1ACCESS(2) Linux Programmer's Manual ACCESS(2)
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6 access, faccessat, faccessat2 - check user's permissions for a file
7
9 #include <unistd.h>
10 int access(const char *pathname, int mode);
12 #include <fcntl.h> /* Definition of AT_* constants */
14 #include <unistd.h>
15 int faccessat(int dirfd, const char *pathname, int mode, int flags);
17 /* But see C library/kernel differences, below */
18 int faccessat2(int dirfd, const char *pathname, int mode, int flags);
20 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
22 faccessat():
24 Since glibc 2.10:
25 _POSIX_C_SOURCE >= 200809L
26 Before glibc 2.10:
27 _ATFILE_SOURCE
28
30 access() checks whether the calling process can access the file path‐
31 name. If pathname is a symbolic link, it is dereferenced.
32 The mode specifies the accessibility check(s) to be performed, and is
34 either the value F_OK, or a mask consisting of the bitwise OR of one or
35 more of R_OK, W_OK, and X_OK. F_OK tests for the existence of the
36 file. R_OK, W_OK, and X_OK test whether the file exists and grants
37 read, write, and execute permissions, respectively.
38 The check is done using the calling process's real UID and GID, rather
40 than the effective IDs as is done when actually attempting an operation
41 (e.g., open(2)) on the file. Similarly, for the root user, the check
42 uses the set of permitted capabilities rather than the set of effective
43 capabilities; and for non-root users, the check uses an empty set of
44 capabilities.
45 This allows set-user-ID programs and capability-endowed programs to
47 easily determine the invoking user's authority. In other words, ac‐
48 cess() does not answer the "can I read/write/execute this file?" ques‐
49 tion. It answers a slightly different question: "(assuming I'm a se‐
50 tuid binary) can the user who invoked me read/write/execute this
51 file?", which gives set-user-ID programs the possibility to prevent ma‐
52 licious users from causing them to read files which users shouldn't be
53 able to read.
54 If the calling process is privileged (i.e., its real UID is zero), then
56 an X_OK check is successful for a regular file if execute permission is
57 enabled for any of the file owner, group, or other.
58 faccessat()
60 faccessat() operates in exactly the same way as access(), except for
61 the differences described here.
62 If the pathname given in pathname is relative, then it is interpreted
64 relative to the directory referred to by the file descriptor dirfd
65 (rather than relative to the current working directory of the calling
66 process, as is done by access() for a relative pathname).
67 If pathname is relative and dirfd is the special value AT_FDCWD, then
69 pathname is interpreted relative to the current working directory of
70 the calling process (like access()).
71 If pathname is absolute, then dirfd is ignored.
73 flags is constructed by ORing together zero or more of the following
75 values:
76 AT_EACCESS
78 Perform access checks using the effective user and group IDs.
79 By default, faccessat() uses the real IDs (like access()).
80 AT_SYMLINK_NOFOLLOW
82 If pathname is a symbolic link, do not dereference it: instead
83 return information about the link itself.
84 See openat(2) for an explanation of the need for faccessat().
86 faccessat2()
88 The description of faccessat() given above corresponds to POSIX.1 and
89 to the implementation provided by glibc. However, the glibc implemen‐
90 tation was an imperfect emulation (see BUGS) that papered over the fact
91 that the raw Linux faccessat() system call does not have a flags argu‐
92 ment. To allow for a proper implementation, Linux 5.8 added the fac‐
93 cessat2() system call, which supports the flags argument and allows a
94 correct implementation of the faccessat() wrapper function.
95
97 On success (all requested permissions granted, or mode is F_OK and the
98 file exists), zero is returned. On error (at least one bit in mode
99 asked for a permission that is denied, or mode is F_OK and the file
100 does not exist, or some other error occurred), -1 is returned, and er‐
101 rno is set appropriately.
102
104 access() and faccessat() shall fail if:
105 EACCES The requested access would be denied to the file, or search per‐
107 mission is denied for one of the directories in the path prefix
108 of pathname. (See also path_resolution(7).)
109 ELOOP Too many symbolic links were encountered in resolving pathname.
111 ENAMETOOLONG
113 pathname is too long.
114 ENOENT A component of pathname does not exist or is a dangling symbolic
116 link.
117 ENOTDIR
119 A component used as a directory in pathname is not, in fact, a
120 directory.
121 EROFS Write permission was requested for a file on a read-only
123 filesystem.
124 access() and faccessat() may fail if:
126 EFAULT pathname points outside your accessible address space.
128 EINVAL mode was incorrectly specified.
130 EIO An I/O error occurred.
132 ENOMEM Insufficient kernel memory was available.
134 ETXTBSY
136 Write access was requested to an executable which is being exe‐
137 cuted.
138 The following additional errors can occur for faccessat():
140 EBADF dirfd is not a valid file descriptor.
142 EINVAL Invalid flag specified in flags.
144 ENOTDIR
146 pathname is relative and dirfd is a file descriptor referring to
147 a file other than a directory.
148
150 faccessat() was added to Linux in kernel 2.6.16; library support was
151 added to glibc in version 2.4.
152 faccessat2() was added to Linux in version 5.8.
154
156 access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
157 faccessat(): POSIX.1-2008.
159 faccessat2(): Linux-specific.
161
163 Warning: Using these calls to check if a user is authorized to, for ex‐
164 ample, open a file before actually doing so using open(2) creates a se‐
165 curity hole, because the user might exploit the short time interval be‐
166 tween checking and opening the file to manipulate it. For this reason,
167 the use of this system call should be avoided. (In the example just
168 described, a safer alternative would be to temporarily switch the
169 process's effective user ID to the real ID and then call open(2).)
170 access() always dereferences symbolic links. If you need to check the
172 permissions on a symbolic link, use faccessat() with the flag AT_SYM‐
173 LINK_NOFOLLOW.
174 These calls return an error if any of the access types in mode is de‐
176 nied, even if some of the other access types in mode are permitted.
177 If the calling process has appropriate privileges (i.e., is superuser),
179 POSIX.1-2001 permits an implementation to indicate success for an X_OK
180 check even if none of the execute file permission bits are set. Linux
181 does not do this.
182 A file is accessible only if the permissions on each of the directories
184 in the path prefix of pathname grant search (i.e., execute) access. If
185 any directory is inaccessible, then the access() call fails, regardless
186 of the permissions on the file itself.
187 Only access bits are checked, not the file type or contents. There‐
189 fore, if a directory is found to be writable, it probably means that
190 files can be created in the directory, and not that the directory can
191 be written as a file. Similarly, a DOS file may be found to be "exe‐
192 cutable," but the execve(2) call will still fail.
193 These calls may not work correctly on NFSv2 filesystems with UID map‐
195 ping enabled, because UID mapping is done on the server and hidden from
196 the client, which checks permissions. (NFS versions 3 and higher per‐
197 form the check on the server.) Similar problems can occur to FUSE
198 mounts.
199 C library/kernel differences
201 The raw faccessat() system call takes only the first three arguments.
202 The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are actually implemented
203 within the glibc wrapper function for faccessat(). If either of these
204 flags is specified, then the wrapper function employs fstatat(2) to de‐
205 termine access permissions, but see BUGS.
206 Glibc notes
208 On older kernels where faccessat() is unavailable (and when the AT_EAC‐
209 CESS and AT_SYMLINK_NOFOLLOW flags are not specified), the glibc wrap‐
210 per function falls back to the use of access(). When pathname is a
211 relative pathname, glibc constructs a pathname based on the symbolic
212 link in /proc/self/fd that corresponds to the dirfd argument.
213
215 Because the Linux kernel's faccessat() system call does not support a
216 flags argument, the glibc faccessat() wrapper function provided in
217 glibc 2.32 and earlier emulates the required functionality using a com‐
218 bination of the faccessat() system call and fstatat(2). However, this
219 emulation does not take ACLs into account. Starting with glibc 2.33,
220 the wrapper function avoids this bug by making use of the faccessat2()
221 system call where it is provided by the underlying kernel.
222 In kernel 2.4 (and earlier) there is some strangeness in the handling
224 of X_OK tests for superuser. If all categories of execute permission
225 are disabled for a nondirectory file, then the only access() test that
226 returns -1 is when mode is specified as just X_OK; if R_OK or W_OK is
227 also specified in mode, then access() returns 0 for such files. Early
228 2.6 kernels (up to and including 2.6.3) also behaved in the same way as
229 kernel 2.4.
230 In kernels before 2.6.20, these calls ignored the effect of the
232 MS_NOEXEC flag if it was used to mount(2) the underlying filesystem.
233 Since kernel 2.6.20, the MS_NOEXEC flag is honored.
234
236 chmod(2), chown(2), open(2), setgid(2), setuid(2), stat(2), euidac‐
237 cess(3), credentials(7), path_resolution(7), symlink(7)
238
240 This page is part of release 5.10 of the Linux man-pages project. A
241 description of the project, information about reporting bugs, and the
242 latest version of this page, can be found at
243 https://www.kernel.org/doc/man-pages/.
244Linux 2020-12-21 ACCESS(2)