1/* Change the protections of file relative to open directory. Linux version.
2 Copyright (C) 2006-2024 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <errno.h>
20#include <fcntl.h>
21#include <fd_to_filename.h>
22#include <not-cancel.h>
23#include <stdio.h>
24#include <sys/stat.h>
25#include <sys/types.h>
26#include <sysdep.h>
27#include <unistd.h>
28
29#if !__ASSUME_FCHMODAT2
30static int
31fchmodat_fallback (int fd, const char *file, mode_t mode, int flag)
32{
33 if (flag != AT_SYMLINK_NOFOLLOW)
34 return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
35
36 /* The kernel system call does not have a mode argument.
37 However, we can create an O_PATH descriptor and change that
38 via /proc (which does not resolve symbolic links). */
39
40 int pathfd = __openat_nocancel (fd, file,
41 O_PATH | O_NOFOLLOW | O_CLOEXEC);
42 if (pathfd < 0)
43 /* This may report errors such as ENFILE and EMFILE. The
44 caller can treat them as temporary if necessary. */
45 return pathfd;
46
47 /* Use fstatat because fstat does not work on O_PATH descriptors
48 before Linux 3.6. */
49 struct __stat64_t64 st;
50 if (__fstatat64_time64 (pathfd, "", &st, AT_EMPTY_PATH) != 0)
51 {
52 __close_nocancel (pathfd);
53 return -1;
54 }
55
56 /* Some Linux versions with some file systems can actually
57 change symbolic link permissions via /proc, but this is not
58 intentional, and it gives inconsistent results (e.g., error
59 return despite mode change). The expected behavior is that
60 symbolic link modes cannot be changed at all, and this check
61 enforces that. */
62 if (S_ISLNK (st.st_mode))
63 {
64 __close_nocancel (pathfd);
65 __set_errno (EOPNOTSUPP);
66 return -1;
67 }
68
69 /* For most file systems, fchmod does not operate on O_PATH
70 descriptors, so go through /proc. */
71 struct fd_to_filename filename;
72 int ret = __chmod (__fd_to_filename (descriptor: pathfd, storage: &filename), mode);
73 if (ret != 0)
74 {
75 if (errno == ENOENT)
76 /* /proc has not been mounted. Without /proc, there is no
77 way to upgrade the O_PATH descriptor to a full
78 descriptor. It is also not possible to re-open the
79 file without O_PATH because the file name may refer to
80 another file, and opening that without O_PATH may have
81 side effects (such as blocking, device rewinding, or
82 releasing POSIX locks). */
83 __set_errno (EOPNOTSUPP);
84 }
85 __close_nocancel (pathfd);
86 return ret;
87}
88#endif
89
90int
91fchmodat (int fd, const char *file, mode_t mode, int flag)
92{
93#if __ASSUME_FCHMODAT2
94 return INLINE_SYSCALL_CALL (fchmodat2, fd, file, mode, flag);
95#else
96 if (flag == 0)
97 return INLINE_SYSCALL_CALL (fchmodat, fd, file, mode);
98
99 int r = INLINE_SYSCALL_CALL (fchmodat2, fd, file, mode, flag);
100 if (r != 0 && errno == ENOSYS)
101 return fchmodat_fallback (fd, file, mode, flag);
102 return r;
103#endif
104}
105libc_hidden_def (fchmodat)
106

source code of glibc/sysdeps/unix/sysv/linux/fchmodat.c