access(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

ACCESS(2)               Linux Programmer's Manual              ACCESS(2)

NAME         top

       access, faccessat, faccessat2 - check user's permissions for a
       file

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 */

       int 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

DESCRIPTION         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)) 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_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.

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 is set appropriately.

ERRORS         top

       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.

VERSIONS         top

       faccessat() was added to Linux in kernel 2.6.16; library support
       was added to glibc in version 2.4.

       faccessat2() was added to Linux in version 5.8.

CONFORMING TO         top

       access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.

       faccessat(): POSIX.1-2008.

       faccessat2(): Linux-specific.

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)
       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.

   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.

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).  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 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.

SEE ALSO         top

       chmod(2), chown(2), open(2), setgid(2), setuid(2), stat(2),
       euidaccess(3), credentials(7), path_resolution(7), symlink(7)

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-12-21                      ACCESS(2)

Pages that refer to this page: find(1)pmseries(1)strace(1)open(2)stat(2)statx(2)syscalls(2)euidaccess(3)cpuset(7)credentials(7)signal-safety(7)spufs(7)symlink(7)lsof(8)