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Man Pages
CHROOT(2) FreeBSD System Calls Manual CHROOT(2)

chroot
change root directory

Standard C Library (libc, -lc)

#include <unistd.h>
int
chroot(const char *dirname);

The dirname argument is the address of the pathname of a directory, terminated by an ASCII NUL. The chroot() system call causes dirname to become the root directory, that is, the starting point for path searches of pathnames beginning with ‘/’.
In order for a directory to become the root directory a process must have execute (search) access for that directory.
It should be noted that chroot() has no effect on the process's current directory.
This call is restricted to the super-user.
Depending on the setting of the ‘kern.chroot_allow_open_directories’ sysctl variable, open filedescriptors which reference directories will make the chroot() fail as follows:
If ‘kern.chroot_allow_open_directories’ is set to zero, chroot() will always fail with EPERM if there are any directories open.
If ‘kern.chroot_allow_open_directories’ is set to one (the default), chroot() will fail with EPERM if there are any directories open and the process is already subject to the chroot() system call.
Any other value for ‘kern.chroot_allow_open_directories’ will bypass the check for open directories

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The chroot() system call will fail and the root directory will be unchanged if:
[]
A component of the path name is not a directory.
[]
The effective user ID is not the super-user, or one or more filedescriptors are open directories.
[]
A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
[]
The named directory does not exist.
[]
Search permission is denied for any component of the path name.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The dirname argument points outside the process's allocated address space.
[]
An I/O error occurred while reading from or writing to the file system.

chdir(2), jail(2)

The chroot() system call appeared in 4.2BSD. It was marked as “legacy” in Version 2 of the Single UNIX Specification (“SUSv2”), and was removed in subsequent standards.

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.

The system have many hardcoded paths to files where it may load after the process starts. It is generally recommended to drop privileges immediately after a successful chroot call, and restrict write access to a limited subtree of the chroot root, for instance, setup the sandbox so that the sandboxed user will have no write access to any well-known system directories.
January 3, 2012 FreeBSD 12.0-RELEASE

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