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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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access(2) System Calls Manual access(2)
access, faccessat, faccessat2 - check user's permissions for a
file
Standard C library (libc, -lc)
#include <unistd.h>
int access(const char *pathname, int mode);
#include <fcntl.h> /* Definition of AT_* constants */
#include <unistd.h>
int faccessat(int dirfd, const char *pathname, int mode, int flags);
/* But see C library/kernel differences, below */
#include <fcntl.h> /* Definition of AT_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_faccessat2,
int dirfd, const char *pathname, int mode, int flags);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
faccessat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
access() checks whether the calling process can access the file
pathname. If pathname is a symbolic link, it is dereferenced.
The mode specifies the accessibility check(s) to be performed,
and is either the value F_OK, or a mask consisting of the bitwise
OR of one or more of R_OK, W_OK, and X_OK. F_OK tests for the
existence of the file. R_OK, W_OK, and X_OK test whether the
file exists and grants read, write, and execute permissions,
respectively.
The check is done using the calling process's real UID and GID,
rather than the effective IDs as is done when actually attempting
an operation (e.g., open(2)) on the file. Similarly, for the
root user, the check uses the set of permitted capabilities
rather than the set of effective capabilities; and for non-root
users, the check uses an empty set of capabilities.
This allows set-user-ID programs and capability-endowed programs
to easily determine the invoking user's authority. In other
words, access() does not answer the "can I read/write/execute
this file?" question. It answers a slightly different question:
"(assuming I'm a setuid binary) can the user who invoked me
read/write/execute this file?", which gives set-user-ID programs
the possibility to prevent malicious users from causing them to
read files which users shouldn't be able to read.
If the calling process is privileged (i.e., its real UID is
zero), then an X_OK check is successful for a regular file if
execute permission is enabled for any of the file owner, group,
or other.
faccessat()
faccessat() operates in exactly the same way as access(), except
for the differences described here.
If the pathname given in pathname is relative, then it is
interpreted relative to the directory referred to by the file
descriptor dirfd (rather than relative to the current working
directory of the calling process, as is done by access() for a
relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD,
then pathname is interpreted relative to the current working
directory of the calling process (like access()).
If pathname is absolute, then dirfd is ignored.
flags is constructed by ORing together zero or more of the
following values:
AT_EACCESS
Perform access checks using the effective user and group
IDs. By default, faccessat() uses the real IDs (like
access()).
AT_EMPTY_PATH (since Linux 5.8)
If pathname is an empty string, operate on the file
referred to by dirfd (which may have been obtained using
the open(2) O_PATH flag). In this case, dirfd can refer
to any type of file, not just a directory. If dirfd is
AT_FDCWD, the call operates on the current working
directory. This flag is Linux-specific; define
_GNU_SOURCE to obtain its definition.
AT_SYMLINK_NOFOLLOW
If pathname is a symbolic link, do not dereference it:
instead return information about the link itself.
See openat(2) for an explanation of the need for faccessat().
faccessat2()
The description of faccessat() given above corresponds to POSIX.1
and to the implementation provided by glibc. However, the glibc
implementation was an imperfect emulation (see BUGS) that papered
over the fact that the raw Linux faccessat() system call does not
have a flags argument. To allow for a proper implementation,
Linux 5.8 added the faccessat2() system call, which supports the
flags argument and allows a correct implementation of the
faccessat() wrapper function.
On success (all requested permissions granted, or mode is F_OK
and the file exists), zero is returned. On error (at least one
bit in mode asked for a permission that is denied, or mode is
F_OK and the file does not exist, or some other error occurred),
-1 is returned, and errno is set to indicate the error.
EACCES The requested access would be denied to the file, or
search permission is denied for one of the directories in
the path prefix of pathname. (See also
path_resolution(7).)
EBADF (faccessat()) pathname is relative but dirfd is neither
AT_FDCWD (faccessat()) nor a valid file descriptor.
EFAULT pathname points outside your accessible address space.
EINVAL mode was incorrectly specified.
EINVAL (faccessat()) Invalid flag specified in flags.
EIO An I/O error occurred.
ELOOP Too many symbolic links were encountered in resolving
pathname.
ENAMETOOLONG
pathname is too long.
ENOENT A component of pathname does not exist or is a dangling
symbolic link.
ENOMEM Insufficient kernel memory was available.
ENOTDIR
A component used as a directory in pathname is not, in
fact, a directory.
ENOTDIR
(faccessat()) pathname is relative and dirfd is a file
descriptor referring to a file other than a directory.
EPERM Write permission was requested to a file that has the
immutable flag set. See also FS_IOC_SETFLAGS(2const).
EROFS Write permission was requested for a file on a read-only
filesystem.
ETXTBSY
Write access was requested to an executable which is being
executed.
If the calling process has appropriate privileges (i.e., is
superuser), POSIX.1-2001 permits an implementation to indicate
success for an X_OK check even if none of the execute file
permission bits are set. Linux does not do this.
C library/kernel differences
The raw faccessat() system call takes only the first three
arguments. The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are
actually implemented within the glibc wrapper function for
faccessat(). If either of these flags is specified, then the
wrapper function employs fstatat(2) to determine access
permissions, but see BUGS.
glibc notes
On older kernels where faccessat() is unavailable (and when the
AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are not specified), the
glibc wrapper function falls back to the use of access(). When
pathname is a relative pathname, glibc constructs a pathname
based on the symbolic link in /proc/self/fd that corresponds to
the dirfd argument.
access()
faccessat()
POSIX.1-2008.
faccessat2()
Linux.
access()
SVr4, 4.3BSD, POSIX.1-2001.
faccessat()
Linux 2.6.16, glibc 2.4.
faccessat2()
Linux 5.8.
Warning: Using these calls to check if a user is authorized to,
for example, open a file before actually doing so using open(2)
creates a security hole, because the user might exploit the short
time interval between checking and opening the file to manipulate
it. For this reason, the use of this system call should be
avoided. (In the example just described, a safer alternative
would be to temporarily switch the process's effective user ID to
the real ID and then call open(2).)
access() always dereferences symbolic links. If you need to
check the permissions on a symbolic link, use faccessat() with
the flag AT_SYMLINK_NOFOLLOW.
These calls return an error if any of the access types in mode is
denied, even if some of the other access types in mode are
permitted.
A file is accessible only if the permissions on each of the
directories in the path prefix of pathname grant search (i.e.,
execute) access. If any directory is inaccessible, then the
access() call fails, regardless of the permissions on the file
itself.
Only access bits are checked, not the file type or contents.
Therefore, if a directory is found to be writable, it probably
means that files can be created in the directory, and not that
the directory can be written as a file. Similarly, a DOS file
may be reported as executable, but the execve(2) call will still
fail.
These calls may not work correctly on NFSv2 filesystems with UID
mapping enabled, because UID mapping is done on the server and
hidden from the client, which checks permissions. (NFS versions
3 and higher perform the check on the server.) Similar problems
can occur to FUSE mounts.
Because the Linux kernel's faccessat() system call does not
support a flags argument, the glibc faccessat() wrapper function
provided in glibc 2.32 and earlier emulates the required
functionality using a combination of the faccessat() system call
and fstatat(2). However, this emulation does not take ACLs into
account. Starting with glibc 2.33, the wrapper function avoids
this bug by making use of the faccessat2() system call where it
is provided by the underlying kernel.
In Linux 2.4 (and earlier) there is some strangeness in the
handling of X_OK tests for superuser. If all categories of
execute permission are disabled for a nondirectory file, then the
only access() test that returns -1 is when mode is specified as
just X_OK; if R_OK or W_OK is also specified in mode, then
access() returns 0 for such files. Early Linux 2.6 (up to and
including Linux 2.6.3) also behaved in the same way as Linux 2.4.
Before Linux 2.6.20, these calls ignored the effect of the
MS_NOEXEC flag if it was used to mount(2) the underlying
filesystem. Since Linux 2.6.20, the MS_NOEXEC flag is honored.
chmod(2), chown(2), open(2), setgid(2), setuid(2), stat(2),
euidaccess(3), credentials(7), path_resolution(7), symlink(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-06-13 access(2)
Pages that refer to this page: find(1), pmseries(1), strace(1), test(1), open(2), stat(2), statx(2), syscalls(2), euidaccess(3), cpuset(7), credentials(7), landlock(7), signal-safety(7), spufs(7), symlink(7), lsof(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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