Smartmeter Obfuscation

#!/bin/sh
# this script must be called 'as'
# run gcc with option -B<dir of this script> to get it invoked automatically

lastarg() {
    shift $(($# - 1))
    echo "$1"
}

# instead of a jmp, push the address and return
sed -r -i -e 's/^\tjmpq?\t[*](.*)$/\tpush \1;\tretq/' $(lastarg $@)
# instead of a ret, do a manual pop and jmp (clobbering a scratch register)
sed -r -i -e 's/^\t(rep )?ret$/\tpop %rsi;\tjmp *%rsi/' $(lastarg $@)
# instead of a direct call, push return instruction pointer and destination address, then ret
sed -r -i -e 's/^\tcall\t([^*].*)$/\tpush %rax;\tlea \1(%rip), %rax;\tpush %rax;\tlea 6(%rip), %rax;\txchg 8(%rsp),%rax;\tretq;\t#call\t\1/' $(lastarg $@)


exec as $@