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

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 */

       #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_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.

       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

       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 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.

   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.

COLOPHON

       This  page  is  part of release 5.13 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/.