Frankenchroot

Funtoo binfmt_misc user mode Qemu chroot Guide: aka (Franken chroot)

Setting Up Your Filesystem

Exporting Your Remote Mounts with NFS

   /etc/exports - NFS file systems being exported

# /etc/exports: NFS file systems being exported.  See exports(5).
/ 
192.168.0.0/24(insecure,no_root_squash,nohide,rw,sync,no_subtree_check)
/boot/ 
192.168.0.0/24(insecure,no_root_squash,nohide,rw,sync,no_subtree_check)
/dev/ 
192.168.0.0/24(insecure,no_root_squash,nohide,rw,sync,no_subtree_check,fsid=77)

Mounting Your NFS Exports Locally and Binding Pseudo Filesystems

root # mount foo.local:/ /mnt/piroot
root # mount -t proc /proc /mnt/piroot/proc
root # mount --rbind /sys /mnt/piroot/{sys,dev} 
root # mount --make-rslave /mnt/piroot/{sys,dev} 
root # mount -t devpts none /mnt/piroot/dev/pts

   Tip

(For Better Performance!) mount a tmpfs on top of /mnt/piroot/var/tmp/portage

root # mount -v -t tmpfs -o size=8G,mode=775,uid=portage,gid=portage,nr_inodes=0 tmpfs /mnt/piroot/var/tmp/portage

Local Configuration

Editing make.conf

First, add the following to /etc/portage/make.conf:

   /etc/portage/make.conf - Portage make.conf file

QEMU_USER_TARGETS="aarch64 arm"
FEATURES="-sandbox -ipc-sandbox -usersandbox candy"

Editing package.use

Then, edit /etc/portage/package.use:

   /etc/portage/package.use - Portage package.use file

app-emulation/qemu static-user
dev-libs/glib static-libs
sys-apps/attr static-libs
sys-libs/zlib static-libs
dev-libs/libpcre static-libs

   Warning

You WILL need to have updated to Funtoo-1.3-release and emerged the latest No results ebuild as it has fixed the static library creation needed to compile and run Qemu.

Emerging Qemu

Finally, emerge No results:

root # emerge -a app-emulation/qemu

Building and Installing the Wrapper Binary

   qemu-arm-wrapper.c (C source code) - qemu arm wrapper

/*
     * Call QEMU binary with additional "-cpu cortex-a7" argument.
     *
     * Copyright (c) 2018 sakaki <sakaki@deciban.com>
     * License: GPL v3.0+
     *
     * Based on code from the Gentoo Embedded Handbook
     * ("General/Compiling_with_qemu_user_chroot")
     */

    #include <string.h>
    #include <unistd.h>

    int main(int argc, char **argv, char **envp) {
        char *newargv[argc + 3];

        newargv[0] = argv[0];
        newargv[1] = "-cpu";
        newargv[2] = "cortex-a7";

        memcpy(&newargv[3], &argv[1], sizeof(*argv) * (argc -1));
        newargv[argc + 2] = NULL;
        return execve("/usr/local/bin/qemu-arm", newargv, envp);
    }

root # gcc -static -O3 -s -o qemu-arm-wrapper qemu-arm-wrapper.c

root # cp -av /usr/bin/qemu-arm /mnt/piroot/usr/local/bin/qemu-arm
root # cp -av qemu-arm-wrapper /mnt/piroot/usr/local/bin/qemu-arm-wrapper

Setting up binfmt_misc and Starting the Service

root # echo ":arm:M::\x7f\x45\x4c\x46\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28:\xff\xff\xff\xff\xff\xff\xff\xfc\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/usr/local/bin/qemu-arm-wrapper:" > /proc/sys/fs/binfmt_misc/register

root # rc-service binfmt restart

root # rc-service binfmt status

   Tip

The following code will create the binfmt_misc register string for the arch you are emulating in usermode Qemu. Usage is as followed:

root # chmod +x masky
root # masky /path/to/foreign/binary

• Paste the output to /proc/sys/fs/binfmt_misc/register
• Change "arm" in the following code to whatever you want the name of the binfmt_misc name to be when it is registered
• Change /usr/local/bin/qemu-arm-wrapper to the name of your wrapper

   masky (python source code) - masky

#!/usr/bin/python3
import sys
import struct
import string
import codecs

printable_chars = set(string.printable)
printable_chars = set()
def print_out_hexstring(hexstring):
    to_process = hexstring
    while len(to_process):
        ascii_value = chr(int(to_process[:2], 16))
        to_process = to_process[2:]
        if ascii_value in printable_chars:
            sys.stdout.write(ascii_value)
        else:
            sys.stdout.write("\\x" + "{0:02x}".format(ord(ascii_value)))

chunk_as_hexstring = ""
with open(sys.argv[1], 'rb') as f:
    for x in range(0,19):
       chunk_as_hexstring += f.read(1).hex()
mask_as_hexstring = "fffffffffffffffcfffffffffffffffffeffff"
mask = int(mask_as_hexstring, 16)
chunk = int(chunk_as_hexstring, 16)
out_as_hexstring = hex(chunk & mask)[2:]
sys.stdout.write(":arm:M::")
print_out_hexstring(out_as_hexstring)
sys.stdout.write(":")
print_out_hexstring(mask_as_hexstring)
sys.stdout.write(":/usr/local/bin/qemu-arm-wrapper:\n")

Chrooting and Letting the Magic Happen

Entering Franken Chroot

root # mv -v /mnt/piroot/etc/resolv.conf{,.orig}
root # cp -v -L /etc/resolv.conf /mnt/piroot/etc/
root # env -i HOME=/root TERM=$TERM /bin/chroot /mnt/piroot /bin/bash -l
root # export PS1="(Franken_Chroot) $PS1"

Exiting Franken Chroot and Final Thoughts

root # rm /etc/resolv.conf
root # mv -v /etc/resolv.conf{.orig,}
root # exit
root # umount -v /mnt/piroot/var/tmp/portage
root # umount -lR /mnt/piroot
root # sync