libbcd0: A Python repository from wodny

Library

This library can be used to edit the Windows Boot Configuration Data (BCD). For example one can add a new entry in the Windows Vista/7 bootloader menu.

Assumptions

The script below assumes that there will be a file called "linux.bin" on the same partition Windows was installed to. Usually it's C: -- the one with the Windows directory, not the one with the BCD file.

That "linux.bin" file is an MBR image. It can be created using the following command:

install-mbr -t 1 -T /dev/sda -p 3 -f /media/sda2/linux.bin

The install-mbr command comes with the mbr package from Debian repositories.

Those particular parameters mean:

timeout is 1 second,
take partitions layout from the real /dev/sda MBR,
Linux bootloader is at partition 3 (/dev/sda3),
Windows partition is mounted at /media/sda2.

Issue that command after installing Linux or at least after creating a new partition layout.

Booting process

BIOS → MBR at /dev/sda → Windows bootloader →
artificial MBR in linux.bin → Linux bootloader → kernel

Example

The example below adds a new BCD entry to boot Linux. The entry will be named "My Linux".

Execute it like that:

./example.py <path_to_the_BCD_file>

And the script:

#!/usr/bin/python

import uuid
import sys

from lib.mappings import *
from lib.BCD import BCD
from lib.BCDMenu import BCDMenu, BCDBootEntry

class BCDMyLinuxUpdater(object):
    mylinux_desc  = "My Linux"
    mylinux_image = "\\linux.bin"
    menu_timeout = 20

    def __init__(self, filename):
        self.__bcdmenu = BCDMenu(filename)

    def __create_mylinux(self):
        guid = "{{{0}}}".format(uuid.uuid1())
        print("New GUID: {0}".format(guid))
        entry = BCDBootEntry(self.__bcdmenu.bcd, guid)
        entry.description = BCDMyLinuxUpdater.mylinux_desc
        win7 = self.__bcdmenu.find_bootentry_by_name("Windows 7")
        entry.boot_device = win7.boot_device
        entry.applicationpath = BCDMyLinuxUpdater.mylinux_image
        return entry

    def update(self):
        mylinux = self.__bcdmenu.find_bootentry_by_name(BCDMyLinuxUpdater.mylinux_desc)
        if mylinux is not None:
            self.__bcdmenu.delete_entry(mylinux)
        mylinux = self.__create_mylinux()
        self.__bcdmenu.add_entry(mylinux)
        self.__bcdmenu.timeout = BCDMyLinuxUpdater.menu_timeout
        self.__bcdmenu.commit()

    def print_info(self):
        print("BCD information:")
        print("Timeout: {0}".format(self.__bcdmenu.timeout))
        print("Active boot entries:")
        for element in self.__bcdmenu.bootentries:
            print("  {0}".format(element.description))
            print("    {0}".format(element.guid))

if __name__ == "__main__":
    if len(sys.argv) != 2:
        exit("Hive filename required.")

    filename = sys.argv[1]

    mylinux = BCDMyLinuxUpdater(filename)
    print("Before changes:")
    mylinux.print_info()
    print("")
    mylinux.update()
    print("")
    print("After changes:")
    mylinux.print_info()

Unknown

That code assumes that the "linux.bin" file is on the Windows partition, so BcdLibraryDevice_ApplicationDevice (11000001) is just copied from the Windows 7 entry.

That element's structure hasn't been decoded yet. It has been tested that at offset 0x20 there is the byte offset of the system partition and at offset 0x38 there is the disk signature from the MBR (offset 0x1B8).

But it is still unknown what do some bytes do:

0x06 at 0x10,
0x48 at 0x18,
0x01 at 0x34.

Example hex dump

0x00:    00,00,00,00,  00,00,00,00,  00,00,00,00,  00,00,00,00,
0x10:    06,00,00,00,  00,00,00,00,
0x18:    48,00,00,00,  00,00,00,00,
0x20:    00,00,50,06,  00,00,00,00,  00,00,00,00,  00,00,00,00,
0x30:    00,00,00,00,
0x34:    01,00,00,00,
0x38:    11,fb,ac,fd,  00,00,00,00,  00,00,00,00,  00,00,00,00,
0x48:    00,00,00,00,  00,00,00,00,  00,00,00,00,  00,00,00,00

Partition offset: 00,00,50,06 == 0x06500000 == 105906176 bytes == 206848 sectors * 512 bytes.

Disk signature: 11fbacfd.

For MBR containing the following layout:

partition 1: ID=0x7,  starthead 32,  startsector 2048,      204800    sectors
partition 2: ID=0x7,  starthead 223, startsector 206848,    522535222 sectors
partition 3: ID=0xb,  starthead 254, startsector 522755100, 4192965   sectors
partition 4: ID=0x83, starthead 254, startsector 526948065, 4192965   sectors