Frankenchroot/Manual Binary Format Setup

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

   Warning

The preceding wrapper and binfmt_misc register string were for arm 32bit. They *MUST* be edited for aarch64.

   Tip

The following code will create the binfmt_misc register string for the arch you are emulating in usermode Qemu. This is only needed for targets other than the arm target already packaged for you above. 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")