faccessat2

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



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

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

       faccessat(): POSIX.1-2008.

       faccessat2(): Linux-specific.

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

COLOPHON
       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)