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CHROOT

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME

chroot - change root directory

SYNOPSIS

#include <unistd.h>

int chroot(const char *path);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

chroot():

Since glibc 2.2.2:

_XOPEN_SOURCE && ! (_POSIX_C_SOURCE >= 200112L)
    || /* Since glibc 2.20: */ _DEFAULT_SOURCE
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE

Before glibc 2.2.2: none

DESCRIPTION

chroot() changes the root directory of the calling process to that specified in path. This directory will be used for pathnames beginning with /. The root directory is inherited by all children of the calling process.

Only a privileged process (Linux: one with the CAP_SYS_CHROOT capability in its user namespace) may call chroot().

This call changes an ingredient in the pathname resolution process and does nothing else. In particular, it is not intended to be used for any kind of security purpose, neither to fully sandbox a process nor to restrict filesystem system calls. In the past, chroot() has been used by daemons to restrict themselves prior to passing paths supplied by untrusted users to system calls such as open(2). However, if a folder is moved out of the chroot directory, an attacker can exploit that to get out of the chroot directory as well. The easiest way to do that is to chdir(2) to the to-be-moved directory, wait for it to be moved out, then open a path like ../../../etc/passwd.

A slightly trickier variation also works under some circumstances if chdir(2) is not permitted. If a daemon allows a "chroot directory" to be specified, that usually means that if you want to prevent remote users from accessing files outside the chroot directory, you must ensure that folders are never moved out of it.

This call does not change the current working directory, so that after the call aq.aq can be outside the tree rooted at aq/aq. In particular, the superuser can escape from a "chroot jail" by doing:

mkdir foo; chroot foo; cd ..

This call does not close open file descriptors, and such file descriptors may allow access to files outside the chroot tree.

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS

Depending on the filesystem, other errors can be returned. The more general errors are listed below:

EACCES: Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
EFAULT: path points outside your accessible address space.
EIO: An I/O error occurred.
ELOOP: Too many symbolic links were encountered in resolving path.
ENAMETOOLONG: path is too long.
ENOENT: The file does not exist.
ENOMEM: Insufficient kernel memory was available.
ENOTDIR: A component of path is not a directory.
EPERM: The caller has insufficient privilege.

CONFORMING TO

SVr4, 4.4BSD, SUSv2 (marked LEGACY). This function is not part of POSIX.1-2001.

NOTES

A child process created via fork(2) inherits its parent's root directory. The root directory is left unchanged by execve(2).

FreeBSD has a stronger jail() system call.

COLOPHON

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