#!/usr/bin/env python3

import sys, os, struct

loaderfile = sys.argv[1]
elffile = sys.argv[2]
outfile = sys.argv[3]

data = open(loaderfile,"rb").read()

hdrlen, loaderlen, elflen, arg = struct.unpack(">IIII",data[:16])

if hdrlen < 0x10:
	print("ERROR: header length is 0x%x, expected at least 0x10"%hdrlen)
	sys.exit(1)

loaderoff = hdrlen
elfoff = loaderoff + loaderlen
elfend = elfoff + elflen

hdr = data[:hdrlen]
loader = data[loaderoff:elfoff]

elf = open(elffile,"rb").read()

if elflen > 0:
	print("WARNING: loader already contains ELF, will replace")

elflen = len(elf)

if loaderlen < len(loader):
	print("ERROR: loader is larger than its reported length")
	sys.exit(1)

if loaderlen > len(loader):
	print("Padding loader with 0x%x zeroes"%(loaderlen-len(loader)))
	loader += b"\0"*(loaderlen-len(loader))

newdata = struct.pack(">IIII", hdrlen, loaderlen, elflen, 0) + hdr[16:]
newdata += loader
newdata += elf

print("Header: 0x%x bytes"%(hdrlen))
print("Loader: 0x%x bytes"%(loaderlen))
print("ELF:    0x%x bytes"%(elflen))

f = open(outfile,"wb")
f.write(newdata)
f.close()