access, faccessat - check user's permissions for a file
#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);
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.
The
faccessat() system call 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_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().
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 appropriately.
access() and
faccessat() shall fail if:
- 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).)
- 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.
- ENOTDIR
- A component used as a directory in pathname is not, in fact, a
directory.
- EROFS
- Write permission was requested for a file on a read-only filesystem.
access() and
faccessat() may fail if:
- EFAULT
- pathname points outside your accessible address space.
- EINVAL
- mode was incorrectly specified.
- EIO
- An I/O error occurred.
- ENOMEM
- Insufficient kernel memory was available.
- ETXTBSY
- Write access was requested to an executable which is being executed.
The following additional errors can occur for
faccessat():
- EBADF
- dirfd is not a valid file descriptor.
- EINVAL
- Invalid flag specified in flags.
- ENOTDIR
- pathname is relative and dirfd is a file descriptor
referring to a file other than a directory.
faccessat() was added to Linux in kernel 2.6.16; library support was
added to glibc in version 2.4.
access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
faccessat(): POSIX.1-2008.
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.
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.
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 found to be "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.
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.
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.
In kernel 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 2.6 kernels (up to and
including 2.6.3) also behaved in the same way as kernel 2.4.
In kernels before 2.6.20, these calls ignored the effect of the
MS_NOEXEC
flag if it was used to
mount(2) the underlying filesystem. Since kernel
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)