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
              ioctl_iflags(2).

       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.