Python idc.PrevHead() Examples

def find_all_ioctls():
    """
    From the currently selected address attempts to traverse all blocks inside the current function to find all immediate values which
    are used for a comparison/sub immediately before a jz. Returns a list of address, second operand pairs.
    """
    
    ioctls = []
    # Find the currently selected function and get a list of all of it's basic blocks
    addr = idc.ScreenEA()
    f = idaapi.get_func(addr)
    fc = idaapi.FlowChart(f, flags=idaapi.FC_PREDS)
    for block in fc:
        # grab the last two instructions in the block 
        last_inst = idc.PrevHead(block.endEA)
        penultimate_inst = idc.PrevHead(last_inst)
        # If the penultimate instruction is cmp or sub against an immediate value immediately preceding a 'jz' 
        # then it's a decent guess that it's an IOCTL code (if this is a dispatch function)
        if idc.GetMnem(penultimate_inst) in ['cmp', 'sub'] and idc.GetOpType(penultimate_inst, 1) == 5:
            if idc.GetMnem(last_inst) == 'jz':
                value = get_operand_value(penultimate_inst)
                ioctls.append((penultimate_inst, value))
                ioctl_tracker.add_ioctl(penultimate_inst, value)
    return ioctls 

def get_function_end_address(ea):
    """
    Get function end address
    @param ea: function start_ea.
    @return: The function end ea. If no function end ea found returns None.
    """
    try:
        if ea is None:
            return None

        func_attr_end = idc.GetFunctionAttr(ea, idc.FUNCATTR_END)
        if func_attr_end == idc.BADADDR:
            return None

        return idc.PrevHead(func_attr_end, ea)

    except Exception as ex:
        raise RuntimeError("Count not locate end address for function %s: %s" % (hex(ea), ex)) 

def get_bbl_head(self, head):
        """
        The function returns address of the head instruction
        for the basic block.
        @head - address of arbitrary instruction in the basic block.
        @return - head address of the basic block.
        """

        while 1:
            prev_head = idc.PrevHead(head, 0)
            if isFlow(idc.GetFlags(prev_head)):
                head = prev_head
                if prev_head >= SegEnd(head):
                    raise Exception("Can't identify bbl head")
                continue
            else:
                if prev_head == hex(0xffffffffL):
                    return head
                else:
                    return prev_head
                break 

def find_arg_ea(ea_call, arg_name):
    """ Return ea of argument by looking backwards from library function
    call.

    Arguments:
    ea_call -- effective address of call
    arg_name -- the argument name to look for
    """
    # the search for previous instruction/data will stop at the specified
    # address (inclusive)
    prev_instr = idc.PrevHead(ea_call, ea_call - PREVIOUS_INSTR_DELTA)
    while prev_instr > (ea_call - ARG_SEARCH_THRESHOLD) and \
            prev_instr != idaapi.BADADDR:
        # False indicates not to look for repeatable comments
        comment = idc.GetCommentEx(prev_instr, False)
        if comment == arg_name:
            return prev_instr
        prev_instr = idc.PrevHead(
            prev_instr, prev_instr - PREVIOUS_INSTR_DELTA)
    raise ArgumentNotFoundException('  Argument {} not found within threshold'
                                    .format(arg_name)) 

def find_pool_tags():
	""" Dirty hack around IDA's type information, find references to tag using functions then the comment marking the tag 
	then add the function caller/tag to output dictionary.
	"""
	
	funcs = [
		'ExAllocatePoolWithTag',
		'ExFreePoolWithTag',
		'ExAllocatePoolWithTagPriority'
	]

	tags = {}

	def imp_cb(ea, name, ord):
		if name in funcs:
			for xref in idautils.XrefsTo(ea):
				call_addr = xref.frm
				caller_name = idc.GetFunctionName(call_addr)
				prev = idc.PrevHead(call_addr)
				for _ in range(10):
					if idc.Comment(prev) == 'Tag' and idc.GetOpType(prev, 1) == 5:
						tag_raw = idc.GetOperandValue(prev, 1)
						tag = ''
						for i in range(3, -1, -1):
							tag += chr((tag_raw >> 8 * i) & 0xFF)
						if tag in tags.keys():
							tags[tag].add(caller_name)
						else:
							tags[tag] = set([caller_name])
						break
					prev = idc.PrevHead(prev)
		return True
	
	nimps = idaapi.get_import_module_qty()

	for i in xrange(0, nimps):
		name = idaapi.get_import_module_name(i)
		if not name:
			continue

		idaapi.enum_import_names(i, imp_cb)
	return tags 

def get_function_args_count(self, function_ea, local_vars):
        """
        The function returns count of function arguments
        @function_ea - function entry point
        @local_vars - local variables dictionary
        @return - function arguments count
        """
        # i#9 Now, we can't identify fastcall functions.

        function_args_count = 0
        args_dict = dict()
        for local_var in local_vars:
            usage_list = local_vars.get(local_var, None)
            if usage_list == None:
                print "WARNING: empty usage list for ", local_var
                continue
            for head in usage_list:
                ops = self.get_instr_operands(int(head, 16))
                for idx, (op,type) in enumerate(ops):
                    if op.count("+") == 1:
                        value = idc.GetOperandValue(int (head, 16), idx)
                        if value < (15 * ARGUMENT_SIZE) and "ebp" in op:
                            args_dict.setdefault(local_var, []).append(head)
                    elif op.count("+") == 2:
                        if "arg" in local_var:
                            args_dict.setdefault(local_var, []).append(head)
                    else:
                        continue

        function_args_count = len(args_dict)
        if function_args_count:
            return function_args_count, args_dict

        #TODO Check previous algorithm here
        f_end = idc.FindFuncEnd(function_ea)
        f_end = idc.PrevHead(f_end, 0)
        instr_mnem = idc.GetMnem(f_end)
        #stdcall ?
        if "ret" in instr_mnem:
            ops = self.get_instr_operands(f_end)
            if len(ops) == 1:
                for op,type in ops:
                    op = op.replace("h", "")
                    function_args_count = int(op,16)/ARGUMENT_SIZE
                    return function_args_count, args_dict
        #cdecl ?
        refs = idautils.CodeRefsTo(function_ea, 0)
        for ref in refs:
            #trying to find add esp,x signature after call
            head = idc.NextHead(ref, 0xFFFFFFFF)
            if head:
                disasm = idc.GetDisasm(head)
                if "add" in disasm and "esp," in disasm:
                    ops = self.get_instr_operands(head)
                    op,type = ops[1]
                    if op:
                        op = op.replace("h", "")
                        function_args_count = int(op,16)/ARGUMENT_SIZE
                        return function_args_count, args_dict
        return function_args_count, args_dict