a symlink to somewhere else and not the
92 device commonly known as "/dev/null" we bail out. */
93 check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);
94 check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);
95 check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);
96}
97