path: root/man2/access.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"             and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2004, 2006, 2007, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu)
.\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
.\"   Removed note about old kernel (pre-1.1.44) using wrong id on path.
.\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
.\"   Stated more clearly how it behaves with symbolic links.
.\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
.\" Modified 1996-09-07 by Michael Haardt:
.\"   Restrictions for NFS
.\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
.\" Modified 1998-01-13 by Michael Haardt:
.\"   Using access is often insecure
.\" Modified 2001-10-16 by aeb
.\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
.\" Modified 2004-06-23 by Michael Kerrisk
.\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
.\"
.TH ACCESS 2 2020-12-21 "Linux" "Linux Programmer's Manual"
.SH NAME
access, faccessat, faccessat2 \- check user's permissions for a file
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "int access(const char *" pathname ", int " mode );

.BR "#include <fcntl.h>           " "/* Definition of AT_* constants */"
.B #include <unistd.h>
.PP
.BI "int faccessat(int " dirfd ", const char *" pathname ", int " \
mode ", int " flags );
                /* But see C library/kernel differences, below */

.BI "int faccessat2(int " dirfd ", const char *" pathname ", int " \
mode ", int " flags );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR faccessat ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.10:
_POSIX_C_SOURCE\ >=\ 200809L
.TP
Before glibc 2.10:
_ATFILE_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
.BR access ()
checks whether the calling process can access the file
.IR pathname .
If
.I pathname
is a symbolic link, it is dereferenced.
.PP
The
.I mode
specifies the accessibility check(s) to be performed,
and is either the value
.BR F_OK ,
.\" F_OK is defined as 0 on every system that I know of.
or a mask consisting of the bitwise OR of one or more of
.BR R_OK ", " W_OK ", and " X_OK .
.B F_OK
tests for the existence of the file.
.BR R_OK ", " W_OK ", and " X_OK
test whether the file exists and grants read, write, and
execute permissions, respectively.
.PP
The check is done using the calling process's
.I real
UID and GID, rather than the effective IDs as is done when
actually attempting an operation (e.g.,
.BR 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.
.PP
This allows set-user-ID programs and capability-endowed programs
to easily determine the invoking user's authority.
In other words,
.BR 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
.I 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.
.PP
If the calling process is privileged (i.e., its real UID is zero),
then an
.B X_OK
check is successful for a regular file if execute permission
is enabled for any of the file owner, group, or other.
.SS faccessat()
.BR faccessat ()
operates in exactly the same way as
.BR access (),
except for the differences described here.
.PP
If the pathname given in
.I pathname
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
.I dirfd
(rather than relative to the current working directory of
the calling process, as is done by
.BR access ()
for a relative pathname).
.PP
If
.I pathname
is relative and
.I dirfd
is the special value
.BR AT_FDCWD ,
then
.I pathname
is interpreted relative to the current working
directory of the calling process (like
.BR access ()).
.PP
If
.I pathname
is absolute, then
.I dirfd
is ignored.
.PP
.I flags
is constructed by ORing together zero or more of the following values:
.TP
.B AT_EACCESS
Perform access checks using the effective user and group IDs.
By default,
.BR faccessat ()
uses the real IDs (like
.BR access ()).
.TP
.B AT_SYMLINK_NOFOLLOW
If
.I pathname
is a symbolic link, do not dereference it:
instead return information about the link itself.
.PP
See
.BR openat (2)
for an explanation of the need for
.BR faccessat ().
.\"
.SS faccessat2()
The description of
.BR 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
.BR faccessat ()
system call does not have a
.I flags
argument.
To allow for a proper implementation, Linux 5.8 added the
.BR faccessat2 ()
system call, which supports the
.I flags
argument and allows a correct implementation of the
.BR faccessat ()
wrapper function.
.SH RETURN VALUE
On success (all requested permissions granted, or
.I mode
is
.B F_OK
and the file exists), zero is returned.
On error (at least one bit in
.I mode
asked for a permission that is denied, or
.I mode
is
.B F_OK
and the file does not exist, or some other error occurred),
\-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.BR access ()
and
.BR faccessat ()
shall fail if:
.TP
.B 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
.IR pathname .
(See also
.BR path_resolution (7).)
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR pathname .
.TP
.B ENAMETOOLONG
.I pathname
is too long.
.TP
.B ENOENT
A component of
.I pathname
does not exist or is a dangling symbolic link.
.TP
.B ENOTDIR
A component used as a directory in
.I pathname
is not, in fact, a directory.
.TP
.B EROFS
Write permission was requested for a file on a read-only filesystem.
.PP
.BR access ()
and
.BR faccessat ()
may fail if:
.TP
.B EFAULT
.I pathname
points outside your accessible address space.
.TP
.B EINVAL
.I mode
was incorrectly specified.
.TP
.B EIO
An I/O error occurred.
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ETXTBSY
Write access was requested to an executable which is being
executed.
.PP
The following additional errors can occur for
.BR faccessat ():
.TP
.B EBADF
.I dirfd
is not a valid file descriptor.
.TP
.B EINVAL
Invalid flag specified in
.IR flags .
.TP
.B ENOTDIR
.I pathname
is relative and
.I dirfd
is a file descriptor referring to a file other than a directory.
.SH VERSIONS
.BR faccessat ()
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
.PP
.BR faccessat2 ()
was added to Linux in version 5.8.
.SH CONFORMING TO
.BR access ():
SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
.PP
.BR faccessat ():
POSIX.1-2008.
.PP
.BR faccessat2 ():
Linux-specific.
.SH NOTES
.BR Warning :
Using these calls to check if a user is authorized to, for example,
open a file before actually doing so using
.BR open (2)
creates a security hole, because the user might exploit the short time
interval between checking and opening the file to manipulate it.
.BR "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
.BR open (2).)
.PP
.BR access ()
always dereferences symbolic links.
If you need to check the permissions on a symbolic link, use
.BR faccessat ()
with the flag
.BR AT_SYMLINK_NOFOLLOW .
.PP
These calls return an error if any of the access types in
.I mode
is denied, even if some of the other access types in
.I mode
are permitted.
.PP
If the calling process has appropriate privileges (i.e., is superuser),
POSIX.1-2001 permits an implementation to indicate success for an
.B X_OK
check even if none of the execute file permission bits are set.
.\" HPU-UX 11 and Tru64 5.1 do this.
Linux does not do this.
.PP
A file is accessible only if the permissions on each of the
directories in the path prefix of
.I pathname
grant search (i.e., execute) access.
If any directory is inaccessible, then the
.BR access ()
call fails, regardless of the permissions on the file itself.
.PP
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
.BR execve (2)
call will still fail.
.PP
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.
.\"
.\"
.SS C library/kernel differences
The raw
.BR faccessat ()
system call takes only the first three arguments.
The
.B AT_EACCESS
and
.B AT_SYMLINK_NOFOLLOW
flags are actually implemented within the glibc wrapper function for
.BR faccessat ().
If either of these flags is specified, then the wrapper function employs
.BR fstatat (2)
to determine access permissions, but see BUGS.
.\"
.SS Glibc notes
On older kernels where
.BR faccessat ()
is unavailable (and when the
.B AT_EACCESS
and
.B AT_SYMLINK_NOFOLLOW
flags are not specified),
the glibc wrapper function falls back to the use of
.BR access ().
When
.I pathname
is a relative pathname,
glibc constructs a pathname based on the symbolic link in
.IR /proc/self/fd
that corresponds to the
.IR dirfd
argument.
.SH BUGS
Because the Linux kernel's
.BR faccessat ()
system call does not support a
.I flags
argument, the glibc
.BR faccessat ()
wrapper function provided in glibc 2.32 and earlier
emulates the required functionality using
a combination of the
.BR faccessat ()
system call and
.BR 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
.BR faccessat2 ()
system call where it is provided by the underlying kernel.
.PP
In kernel 2.4 (and earlier) there is some strangeness in the handling of
.B X_OK
tests for superuser.
If all categories of execute permission are disabled
for a nondirectory file, then the only
.BR access ()
test that returns \-1 is when
.I mode
is specified as just
.BR X_OK ;
if
.B R_OK
or
.B W_OK
is also specified in
.IR mode ,
then
.BR access ()
returns 0 for such files.
.\" This behavior appears to have been an implementation accident.
Early 2.6 kernels (up to and including 2.6.3)
also behaved in the same way as kernel 2.4.
.PP
In kernels before 2.6.20,
these calls ignored the effect of the
.B MS_NOEXEC
flag if it was used to
.BR mount (2)
the underlying filesystem.
Since kernel 2.6.20, the
.B MS_NOEXEC
flag is honored.
.SH SEE ALSO
.BR chmod (2),
.BR chown (2),
.BR open (2),
.BR setgid (2),
.BR setuid (2),
.BR stat (2),
.BR euidaccess (3),
.BR credentials (7),
.BR path_resolution (7),
.BR symlink (7)
.SH COLOPHON
This page is part of release 5.10 of the Linux
.I 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/.