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

NAME

       access, faccessat - 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);

   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()
The faccessat() system call 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().

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.

CONFORMING TO

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

       faccessat(): POSIX.1-2008.

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.

   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

       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 5.05 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/.