1/* Copyright (C) 2000-2016 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 <http://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 /* Note that fcntl() with this parameter is not a cancellation point. */
43 if (__builtin_expect (__libc_fcntl (fd, F_GETFD), 0) == -1
44 && errno == EBADF)
45 {
46 const char *name;
47 dev_t dev;
48
49 /* For writable descriptors we use /dev/full. */
50 if ((mode & O_ACCMODE) == O_WRONLY)
51 {
52 name = _PATH_DEV "full";
53 dev = makedev (DEV_FULL_MAJOR, DEV_FULL_MINOR);
54 }
55 else
56 {
57 name = _PATH_DEVNULL;
58 dev = makedev (DEV_NULL_MAJOR, DEV_NULL_MINOR);
59 }
60
61 /* Something is wrong with this descriptor, it's probably not
62 opened. Open /dev/null so that the SUID program we are
63 about to start does not accidentally use this descriptor. */
64 int nullfd = open_not_cancel (name, mode, 0);
65
66 /* We are very paranoid here. With all means we try to ensure
67 that we are actually opening the /dev/null device and nothing
68 else.
69
70 Note that the following code assumes that STDIN_FILENO,
71 STDOUT_FILENO, STDERR_FILENO are the three lowest file
72 decsriptor numbers, in this order. */
73 struct stat64 st;
74 if (__builtin_expect (nullfd != fd, 0)
75 || __builtin_expect (__fxstat64 (_STAT_VER, fd, &st), 0) != 0
76 || __builtin_expect (S_ISCHR (st.st_mode), 1) == 0
77 || st.st_rdev != dev)
78 /* We cannot even give an error message here since it would
79 run into the same problems. */
80 while (1)
81 /* Try for ever and ever. */
82 ABORT_INSTRUCTION;
83 }
84}
85
86
87void
88__libc_check_standard_fds (void)
89{
90 /* This is really paranoid but some people actually are. If /dev/null
91 should happen to be a symlink to somewhere else and not the device
92 commonly known as "/dev/null" we bail out. We can detect this with
93 the O_NOFOLLOW flag for open() but only on some system. */
94#ifndef O_NOFOLLOW
95# define O_NOFOLLOW 0
96#endif
97 /* Check all three standard file descriptors. */
98 check_one_fd (STDIN_FILENO, O_WRONLY | O_NOFOLLOW);
99 check_one_fd (STDOUT_FILENO, O_RDONLY | O_NOFOLLOW);
100 check_one_fd (STDERR_FILENO, O_RDONLY | O_NOFOLLOW);
101}
102