access(2) - Linux manual page (original) (raw)
access(2) System Calls Manual access(2)
NAME top
access, faccessat, faccessat2 - check user's permissions for a
fileLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_SOURCEDESCRIPTION top
**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)](../man2/open.2.html)) 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)](../man2/open.2.html) **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)](../man2/openat.2.html) 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.
RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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)](../man7/path%5Fresolution.7.html).)
**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)](../man2/FS%5FIOC%5FSETFLAGS.2const.html).
**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.VERSIONS top
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.
STANDARDS top
**access**()
**faccessat**()
POSIX.1-2008.
**faccessat2**()
Linux.HISTORY top
**access**()
SVr4, 4.3BSD, POSIX.1-2001.
**faccessat**()
Linux 2.6.16, glibc 2.4.
**faccessat2**()
Linux 5.8.NOTES top
**Warning**: Using these calls to check if a user is authorized to,
for example, open a file before actually doing so using [open(2)](../man2/open.2.html)
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)](../man2/open.2.html).)
**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)](../man2/execve.2.html) 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.BUGS top
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)](../man2/fstatat.2.html). 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)](../man2/mount.2.html) the underlying
filesystem. Since Linux 2.6.20, the **MS_NOEXEC** flag is honored.SEE ALSO top
[chmod(2)](../man2/chmod.2.html), [chown(2)](../man2/chown.2.html), [open(2)](../man2/open.2.html), [setgid(2)](../man2/setgid.2.html), [setuid(2)](../man2/setuid.2.html), [stat(2)](../man2/stat.2.html),
[euidaccess(3)](../man3/euidaccess.3.html), [credentials(7)](../man7/credentials.7.html), [path_resolution(7)](../man7/path%5Fresolution.7.html), [symlink(7)](../man7/symlink.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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)