Script to undo the VirtualBox broken .vhd file resize

vhdresize.py

#!/usr/bin/python                                                                                                                                                                                                    
                                                                                                                                                                                                                     
import struct                                                                                                                                                                                                        
import sys                                                                                                                                                                                                           
                                                                                                                                                                                                                     
def get_chksum(data):                                                                                                                                                                                                
    chksum = 0                                                                                                                                                                                                       
    for c in data:                                                                                                                                                                                                   
        chksum += ord(c)                                                                                                                                                                                             
    return (~chksum & 0xffffffff)                                                                                                                                                                                    
                                                                                                                                                                                                                     
def resize_footer(footer):                                                                                                                                                                                           
    fields = list(struct.unpack('>8s2LQL4sLL2Q3L16sB427s', footer))                                                                                                                                                  
    fields[9] = fields[8] # current_size = original_size                                                                                                                                                             
    fields[12] = 0 # chksum = 0                                                                                                                                                                                      
    new_footer = struct.pack('>8s2LQL4sLL2Q3L16sB427s', *fields)                                                                                                                                                     
    chksum = struct.pack('>L', get_chksum(new_footer))                                                                                                                                                               
    new_footer = new_footer[:64] + chksum + new_footer[68:]                                                                                                                                                          
    return new_footer, fields[8]                                                                                                                                                                                     
                                                                                                                                                                                                                     
def resize_header(header, new_table_entries):                                                                                                                                                                        
    fields = list(struct.unpack('>8s2Q4L16s2L512s24s24s24s24s24s24s24s24s256s', header))                                                                                                                             
    fields[4] = new_table_entries                                                                                                                                                                                    
    fields[6] = 0                                                                                                                                                                                                    
    new_header = struct.pack('>8s2Q4L16s2L512s24s24s24s24s24s24s24s24s256s', *fields)                                                                                                                                
    chksum = struct.pack('>L', get_chksum(new_header))                                                                                                                                                               
    new_header = new_header[:36] + chksum + new_header[40:]                                                                                                                                                          
    return new_header                                                                                                                                                                                                
                                                                                                                                                                                                                     
with open(sys.argv[1], 'rb+') as f:                                                                                                                                                                                  
    footer = f.read(512)                                                                                                                                                                                             
    header = f.read(1024)                                                                                                                                                                                            
                                                                                                                                                                                                                     
    new_footer, original_size = resize_footer(footer)                                                                                                                                                                
    original_table_entries = original_size / 0x200000                                                                                                                                                                
    new_header = resize_header(header, original_table_entries)                                                                                                                                                       
                                                                                                                                                                                                                     
    f.seek(0)                                                                                                                                                                                                        
    f.write(new_footer)                                                                                                                                                                                              
    f.write(new_header)                                                                                                                                                                                              
    f.seek(512, 2)                                                                                                                                                                                                   
    f.write(new_footer)