mirror of https://github.com/mkerrisk/man-pages
252 lines
7.3 KiB
Groff
252 lines
7.3 KiB
Groff
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
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.\" 1993 Michael Haardt, Ian Jackson.
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.\" and Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu)
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.\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
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.\" Removed note about old kernel (pre-1.1.44) using wrong id on path.
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.\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
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.\" Stated more clearly how it behaves with symbolic links.
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.\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
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.\" Modified 1996-09-07 by Michael Haardt:
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.\" Restrictions for NFS
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.\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
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.\" Modified 1998-01-13 by Michael Haardt:
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.\" Using access is often insecure
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.\" Modified 2001-10-16 by aeb
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.\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
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.\" Modified 2004-06-23 by Michael Kerrisk
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.\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
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.\"
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.TH ACCESS 2 2013-02-28
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inux" "Linux Programmer's Manual"
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.SH NAME
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access \- check real user's permissions for a file
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.SH SYNOPSIS
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.nf
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.B #include <unistd.h>
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.sp
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.BI "int access(const char *" pathname ", int " mode );
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.fi
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.SH DESCRIPTION
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.BR access ()
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checks whether the calling process can access the file
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.IR pathname .
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If
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.I pathname
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is a symbolic link, it is dereferenced.
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The
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.I mode
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specifies the accessibility check(s) to be performed,
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and is either the value
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.BR F_OK ,
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.\" F_OK is defined as 0 on every system that I know of.
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or a mask consisting of the bitwise OR of one or more of
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.BR R_OK ", " W_OK ", and " X_OK .
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.B F_OK
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tests for the existence of the file.
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.BR R_OK ", " W_OK ", and " X_OK
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test whether the file exists and grants read, write, and
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execute permissions, respectively.
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The check is done using the calling process's
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.I real
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UID and GID, rather than the effective IDs as is done when
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actually attempting an operation (e.g.,
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.BR open (2))
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on the file.
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This allows set-user-ID programs to
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easily determine the invoking user's authority.
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If the calling process is privileged (i.e., its real UID is zero),
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then an
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.B X_OK
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check is successful for a regular file if execute permission
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is enabled for any of the file owner, group, or other.
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.SH "RETURN VALUE"
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On success (all requested permissions granted), zero is returned.
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On error (at least one bit in
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.I mode
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asked for a permission that is denied, or some other error occurred),
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\-1 is returned, and
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.I errno
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is set appropriately.
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.SH ERRORS
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.BR access ()
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shall fail if:
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.TP
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.B EACCES
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The requested access would be denied to the file, or search permission
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is denied for one of the directories in the path prefix of
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.IR pathname .
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(See also
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.BR path_resolution (7).)
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.TP
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.B ELOOP
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Too many symbolic links were encountered in resolving
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.IR pathname .
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.TP
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.B ENAMETOOLONG
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.I pathname
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is too long.
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.TP
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.B ENOENT
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A component of
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.I pathname
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does not exist or is a dangling symbolic link.
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.TP
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.B ENOTDIR
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A component used as a directory in
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.I pathname
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is not, in fact, a directory.
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.TP
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.B EROFS
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Write permission was requested for a file on a read-only file system.
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.PP
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.BR access ()
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may fail if:
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.TP
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.B EFAULT
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.I pathname
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points outside your accessible address space.
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.TP
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.B EINVAL
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.I mode
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was incorrectly specified.
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.TP
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.B EIO
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An I/O error occurred.
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.TP
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.B ENOMEM
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Insufficient kernel memory was available.
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.TP
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.B ETXTBSY
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Write access was requested to an executable which is being
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executed.
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.SH "CONFORMING TO"
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SVr4, 4.3BSD, POSIX.1-2001.
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.SH NOTES
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.PP
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.BR Warning :
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Using
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.BR access ()
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to check if a user is authorized to, for example,
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open a file before actually doing so using
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.BR open (2)
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creates a security hole, because the user might exploit the short time
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interval between checking and opening the file to manipulate it.
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.BR "For this reason, the use of this system call should be avoided" .
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(In the example just described,
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a safer alternative would be to temporarily switch the process's
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effective user ID to the real ID and then call
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.BR open (2).)
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.PP
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.BR access ()
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always dereferences symbolic links.
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If you need to check the permissions on a symbolic link, use
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.BR faccessat (2)
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with the flag
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.BR AT_SYMLINK_NOFOLLOW .
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.PP
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.BR access ()
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returns an error if any of the access types in
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.I mode
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is denied, even if some of the other access types in
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.I mode
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are permitted.
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.PP
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If the calling process has appropriate privileges (i.e., is superuser),
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POSIX.1-2001 permits an implementation to indicate success for an
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.B X_OK
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check even if none of the execute file permission bits are set.
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.\" HPU-UX 11 and Tru64 5.1 do this.
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Linux does not do this.
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.PP
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A file is only accessible if the permissions on each of the
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directories in the path prefix of
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.I pathname
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grant search (i.e., execute) access.
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If any directory is inaccessible, then the
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.BR access ()
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call will fail, regardless of the permissions on the file itself.
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.PP
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Only access bits are checked, not the file type or contents.
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Therefore, if a directory is found to be writable,
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it probably means that files can be created in the directory,
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and not that the directory can be written as a file.
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Similarly, a DOS file may be found to be "executable," but the
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.BR execve (2)
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call will still fail.
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.PP
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.BR access ()
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may not work correctly on NFS file systems with UID mapping enabled,
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because UID mapping is done on the server and hidden from the client,
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which checks permissions.
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Similar problems can occur to FUSE mounts.
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.SH BUGS
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In kernel 2.4 (and earlier) there is some strangeness in the handling of
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.B X_OK
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tests for superuser.
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If all categories of execute permission are disabled
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for a nondirectory file, then the only
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.BR access ()
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test that returns \-1 is when
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.I mode
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is specified as just
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.BR X_OK ;
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if
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.B R_OK
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or
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.B W_OK
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is also specified in
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.IR mode ,
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then
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.BR access ()
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returns 0 for such files.
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.\" This behavior appears to have been an implementation accident.
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Early 2.6 kernels (up to and including 2.6.3)
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also behaved in the same way as kernel 2.4.
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In kernels before 2.6.20,
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.BR access ()
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ignored the effect of the
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.B MS_NOEXEC
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flag if it was used to
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.BR mount (2)
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the underlying file system.
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Since kernel 2.6.20,
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.BR access ()
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honors this flag.
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.SH "SEE ALSO"
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.BR chmod (2),
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.BR chown (2),
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.BR faccessat (2),
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.BR open (2),
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.BR setgid (2),
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.BR setuid (2),
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.BR stat (2),
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.BR euidaccess (3),
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.BR credentials (7),
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.BR path_resolution (7)
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