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Creating a cross-compile chroot environment using Gentoo.

1. Create destination directory.  Referenced as <chroot> below.
    Ex. /esw/powerpc64-gcc-20130412

2. Configure portage options.

    <chroot>/etc/make.conf:
        MAKEOPTS="-j5"
        CFLAGS="-O2 -pipe"
        CXXFLAGS="${CFLAGS}"

    <chroot>/etc/portage/make.profile/parent:
        gentoo:default/linux/amd64
        # OR
        gentoo:funtoo/1.0/linux-gnu/arch/x86-64bit
        gentoo:funtoo/1.0/linux-gnu/build/current
        gentoo:funtoo/1.0/linux-gnu/flavor/minimal

    <chroot>/etc/portage/package.use/net-misc:
        net-misc/curl ssl

    <chroot>/etc/portage/package.use/sys-devel:
        sys-devel/binutils multitarget

3. Install 'system'.

    cd <chroot>
    ROOT=`pwd` PORTAGE_CONFIGROOT=`pwd` emerge @system --jobs

4. Install cross-compiler and extra packages.

    cp /etc/resolv.conf <chroot>/etc/resolv.conf
    unshare -m /bin/bash
    mount -n --bind /dev <chroot>/dev
    mount -n --bind /proc <chroot>/proc
    mkdir <chroot>/usr/portage
    mount -n --bind /usr/portage <chroot>/usr/portage
    chroot <chroot> /bin/bash

    emerge crossdev dwarves --jobs
    emerge strace                       # optional

    mkdir /usr/local/portage
    crossdev powerpc64-unknown-linux-gnu --ov-output /usr/local/portage

    exit
    exit

5. Install mchroot and wrappers.

    cd /tmp
    git clone http://github.com/williamspatrick/mchroot
    cd mchroot
    gcc mchroot.c -Os -static -o mchroot

    mkdir <chroot>/wrappers
    cp mchroot mchroot_wrapper.sh jail_wrapper.sh <chroot>/wrappers
    
    cd <chroot>/wrappers
    chmod u+s mchroot
    /tmp/mchroot/create_wrapper_links.sh

6. Configure mchroot

    Add file systems you wish to bind-mount into the chroot to 
    <chroot>/.mchroot.cfg with one mount path per line.

    <chroot>/.mchroot.cfg:
        /dev
        /proc
        /tmp
        /home
        (...etc...)