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

NAME

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

LIBRARY

       Standard C library (libc, -lc)

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 */

       #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

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_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.

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 to indicate the error.

ERRORS

       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.

VERSIONS

       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

       access()
       faccessat()
              POSIX.1-2008.

       faccessat2()
              Linux.

HISTORY

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

       faccessat()
              Linux 2.6.16, glibc 2.4.

       faccessat2()
              Linux 5.8.

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.

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.

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 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.