155 lines
5 KiB
Groff
155 lines
5 KiB
Groff
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.\" $NetBSD: access.2,v 1.26 2010/05/03 05:53:56 jruoho Exp $
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.\"
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.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)access.2 8.2 (Berkeley) 4/1/94
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.\"
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.Dd May 3, 2010
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.Dt ACCESS 2
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.Os
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.Sh NAME
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.Nm access
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.Nd check access permissions of a file or pathname
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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.Ft int
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.Fn access "const char *path" "int mode"
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.Sh DESCRIPTION
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The
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.Fn access
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function checks the accessibility of the
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file named by
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.Fa path
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for the access permissions indicated by
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.Fa mode .
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The value of
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.Fa mode
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is the bitwise inclusive OR of the access permissions to be
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checked
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.Pf ( Dv R_OK
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for read permission,
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.Dv W_OK
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for write permission and
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.Dv X_OK
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for execute/search permission) or the existence test,
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.Dv F_OK .
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All components of the pathname
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.Fa path
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are checked for access permissions (including
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.Dv F_OK ) .
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.Pp
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The real user ID is used in place of the effective user ID
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and the real group access list
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(including the real group ID) are
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used in place of the effective ID for verifying permission.
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.Pp
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If a process has super-user privileges and indicates success for
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.Dv R_OK
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or
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.Dv W_OK ,
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the file may not actually have read or write permission bits set.
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If a process has super-user privileges and indicates success for
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.Dv X_OK ,
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at least one of the user, group, or other execute bits is set.
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(However, the file may still not be executable.
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See
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.Xr execve 2 . )
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.Sh RETURN VALUES
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If
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.Fa path
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cannot be found or if any of the desired access modes would
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not be granted, then a \-1 value is returned; otherwise
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a 0 value is returned.
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.Sh ERRORS
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Access to the file is denied if:
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.Bl -tag -width Er
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.It Bq Er EACCES
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Permission bits of the file mode do not permit the requested
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access, or search permission is denied on a component of the
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path prefix.
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The owner of a file has permission checked with respect to the
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.Dq owner
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read, write, and execute mode bits, members of the file's group
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other than the owner have permission checked with respect to the
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.Dq group
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mode bits, and all others have permissions checked with respect to
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the
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.Dq other
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mode bits.
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.It Bq Er EFAULT
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.Fa path
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points outside the process's allocated address space.
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.It Bq Er EIO
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An I/O error occurred while reading from or writing to the file system.
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.It Bq Er ELOOP
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Too many symbolic links were encountered in translating the pathname.
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.It Bq Er ENAMETOOLONG
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A component of a pathname exceeded
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.Brq Dv NAME_MAX
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characters, or an entire path name exceeded
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.Brq Dv PATH_MAX
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characters.
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.It Bq Er ENOENT
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The named file does not exist.
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.It Bq Er ENOTDIR
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A component of the path prefix is not a directory.
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.It Bq Er EROFS
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Write access is requested for a file on a read-only file system.
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.It Bq Er ETXTBSY
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Write access is requested for a pure procedure (shared text)
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file presently being executed.
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.El
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.Sh SEE ALSO
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.Xr chmod 2 ,
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.Xr execve 2 ,
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.Xr stat 2 ,
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.Xr secure_path 3
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.Sh STANDARDS
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The
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.Fn access
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function conforms to
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.St -p1003.1-90 .
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.Sh SECURITY CONSIDERATIONS
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The
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.Fn access
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system call is a potential security hole due to race conditions.
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It should never be used.
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Set-user-ID and set-group-ID applications should restore the
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effective user or group ID, and perform actions directly rather than use
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.Fn access
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to simulate access checks for the real user or group ID.
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.Pp
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The
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.Fn access
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system call may however have some value in providing clues to users as to
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whether certain operations make sense for a particular filesystem object.
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Arguably it also allows a cheaper file existence test than
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.Xr stat 2 .
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