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Include errors common to chroot and fchroot in a single list, followed by errors unique to each. Unprivileged chroot is permitted if the security.bsd.unprivileged_chroot sysctl is set to 1. Make note of this and update the EPERM description. Reported by: kevans Reviewed by: kevans, kib Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D51703
215 lines
5.9 KiB
Groff
215 lines
5.9 KiB
Groff
.\" Copyright (c) 1983, 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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.\" 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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.\" SUCH DAMAGE.
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.\"
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.Dd August 2, 2025
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.Dt CHROOT 2
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.Os
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.Sh NAME
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.Nm chroot ,
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.Nm fchroot
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.Nd change root directory
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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 chroot "const char *dirname"
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.Ft int
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.Fn fchroot "int fd"
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.Sh DESCRIPTION
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The
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.Fa dirname
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argument
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is the address of the pathname of a directory, terminated by an ASCII NUL.
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The
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.Fn chroot
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system call causes
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.Fa dirname
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to become the root directory,
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that is, the starting point for path searches of pathnames
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beginning with
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.Ql / .
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.Pp
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In order for a directory to become the root directory
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a process must have execute (search) access for that directory.
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.Pp
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It should be noted that
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.Fn chroot
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has no effect on the process's current directory.
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.Pp
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This call is restricted to the super-user, unless the
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.Ql security.bsd.unprivileged_chroot
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sysctl variable is set to 1
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and the process has enabled the
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.Dv PROC_NO_NEW_PRIVS_CTL
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.Xr procctl 2 .
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.Pp
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Depending on the setting of the
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.Ql kern.chroot_allow_open_directories
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sysctl variable, open filedescriptors which reference directories
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will make the
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.Fn chroot
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fail as follows:
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.Pp
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If
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.Ql kern.chroot_allow_open_directories
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is set to zero,
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.Fn chroot
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will always fail with
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.Er EPERM
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if there are any directories open.
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.Pp
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If
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.Ql kern.chroot_allow_open_directories
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is set to one (the default),
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.Fn chroot
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will fail with
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.Er EPERM
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if there are any directories open and the
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process is already subject to the
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.Fn chroot
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system call.
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.Pp
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Any other value for
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.Ql kern.chroot_allow_open_directories
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will bypass the check for open directories,
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mimicking the historic insecure behavior of
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.Fn chroot
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still present on other systems.
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.Pp
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The
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.Fn fchroot
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system call is identical to
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.Fn chroot
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except it takes a file descriptor instead of path.
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.Sh RETURN VALUES
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.Rv -std
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.Sh ERRORS
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The
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.Fn chroot
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and
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.Fn fchroot
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system calls
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will fail and the root directory will be unchanged if:
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.Bl -tag -width Er
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.It Bq Er EPERM
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The effective user ID is not the super-user and the
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.Ql security.bsd.unprivileged_chroot
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sysctl is 0.
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.It Bq Er EPERM
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The effective user ID is not the super-user and the
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process has not enabled the
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.Dv PROC_NO_NEW_PRIVS_CTL
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.Xr procctl 2 .
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.It Bq Er EPERM
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One or more filedescriptors are open directories and the
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.Ql kern.chroot_allow_open_directories
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sysctl is not set to permit this.
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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 EINTEGRITY
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Corrupted data was detected while reading from the file system.
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.El
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.Pp
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The
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.Fn chroot
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system call
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will fail and the root directory will be unchanged if:
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.Bl -tag -width Er
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.It Bq Er ENOTDIR
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A component of the path name is not a directory.
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.It Bq Er ENAMETOOLONG
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A component of a pathname exceeded 255 characters,
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or an entire path name exceeded 1023 characters.
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.It Bq Er ENOENT
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The named directory does not exist.
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.It Bq Er EACCES
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Search permission is denied for any component of the path name.
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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 EFAULT
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The
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.Fa dirname
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argument
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points outside the process's allocated address space.
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.El
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.Pp
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The
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.Fn fchroot
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system call
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will fail and the root directory will be unchanged if:
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.Bl -tag -width Er
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.It Bq Er EACCES
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Search permission is denied for the directory referenced by the
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file descriptor.
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.It Bq Er EBADF
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The argument
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.Fa fd
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is not a valid file descriptor.
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.It Bq Er ENOTDIR
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The file descriptor does not reference a directory.
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.El
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.Sh SEE ALSO
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.Xr chdir 2 ,
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.Xr jail 2
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.Sh HISTORY
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The
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.Fn chroot
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system call appeared in
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.At v7 .
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It was marked as
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.Dq legacy
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in
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.St -susv2 ,
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and was removed in subsequent standards.
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The
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.Fn fchroot
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system call first appeared in
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.Fx 15.0 .
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.Sh BUGS
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If the process is able to change its working directory to the target
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directory, but another access control check fails (such as a check for
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open directories, or a MAC check), it is possible that this system
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call may return an error, with the working directory of the process
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left changed.
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.Sh SECURITY CONSIDERATIONS
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The system has many hardcoded paths to files which it may load after
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the process starts.
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It is generally recommended to drop privileges immediately after a
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successful
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.Nm
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call,
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and restrict write access to a limited subtree of the
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.Nm
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root.
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For instance,
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setup the sandbox so that the sandboxed user will have no write
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access to any well-known system directories.
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.Pp
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For complete isolation from the rest of the system, use
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.Xr jail 2
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instead.
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