lib/libsys/chroot.2

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.Dd July 15, 2024
.Dt CHROOT 2
.Os
.Sh NAME
.Nm chroot ,
.Nm fchroot
.Nd change root directory
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft int
.Fn chroot "const char *dirname"
.Ft int
.Fn fchroot "int fd"
.Sh DESCRIPTION
The
.Fa dirname
argument
is the address of the pathname of a directory, terminated by an ASCII NUL.
The
.Fn chroot
system call causes
.Fa dirname
to become the root directory,
that is, the starting point for path searches of pathnames
beginning with
.Ql / .
.Pp
In order for a directory to become the root directory
a process must have execute (search) access for that directory.
.Pp
It should be noted that
.Fn chroot
has no effect on the process's current directory.
.Pp
This call is restricted to the super-user.
.Pp
Depending on the setting of the
.Ql kern.chroot_allow_open_directories
sysctl variable, open filedescriptors which reference directories
will make the
.Fn chroot
fail as follows:
.Pp
If
.Ql kern.chroot_allow_open_directories
is set to zero,
.Fn chroot
will always fail with
.Er EPERM
if there are any directories open.
.Pp
If
.Ql kern.chroot_allow_open_directories
is set to one (the default),
.Fn chroot
will fail with
.Er EPERM
if there are any directories open and the
process is already subject to the
.Fn chroot
system call.
.Pp
Any other value for
.Ql kern.chroot_allow_open_directories
will bypass the check for open directories,
mimicking the historic insecure behavior of
.Fn chroot
still present on other systems.
.Pp
The
.Fn fchroot
system call is identical to
.Fn chroot
except it takes a file descriptor instead of path.
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn chroot
system call
will fail and the root directory will be unchanged if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path name is not a directory.
.It Bq Er EPERM
The effective user ID is not the super-user, or one or more
filedescriptors are open directories.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
.It Bq Er ENOENT
The named directory does not exist.
.It Bq Er EACCES
Search permission is denied for any component of the path name.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EFAULT
The
.Fa dirname
argument
points outside the process's allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.El
.Pp
The
.Fn fchroot
system call
will fail and the root directory will be unchanged if:
.Bl -tag -width Er
.It Bq Er EACCES
Search permission is denied for the directory referenced by the
file descriptor.
.It Bq Er EBADF
The argument
.Fa fd
is not a valid file descriptor.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.It Bq Er ENOTDIR
The file descriptor does not reference a directory.
.It Bq Er EPERM
The effective user ID is not the super-user, or one or more
filedescriptors are open directories.
.El
.Sh SEE ALSO
.Xr chdir 2 ,
.Xr jail 2
.Sh HISTORY
The
.Fn chroot
system call appeared in
.At v7 .
It was marked as
.Dq legacy
in
.St -susv2 ,
and was removed in subsequent standards.
The
.Fn fchroot
system call first appeared in
.Fx 15.0 .
.Sh BUGS
If the process is able to change its working directory to the target
directory, but another access control check fails (such as a check for
open directories, or a MAC check), it is possible that this system
call may return an error, with the working directory of the process
left changed.
.Sh SECURITY CONSIDERATIONS
The system has many hardcoded paths to files which it may load after
the process starts.
It is generally recommended to drop privileges immediately after a
successful
.Nm
call,
and restrict write access to a limited subtree of the
.Nm
root.
For instance,
setup the sandbox so that the sandboxed user will have no write
access to any well-known system directories.
.Pp
For complete isolation from the rest of the system, use
.Xr jail 2
instead.