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fea681da | 1 | .\" This manpage is Copyright (C) 1992 Drew Eckhardt; |
ac56b6a8 | 2 | .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson. |
687dcec2 | 3 | .\" and Copyright (C) 2004, 2006, 2007, 2014 Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 4 | .\" |
93015253 | 5 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
6 | .\" Permission is granted to make and distribute verbatim copies of this |
7 | .\" manual provided the copyright notice and this permission notice are | |
8 | .\" preserved on all copies. | |
9 | .\" | |
10 | .\" Permission is granted to copy and distribute modified versions of this | |
11 | .\" manual under the conditions for verbatim copying, provided that the | |
12 | .\" entire resulting derived work is distributed under the terms of a | |
13 | .\" permission notice identical to this one. | |
c13182ef | 14 | .\" |
fea681da MK |
15 | .\" Since the Linux kernel and libraries are constantly changing, this |
16 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
17 | .\" responsibility for errors or omissions, or for damages resulting from | |
18 | .\" the use of the information contained herein. The author(s) may not | |
19 | .\" have taken the same level of care in the production of this manual, | |
20 | .\" which is licensed free of charge, as they might when working | |
21 | .\" professionally. | |
c13182ef | 22 | .\" |
fea681da MK |
23 | .\" Formatted or processed versions of this manual, if unaccompanied by |
24 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 25 | .\" %%%LICENSE_END |
fea681da MK |
26 | .\" |
27 | .\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu) | |
28 | .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com): | |
29 | .\" Removed note about old kernel (pre-1.1.44) using wrong id on path. | |
30 | .\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de): | |
31 | .\" Stated more clearly how it behaves with symbolic links. | |
32 | .\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426 | |
33 | .\" Modified 1996-09-07 by Michael Haardt: | |
34 | .\" Restrictions for NFS | |
35 | .\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk> | |
36 | .\" Modified 1998-01-13 by Michael Haardt: | |
37 | .\" Using access is often insecure | |
38 | .\" Modified 2001-10-16 by aeb | |
39 | .\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch> | |
40 | .\" Modified 2004-06-23 by Michael Kerrisk | |
02d79ece | 41 | .\" 2007-06-10, mtk, various parts rewritten, and added BUGS section. |
fea681da | 42 | .\" |
6d322d5f | 43 | .TH ACCESS 2 2015-02-21 "Linux" "Linux Programmer's Manual" |
fea681da | 44 | .SH NAME |
687dcec2 | 45 | access, faccessat \- check user's permissions for a file |
fea681da MK |
46 | .SH SYNOPSIS |
47 | .nf | |
48 | .B #include <unistd.h> | |
49 | .sp | |
50 | .BI "int access(const char *" pathname ", int " mode ); | |
687dcec2 | 51 | |
2626921f | 52 | .BR "#include <fcntl.h> " "/* Definition of AT_* constants */" |
687dcec2 MK |
53 | .B #include <unistd.h> |
54 | .sp | |
55 | .BI "int faccessat(int " dirfd ", const char *" pathname ", int " \ | |
56 | mode ", int " flags ); | |
57 | .fi | |
58 | .sp | |
59 | .in -4n | |
60 | Feature Test Macro Requirements for glibc (see | |
61 | .BR feature_test_macros (7)): | |
62 | .in | |
63 | .sp | |
64 | .BR faccessat (): | |
65 | .PD 0 | |
66 | .ad l | |
67 | .RS 4 | |
68 | .TP 4 | |
69 | Since glibc 2.10: | |
70 | _XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L | |
71 | .TP | |
72 | Before glibc 2.10: | |
73 | _ATFILE_SOURCE | |
74 | .RE | |
75 | .ad | |
76 | .PD | |
fea681da MK |
77 | .fi |
78 | .SH DESCRIPTION | |
e511ffb6 | 79 | .BR access () |
02d79ece | 80 | checks whether the calling process can access the file |
fea681da MK |
81 | .IR pathname . |
82 | If | |
83 | .I pathname | |
02d79ece | 84 | is a symbolic link, it is dereferenced. |
fea681da | 85 | |
02d79ece | 86 | The |
fea681da | 87 | .I mode |
02d79ece MK |
88 | specifies the accessibility check(s) to be performed, |
89 | and is either the value | |
90 | .BR F_OK , | |
60948d68 | 91 | .\" F_OK is defined as 0 on every system that I know of. |
b0742c81 | 92 | or a mask consisting of the bitwise OR of one or more of |
02d79ece | 93 | .BR R_OK ", " W_OK ", and " X_OK . |
fea681da | 94 | .B F_OK |
02d79ece MK |
95 | tests for the existence of the file. |
96 | .BR R_OK ", " W_OK ", and " X_OK | |
97 | test whether the file exists and grants read, write, and | |
98 | execute permissions, respectively. | |
fea681da | 99 | |
02d79ece | 100 | The check is done using the calling process's |
fea681da | 101 | .I real |
02d79ece MK |
102 | UID and GID, rather than the effective IDs as is done when |
103 | actually attempting an operation (e.g., | |
104 | .BR open (2)) | |
cae279c4 MK |
105 | on the file. |
106 | Similarly, for the root user, the check uses the set of | |
062d1cb3 | 107 | permitted capabilities rather than the set of effective |
cae279c4 | 108 | capabilities; and for non-root users, the check uses an empty set |
062d1cb3 DV |
109 | of capabilities. |
110 | ||
111 | This allows set-user-ID programs and capability-endowed programs | |
cae279c4 MK |
112 | to easily determine the invoking user's authority. |
113 | In other words, | |
062d1cb3 DV |
114 | .BR access () |
115 | does not answer the "can I read/write/execute this file?" question. | |
cae279c4 MK |
116 | It answers a slightly different question: |
117 | "(assuming I'm a setuid binary) can | |
062d1cb3 DV |
118 | .I the user who invoked me |
119 | read/write/execute this file?", | |
cae279c4 MK |
120 | which gives set-user-ID programs the possibility to |
121 | prevent malicious users from causing them to read files | |
122 | which users shouldn't be able to read. | |
fea681da | 123 | |
02d79ece MK |
124 | If the calling process is privileged (i.e., its real UID is zero), |
125 | then an | |
fea681da | 126 | .B X_OK |
02d79ece MK |
127 | check is successful for a regular file if execute permission |
128 | is enabled for any of the file owner, group, or other. | |
e04fe041 | 129 | .SS faccessat() |
687dcec2 MK |
130 | The |
131 | .BR faccessat () | |
132 | system call operates in exactly the same way as | |
cadd38ba | 133 | .BR access (), |
687dcec2 MK |
134 | except for the differences described here. |
135 | ||
136 | If the pathname given in | |
137 | .I pathname | |
138 | is relative, then it is interpreted relative to the directory | |
139 | referred to by the file descriptor | |
140 | .I dirfd | |
141 | (rather than relative to the current working directory of | |
142 | the calling process, as is done by | |
9d3ea7a2 | 143 | .BR access () |
687dcec2 MK |
144 | for a relative pathname). |
145 | ||
146 | If | |
147 | .I pathname | |
148 | is relative and | |
149 | .I dirfd | |
150 | is the special value | |
151 | .BR AT_FDCWD , | |
152 | then | |
153 | .I pathname | |
154 | is interpreted relative to the current working | |
155 | directory of the calling process (like | |
98d3114d | 156 | .BR access ()). |
687dcec2 MK |
157 | |
158 | If | |
159 | .I pathname | |
160 | is absolute, then | |
161 | .I dirfd | |
162 | is ignored. | |
163 | ||
164 | .I flags | |
165 | is constructed by ORing together zero or more of the following values: | |
166 | .TP | |
167 | .B AT_EACCESS | |
168 | Perform access checks using the effective user and group IDs. | |
169 | By default, | |
170 | .BR faccessat () | |
171 | uses the real IDs (like | |
98d3114d | 172 | .BR access ()). |
687dcec2 MK |
173 | .TP |
174 | .B AT_SYMLINK_NOFOLLOW | |
175 | If | |
176 | .I pathname | |
177 | is a symbolic link, do not dereference it: | |
178 | instead return information about the link itself. | |
2907fa3e MK |
179 | .PP |
180 | See | |
181 | .BR openat (2) | |
182 | for an explanation of the need for | |
183 | .BR faccessat (). | |
d282bb24 | 184 | .SH RETURN VALUE |
72d34312 MK |
185 | On success (all requested permissions granted, or |
186 | .I mode | |
187 | is | |
188 | .B F_OK | |
189 | and the file exists), zero is returned. | |
fea681da MK |
190 | On error (at least one bit in |
191 | .I mode | |
72d34312 MK |
192 | asked for a permission that is denied, or |
193 | .I mode | |
194 | is | |
195 | .B F_OK | |
196 | and the file does not exist, or some other error occurred), | |
fea681da MK |
197 | \-1 is returned, and |
198 | .I errno | |
199 | is set appropriately. | |
200 | .SH ERRORS | |
e511ffb6 | 201 | .BR access () |
32e370f2 MK |
202 | and |
203 | .BR faccessat () | |
fea681da MK |
204 | shall fail if: |
205 | .TP | |
206 | .B EACCES | |
dfaae362 | 207 | The requested access would be denied to the file, or search permission |
fea681da MK |
208 | is denied for one of the directories in the path prefix of |
209 | .IR pathname . | |
210 | (See also | |
ad7cc990 | 211 | .BR path_resolution (7).) |
fea681da MK |
212 | .TP |
213 | .B ELOOP | |
214 | Too many symbolic links were encountered in resolving | |
215 | .IR pathname . | |
216 | .TP | |
217 | .B ENAMETOOLONG | |
218 | .I pathname | |
219 | is too long. | |
220 | .TP | |
221 | .B ENOENT | |
33e820cb | 222 | A component of |
fea681da | 223 | .I pathname |
33e820cb | 224 | does not exist or is a dangling symbolic link. |
fea681da MK |
225 | .TP |
226 | .B ENOTDIR | |
227 | A component used as a directory in | |
228 | .I pathname | |
229 | is not, in fact, a directory. | |
230 | .TP | |
231 | .B EROFS | |
9ee4a2b6 | 232 | Write permission was requested for a file on a read-only filesystem. |
fea681da | 233 | .PP |
e511ffb6 | 234 | .BR access () |
32e370f2 MK |
235 | and |
236 | .BR faccessat () | |
fea681da MK |
237 | may fail if: |
238 | .TP | |
239 | .B EFAULT | |
240 | .I pathname | |
241 | points outside your accessible address space. | |
242 | .TP | |
243 | .B EINVAL | |
244 | .I mode | |
245 | was incorrectly specified. | |
246 | .TP | |
247 | .B EIO | |
248 | An I/O error occurred. | |
249 | .TP | |
250 | .B ENOMEM | |
251 | Insufficient kernel memory was available. | |
252 | .TP | |
253 | .B ETXTBSY | |
254 | Write access was requested to an executable which is being | |
255 | executed. | |
687dcec2 MK |
256 | .PP |
257 | The following additional errors can occur for | |
258 | .BR faccessat (): | |
259 | .TP | |
260 | .B EBADF | |
261 | .I dirfd | |
262 | is not a valid file descriptor. | |
263 | .TP | |
264 | .B EINVAL | |
265 | Invalid flag specified in | |
266 | .IR flags . | |
267 | .TP | |
268 | .B ENOTDIR | |
269 | .I pathname | |
270 | is relative and | |
271 | .I dirfd | |
272 | is a file descriptor referring to a file other than a directory. | |
273 | .SH VERSIONS | |
274 | .BR faccessat () | |
275 | was added to Linux in kernel 2.6.16; | |
276 | library support was added to glibc in version 2.4. | |
d282bb24 | 277 | .SH CONFORMING TO |
9d3ea7a2 | 278 | .BR access (): |
b9f4755d | 279 | SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008. |
687dcec2 | 280 | |
cadd38ba | 281 | .BR faccessat (): |
687dcec2 | 282 | POSIX.1-2008. |
b07cd0a9 | 283 | .SH NOTES |
fea681da | 284 | .PP |
4df883b9 | 285 | .BR Warning : |
32e370f2 | 286 | Using these calls to check if a user is authorized to, for example, |
dfaae362 | 287 | open a file before actually doing so using |
fea681da MK |
288 | .BR open (2) |
289 | creates a security hole, because the user might exploit the short time | |
290 | interval between checking and opening the file to manipulate it. | |
02d79ece | 291 | .BR "For this reason, the use of this system call should be avoided" . |
47135643 MK |
292 | (In the example just described, |
293 | a safer alternative would be to temporarily switch the process's | |
294 | effective user ID to the real ID and then call | |
295 | .BR open (2).) | |
02d79ece | 296 | .PP |
27d47e71 | 297 | .BR access () |
05b92251 MK |
298 | always dereferences symbolic links. |
299 | If you need to check the permissions on a symbolic link, use | |
6fdbc779 | 300 | .BR faccessat (2) |
05b92251 MK |
301 | with the flag |
302 | .BR AT_SYMLINK_NOFOLLOW . | |
303 | .PP | |
32e370f2 | 304 | These calls return an error if any of the access types in |
02d79ece MK |
305 | .I mode |
306 | is denied, even if some of the other access types in | |
307 | .I mode | |
308 | are permitted. | |
309 | .PP | |
310 | If the calling process has appropriate privileges (i.e., is superuser), | |
7a35b981 | 311 | POSIX.1-2001 permits an implementation to indicate success for an |
02d79ece MK |
312 | .B X_OK |
313 | check even if none of the execute file permission bits are set. | |
d8ccd692 | 314 | .\" HPU-UX 11 and Tru64 5.1 do this. |
02d79ece MK |
315 | Linux does not do this. |
316 | .PP | |
33a0ccb2 | 317 | A file is accessible only if the permissions on each of the |
02d79ece MK |
318 | directories in the path prefix of |
319 | .I pathname | |
320 | grant search (i.e., execute) access. | |
321 | If any directory is inaccessible, then the | |
322 | .BR access () | |
323 | call will fail, regardless of the permissions on the file itself. | |
324 | .PP | |
325 | Only access bits are checked, not the file type or contents. | |
326 | Therefore, if a directory is found to be writable, | |
327 | it probably means that files can be created in the directory, | |
328 | and not that the directory can be written as a file. | |
329 | Similarly, a DOS file may be found to be "executable," but the | |
330 | .BR execve (2) | |
331 | call will still fail. | |
332 | .PP | |
32e370f2 | 333 | These calls |
1a7d4eb7 | 334 | may not work correctly on NFSv2 filesystems with UID mapping enabled, |
02d79ece | 335 | because UID mapping is done on the server and hidden from the client, |
1a7d4eb7 BF |
336 | which checks permissions. (NFS versions 3 and higher perform the check on |
337 | the server.) | |
e0a699c1 | 338 | Similar problems can occur to FUSE mounts. |
92692952 MK |
339 | .\" |
340 | .\" | |
8f692313 | 341 | .SS C library/kernel ABI differences |
2907fa3e MK |
342 | The raw |
343 | .BR faccessat () | |
344 | system call takes only the first three arguments. | |
687dcec2 MK |
345 | The |
346 | .B AT_EACCESS | |
347 | and | |
348 | .B AT_SYMLINK_NOFOLLOW | |
349 | flags are actually implemented within the glibc wrapper function for | |
350 | .BR faccessat (). | |
351 | If either of these flags is specified, then the wrapper function employs | |
352 | .BR fstatat (2) | |
353 | to determine access permissions. | |
cf81f364 MK |
354 | .SS Glibc notes |
355 | On older kernels where | |
356 | .BR faccessat () | |
357 | is unavailable (and when the | |
358 | .B AT_EACCESS | |
359 | and | |
360 | .B AT_SYMLINK_NOFOLLOW | |
361 | flags are not specified), | |
362 | the glibc wrapper function falls back to the use of | |
363 | .BR access (). | |
364 | When | |
365 | .I pathname | |
366 | is a relative pathname, | |
367 | glibc constructs a pathname based on the symbolic link in | |
368 | .IR /proc/self/fd | |
369 | that corresponds to the | |
370 | .IR dirfd | |
371 | argument. | |
02d79ece MK |
372 | .SH BUGS |
373 | In kernel 2.4 (and earlier) there is some strangeness in the handling of | |
374 | .B X_OK | |
375 | tests for superuser. | |
376 | If all categories of execute permission are disabled | |
24b74457 | 377 | for a nondirectory file, then the only |
02d79ece MK |
378 | .BR access () |
379 | test that returns \-1 is when | |
380 | .I mode | |
381 | is specified as just | |
382 | .BR X_OK ; | |
383 | if | |
384 | .B R_OK | |
385 | or | |
386 | .B W_OK | |
387 | is also specified in | |
388 | .IR mode , | |
389 | then | |
390 | .BR access () | |
391 | returns 0 for such files. | |
392 | .\" This behavior appears to have been an implementation accident. | |
393 | Early 2.6 kernels (up to and including 2.6.3) | |
394 | also behaved in the same way as kernel 2.4. | |
395 | ||
a1d5f77c | 396 | In kernels before 2.6.20, |
32e370f2 | 397 | these calls ignored the effect of the |
a1d5f77c MK |
398 | .B MS_NOEXEC |
399 | flag if it was used to | |
400 | .BR mount (2) | |
9ee4a2b6 | 401 | the underlying filesystem. |
32e370f2 MK |
402 | Since kernel 2.6.20, the |
403 | .B MS_NOEXEC | |
8d0418af | 404 | flag is honored. |
d282bb24 | 405 | .SH SEE ALSO |
fea681da MK |
406 | .BR chmod (2), |
407 | .BR chown (2), | |
408 | .BR open (2), | |
fea681da MK |
409 | .BR setgid (2), |
410 | .BR setuid (2), | |
d975050c | 411 | .BR stat (2), |
70d1cb2d | 412 | .BR euidaccess (3), |
53a1443c | 413 | .BR credentials (7), |
687dcec2 MK |
414 | .BR path_resolution (7), |
415 | .BR symlink (7) |