#!/usr/bin/python

"""A kernel module rewriter.

This is a hack that rewrites kernel modules such that they can be loaded on
kernels they were not compiled for.
"""

__author__ = "Andreas Moser"
import logging
import os
import platform
import struct
import sys


class KernelObjectPatcher(object):
  """The kernel object patching class."""

  ELF_MAGIC = "\x7F\x45\x4C\x46"

  def __init__(self, log=False):
    self.log = log

  def GetSectionOffsets(self, file_data):
    """Returns offsets and lengths of all the sections of this elf file."""

    if file_data[:4] != self.ELF_MAGIC:
      raise RuntimeError("Not an elf file.")

    section_header_offset = struct.unpack("<Q", file_data[40:40+8])[0]
    (section_header_size, num_section_headers,
     string_table) = struct.unpack("<HHH", file_data[58:58+6])

    # Read the string table first.
    start = section_header_offset + string_table * section_header_size
    header_data = file_data[start:start + section_header_size]
    offset, size = struct.unpack("<IIQQQQIIQQ", header_data)[4:6]

    string_data = file_data[offset:offset+size]

    sections = {}

    for start in xrange(section_header_offset,
                        section_header_offset + (
                            num_section_headers * section_header_size),
                        section_header_size):
      header_data = file_data[start: start + section_header_size]

      header = struct.unpack("<IIQQQQIIQQ", header_data)
      name_offset, data_offset, data_size = header[0], header[4], header[5]
      name = string_data[name_offset:string_data.find("\x00", name_offset)]

      if data_size:
        sections[name] = (data_offset, data_size)

    return sections

  def ParseVersionSection(self, version_data):
    """Returns the checksums found for all the imports."""
    checksums = {}
    while version_data:
      act_version = version_data[:0x40]
      version_data = version_data[0x40:]

      function = act_version[8:]
      chksum = act_version[:8]
      checksums[function] = chksum
    return checksums

  def GetImportedVersions(self, file_data, sections):
    if "__versions" not in sections:
      return {}
    start, length = sections["__versions"]
    version_data = file_data[start:start+length]
    return self.ParseVersionSection(version_data)

  def GetModuleVersion(self, file_data, sections):
    info_start, info_length = sections[".modinfo"]
    modinfo = file_data[info_start:info_start + info_length]

    for line in modinfo.split("\x00"):
      if line.startswith("vermagic"):
        return line[len("vermagic") + 1:]
    msg = "Could not find vermagic string."
    logging.info(msg)
    raise RuntimeError(msg)

  def _RewriteModinfo(self, modinfo, obj_kernel_version, this_kernel_version,
                      info_strings=None, to_remove=None):
    new_modinfo = ""
    for line in modinfo.split("\x00"):
      if not line:
        continue
      if to_remove and line.split("=")[0] == to_remove:
        continue
      if info_strings is not None:
        info_strings.add(line.split("=")[0])
      if line.startswith("vermagic"):
        line = line.replace(obj_kernel_version, this_kernel_version)
      new_modinfo += line + "\x00"
    return new_modinfo

  def RewriteModinfo(self, file_data, sections, obj_kernel_version,
                     this_kernel_version):
    """This rewrites the modinfo section and updates the kernel version."""
    info_start, info_length = sections[".modinfo"]
    modinfo = file_data[info_start:info_start + info_length]

    info_strings = set()
    new_modinfo = self._RewriteModinfo(modinfo, obj_kernel_version,
                                       this_kernel_version, info_strings)

    if len(new_modinfo) <= info_length:
      new_modinfo += "\x00" * (info_length - len(new_modinfo))
      return new_modinfo

    logging.info("Rewritten modinfo section is too big.")
    info_strings -= set(["vermagic", "srcversion", "depends"])
    try:
      to_remove = info_strings.pop()
    except KeyError:
      msg = "Could not remove anything from modinfo, giving up."
      logging.info(msg)
      raise RuntimeError(msg)
    logging.info("Will try to remove %s from modinfo.", to_remove)

    return self._RewriteModinfo(modinfo, obj_kernel_version,
                                this_kernel_version, to_remove=to_remove)

  def GetKnownImports(self, needed_versions):
    """Parses the driver directory to find valid import checksums."""
    needed_versions = set(needed_versions)
    found_versions = {}

    driver_path = "/lib/modules/%s/kernel/drivers" % platform.uname()[2]
    num_files = 0
    for (directory, _, files) in os.walk(driver_path):
      for filename in files:
        if filename[-3:] == ".ko":
          try:
            fd = open("%s/%s" % (directory, filename), "rb")
            num_files += 1
            data = fd.read()
            sections = self.GetSectionOffsets(data)
            versions = self.GetImportedVersions(data, sections)
            found_versions.update(versions)
            if set(found_versions.keys()) >= needed_versions:
              logging.info("All imports found, gathered data from %d modules.",
                           num_files)
              return found_versions
          except IOError:
            pass

    missing = needed_versions - set(found_versions.keys())
    msg = "Imports %s could not be found." % ",".join(missing)
    logging.info(msg)
    raise RuntimeError(msg)

  def ReplaceSection(self, file_data, offset, new_section_data):
    result = file_data[:offset]
    result += new_section_data
    result += file_data[offset + len(new_section_data):]
    return result

  def Patch(self, file_data, force_patch=False):
    try:
      sections = self.GetSectionOffsets(file_data)
      obj_version = self.GetModuleVersion(file_data, sections)
      obj_kernel_version = obj_version.split(" ")[0]
      this_kernel_version = platform.uname()[2]
      logging.info("Module version is %s, kernel version is %s.",
                   obj_kernel_version, this_kernel_version)
      if obj_kernel_version == this_kernel_version and not force_patch:
        return file_data

      needed_imports = self.GetImportedVersions(file_data, sections)
      good_imports = self.GetKnownImports(needed_imports)

      rewritten_version_data = ""
      for function in needed_imports.keys():
        if needed_imports[function] == good_imports[function]:
          logging.info("Matching checksum for %s.",
                       function.replace("\x00", ""))
        else:
          logging.info("Checksum mismatch for %s.",
                       function.replace("\x00", ""))
        rewritten_version_data += good_imports[function] + function

      rewritten_modinfo_data = self.RewriteModinfo(
          file_data, sections, obj_kernel_version, this_kernel_version)

      file_data = self.ReplaceSection(file_data, sections["__versions"][0],
                                      rewritten_version_data)
      file_data = self.ReplaceSection(file_data, sections[".modinfo"][0],
                                      rewritten_modinfo_data)
      return file_data

    except (RuntimeError, KeyError) as e:
      logging.info(str(e))
      # Something didn't work, we can just use the data we were sent.
      return file_data


def main():
  if len(sys.argv) < 3:
    print("Usage: python %s <kernel_module> <outfile>" % sys.argv[0])
    exit()

  in_fd = open(sys.argv[1], "rb")

  out_data = KernelObjectPatcher(log=True).Patch(in_fd.read(), force_patch=True)

  with open(sys.argv[2], "wb") as out_fd:
    out_fd.write(out_data)

  logging.info("Kernel Object patched.")

if __name__ == "__main__":
   logging.basicConfig(level=logging.DEBUG)
   main()
