fakechroot man page

fakechroot — gives a fake chroot environment

Synopsis

fakechroot [-s|--use-system-libs] [-l|--lib library] [-d|--elfloader ldso] [-e|--environment type] [-c|--config-dir directory] [-b|--bindir directory] [--] [command]

fakechroot -h|--help

fakechroot -v|--version

Description

fakechroot runs a command in an environment where is additional possibility to use chroot(8) command without root privileges. This is useful for allowing users to create own chrooted environment with possibility to install another packages without need for root privileges.

fakechroot replaces some C library functions (chroot(2), open(2), etc.) by ones that simulate the effect of being called with root privileges.

These wrapper functions are provided as a shared library libfakechroot.so which is loaded through the "LD_PRELOAD" mechanism of the dynamic loader. (See ld.so(8))

In fake chroot you can install i.e. Debian bootstrap with debootstrap(8) command. In such environment you can run i.e. apt-get(8) command to install another packages. You don't need a special privileges and you can run it on common user's account.

Options

-l library|--lib library
Specify an alternative wrapper library. The default is libfakechroot.so
-d ldso|--elfloader ldso
Specify an alternative dynamic linker. This dynamic linker will be invoked directly.
-s|--use-system-libs

Use system libraries before chroot's libraries. This might be a workaround if system dynamic linker can not load libc.so from fake chroot.

Try this setting if you noticed following errors:

$ fakechroot /usr/sbin/chroot /tmp/sarge /bin/true
/bin/true: relocation error: /srv/sarge/lib/tls/libc.so.6: symbol _dl
_starting_up, version GLIBC_PRIVATE not defined in file ld-linux.so.2
 with link time reference
$ fakechroot /usr/sbin/chroot /tmp/centos4 /bin/true
Segmentation fault
-e|--environment type
Load additional configuration with environment. This configuration file is a shell script which is executed before calling command. The script can set additional environment variables, like i.e.: "FAKECHROOT_EXCLUDE_PATH", "FAKECHROOT_CMD_SUBST" or "LD_LIBRARY_PATH".

The environment type is guessed based on command name with optional extension removed (e.g. running gettext.sh loads "gettext" environment file). If command argument is fakeroot(1) this argument is omitted and next argument is taken as environment type.

The configuration file name is type.env and is searched at $HOME/.fakechroot and /etc/fakechroot directories.

The default environment type is default and its configuration file name is "default.env".

The special environment none means that no environment settings are loaded at all.
-c|--config-dir directory
Specify a directory which contains additional configuration for fakechroot. The default directory are "$HOME/.fakechroot" and "/etc/fakechroot".
-b|--bindir directory
Specify a directory which contains the replacement executables shipped with fakechroot. Those are script files with the extension ".fakechroot". By default they are distributed over the "bin" and "sbin" directories in fakechroot's install prefix, but sometimes a fixed location at build time is not feasible.
[--] command
Any command you want to be ran as fakechroot. Use '--' if in the command you have other options that may confuse fakechroot's option parsing.
-h
Display help.
-v
Display version.

Examples

An example session with fakechroot:

$ id
uid=1000(dexter) gid=1000(dexter) groups=1000(dexter)
$ fakechroot fakeroot debootstrap sid /tmp/sid
I: Retrieving Release 
I: Retrieving Release.gpg 
I: Checking Release signature
...
I: Base system installed successfully.
$ fakechroot fakeroot chroot /tmp/sid apt-get install -q hello
Reading package lists...
Building dependency tree...
Reading state information...
The following NEW packages will be installed:
  hello
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 57.4 kB of archives.
After this operation, 558 kB of additional disk space will be used.
Get:1 http://ftp.us.debian.org/debian/ sid/main hello amd64 2.8-4 [57.4 kB]
Fetched 57.4 kB in 0s (127 kB/s)
Selecting previously unselected package hello.
(Reading database ... 24594 files and directories currently installed.)
Unpacking hello (from .../archives/hello_2.8-4_amd64.deb) ...
Processing triggers for man-db ...
Processing triggers for install-info ...
Setting up hello (2.8-4) ...
$ fakechroot chroot /tmp/sid hello
Hello, world!

Fakeroot

fakeroot(1) is a complementary tool which emulates root environment. fakeroot and fakechroot might wrap the same C library functions, i.e. mknod(2) function. It is important to start fake environment in proper order. fakeroot should be started inside fakechroot:

$ fakechroot fakeroot chroot /tmp/sid /bin/mknod /tmp/device c 1 2

Security Aspects

fakechroot is a regular, non-setuid program. It does not enhance a user's privileges.

fakechroot should not be used as a tool for enhancing system security i.e. by separating (sandboxing) applications. It is very easy to escape from a fake chroot environment.

fakechroot should not be run with real root privileges. It might decrease the security of the system because the fakechroot provides own version of core functions with behavior depended on some environment variables.

Files

libfakechroot.so
The shared library containing the wrapper functions.

Environment

FAKECHROOT
The value is true for fake chroot environment.
FAKECHROOT_VERSION
The version number of the current fakechroot library.
FAKECHROOT_BASE
The root directory of fake chroot environment.
FAKECHROOT_DETECT

If this variable is set then "fakechroot version" string is printed to standard output and the current process is terminated with status taken from this variable. It can be a method to check if fakechroot is preloaded correctly.

$ case "`FAKECHROOT_DETECT=1 /bin/echo`" in fakechroot*) echo LOADED;; esac
FAKECHROOT_DEBUG
The fakechroot library will dump some debugging info if this variable is set.
FAKECHROOT_AF_UNIX_PATH
The root directory for unix sockets. The default value is the same as "FAKECHROOT_BASE" and it can be set separately if the "FAKECHROOT_BASE" is too long and the unix socket path could exceed the limit of 108 bytes.
FAKECHROOT_EXCLUDE_PATH
The list of directories which are excluded from being chrooted. The elements of list are separated with colon.

The /dev, /proc and /sys directories are excluded by default if this environment variable is not set.
FAKECHROOT_ELFLOADER
A path to another dynamic linker (i.e. /lib/ld-linux.so.2 for i386 architecture, /lib64/ld-linux-x86-64.so.2 for x86_64 architecture).

This dynamic linker will be invoked directly. The dynamic linker don't allow to change "argv[0]" besides the file name of the executable file, so some application won't work correctly, i.e. busybox(1).
FAKECHROOT_CMD_SUBST

A list of command substitutions. If a program tries to execute one of the commands given (path relative to the chroot, trailing dot is removed) then the substitute command runs instead (path to substitute command is not chrooted).

The substituted command inherits "FAKECHROOT_*" variables but the original "FAKECHROOT_BASE" variable which is saved as "FAKECHROOT_BASE_ORIG". It means that substituted command runs outside fakechroot environment. Also original command name is saved as "FAKECHROOT_CMD_ORIG".

For example:

export FAKECHROOT_CMD_SUBST=/usr/bin/mkfifo=/bin/true

will substitute "/bin/true" for "/usr/bin/mkfifo" and will make possible to install sysvinit binary package.

Give as many substitute commands as you want, separated by ":" (colon) characters.

It is suggested to substitute at least:

·
"/bin/mount=/bin/true"
·
"/sbin/insserv=/bin/true"
·
"/sbin/ldconfig=/bin/ldconfig"
·
"/usr/bin/env=/usr/bin/env.fakechroot"
·
"/usr/bin/ischroot=/bin/true"
·
"/usr/bin/ldd=/usr/bin/ldd.fakechroot"
·
"/usr/bin/mkfifo=/bin/true"

to make debootstrap(8) working correctly.

To prevent some looping, the command substitution is done only if "FAKECHROOT_CMD_ORIG" variable is not set currently.

LD_LIBRARY_PATH, LD_PRELOAD
Fakechroot is implemented by wrapping system calls. This is accomplished by setting "LD_PRELOAD=libfakechroot.so". If this library can't be found by dynamic linker, the /etc/ld.so.conf file or "LD_LIBRARY_PATH" variable have to be modified.

Limitations

·
/lib/ld-linux.so.2 and /lib64/ld-linux-x86-64.so.2 are always loaded from real environment. This path is hardcoded by linker for all binaries. You can set the "FAKECHROOT_ELFLOADER" environment variable or use "--elfloader" option.
·
Every command executed within fakechroot needs to be linked to the same version of the dynamic linker from real environment. If the libraries in chroot are not compatible, try to use "--use-system-libs" option.
·

You can provide symlinks to the outside. The symlink have to be created before chroot is called. It can be useful for accessing the real /proc and /dev directory. You can also set the "FAKECHROOT_EXCLUDE_PATH" environment variable:

$ export FAKECHROOT_EXCLUDE_PATH=/tmp:/proc:/dev:/sys:/var/run:/home
·
Statically linked binaries doesn't work, especially ldconfig(8), so you have to wrap this command with dummy version and set the proper "FAKECHROOT_CMD_SUBST" environment variable.
·
ldd(1) also doesn't work. You have to use "alias ldd='LD_TRACE_LOADED_OBJECTS=1'" or to use a wrapper instead. The wrapper is installed as ldd.fakechroot and can be used with "FAKECHROOT_CMD_SUBST" environment variable.
·
The full screen applications hangs up if /dev/tty file is not a real device. Link /dev/tty file or whole /dev directory to the real one or remove it from fake chroot environment with "FAKECHROOT_EXCLUDE_PATH" variable.
·
lckpwdf(3) and ulckpwdf(3) are ignored so passwd(1) command should work
·
Your real uid should exist in /etc/passwd. Create it with adduser --uid realuid realuser inside fake chroot environment.
·
debuild(1) cleans environment. Use --preserve-env option to prevent this behavior.
·
rpmbuild(8) uses own glob(3) implementation which breaks fakechroot so buildroot directory have to be the same inside and outside fakechroot.

See Also

fakeroot(1), debuild(1), debootstrap(8), rinse(8), http://fakechroot.alioth.debian.org/

Bugs

If you find the bug or want to implement new features, please report it at <https://github.com/fakechroot/fakechroo…>

Authors

Copyright (c) 2003, 2005, 2007-2011, 2013 Piotr Roszatycki <dexter@debian.org>

Copyright (c) 2007 Mark Eichin <eichin@metacarta.com>

Copyright (c) 2006, 2007 Alexander Shishkin <virtuoso@slind.org>

Copyright (c) 2006, 2007 Lionel Tricon <lionel.tricon@free.fr>

Copying

fakechroot is distributed under the GNU Lesser General Public License (LGPL 2.1 or greater).

Info

29 Nov 2013 fakechroot