webassembly.py

# Copyright 2011 The Emscripten Authors.  All rights reserved.
# Emscripten is available under two separate licenses, the MIT license and the
# University of Illinois/NCSA Open Source License.  Both these licenses can be
# found in the LICENSE file.

"""Utilties for manipulating WebAssembly binaries from python.
"""

from collections import namedtuple
from enum import IntEnum
import logging
import os
import sys

from . import utils
from .settings import settings

sys.path.append(utils.path_from_root('third_party'))

import leb128

logger = logging.getLogger('webassembly')


# For the Emscripten-specific WASM metadata section, follows semver, changes
# whenever metadata section changes structure.
# NB: major version 0 implies no compatibility
# NB: when changing the metadata format, we should only append new fields, not
#     reorder, modify, or remove existing ones.
EMSCRIPTEN_METADATA_MAJOR, EMSCRIPTEN_METADATA_MINOR = (0, 3)
# For the JS/WASM ABI, specifies the minimum ABI version required of
# the WASM runtime implementation by the generated WASM binary. It follows
# semver and changes whenever C types change size/signedness or
# syscalls change signature. By semver, the maximum ABI version is
# implied to be less than (EMSCRIPTEN_ABI_MAJOR + 1, 0). On an ABI
# change, increment EMSCRIPTEN_ABI_MINOR if EMSCRIPTEN_ABI_MAJOR == 0
# or the ABI change is backwards compatible, otherwise increment
# EMSCRIPTEN_ABI_MAJOR and set EMSCRIPTEN_ABI_MINOR = 0.
EMSCRIPTEN_ABI_MAJOR, EMSCRIPTEN_ABI_MINOR = (0, 29)

WASM_PAGE_SIZE = 65536

MAGIC = b'\0asm'

VERSION = b'\x01\0\0\0'

HEADER_SIZE = 8

LIMITS_HAS_MAX = 0x1


def toLEB(num):
  return leb128.u.encode(num)


def readULEB(iobuf):
  return leb128.u.decode_reader(iobuf)[0]


def readSLEB(iobuf):
  return leb128.i.decode_reader(iobuf)[0]


def add_emscripten_metadata(wasm_file):
  mem_size = settings.INITIAL_MEMORY // WASM_PAGE_SIZE
  global_base = settings.GLOBAL_BASE

  logger.debug('creating wasm emscripten metadata section with mem size %d' % mem_size)
  name = b'\x13emscripten_metadata' # section name, including prefixed size
  contents = (
    # metadata section version
    toLEB(EMSCRIPTEN_METADATA_MAJOR) +
    toLEB(EMSCRIPTEN_METADATA_MINOR) +

    # NB: The structure of the following should only be changed
    #     if EMSCRIPTEN_METADATA_MAJOR is incremented
    # Minimum ABI version
    toLEB(EMSCRIPTEN_ABI_MAJOR) +
    toLEB(EMSCRIPTEN_ABI_MINOR) +

    # Wasm backend, always 1 now
    toLEB(1) +

    toLEB(mem_size) +
    toLEB(0) +
    toLEB(global_base) +
    toLEB(0) +
    # dynamictopPtr, always 0 now
    toLEB(0) +

    # tempDoublePtr, always 0 in wasm backend
    toLEB(0) +

    toLEB(int(settings.STANDALONE_WASM))

    # NB: more data can be appended here as long as you increase
    #     the EMSCRIPTEN_METADATA_MINOR
  )

  orig = utils.read_binary(wasm_file)
  with open(wasm_file, 'wb') as f:
    f.write(orig[0:8]) # copy magic number and version
    # write the special section
    f.write(b'\0') # user section is code 0
    # need to find the size of this section
    size = len(name) + len(contents)
    f.write(toLEB(size))
    f.write(name)
    f.write(contents)
    f.write(orig[8:])


class SecType(IntEnum):
  CUSTOM = 0
  TYPE = 1
  IMPORT = 2
  FUNCTION = 3
  TABLE = 4
  MEMORY = 5
  EVENT = 13
  GLOBAL = 6
  EXPORT = 7
  START = 8
  ELEM = 9
  DATACOUNT = 12
  CODE = 10
  DATA = 11


class ExternType(IntEnum):
  FUNC = 0
  TABLE = 1
  MEMORY = 2
  GLOBAL = 3
  EVENT = 4


class DylinkType(IntEnum):
  MEM_INFO = 1
  NEEDED = 2


Section = namedtuple('Section', ['type', 'size', 'offset'])
Limits = namedtuple('Limits', ['flags', 'initial', 'maximum'])
Import = namedtuple('Import', ['kind', 'module', 'field'])
Export = namedtuple('Export', ['name', 'kind', 'index'])
Dylink = namedtuple('Dylink', ['mem_size', 'mem_align', 'table_size', 'table_align', 'needed'])


class Module:
  """Extremely minimal wasm module reader.  Currently only used
  for parsing the dylink section."""
  def __init__(self, filename):
    self.size = os.path.getsize(filename)
    self.buf = open(filename, 'rb')
    magic = self.buf.read(4)
    version = self.buf.read(4)
    assert magic == MAGIC
    assert version == VERSION

  def __del__(self):
    self.buf.close()

  def readByte(self):
    return self.buf.read(1)[0]

  def readULEB(self):
    return readULEB(self.buf)

  def readSLEB(self):
    return readSLEB(self.buf)

  def readString(self):
    size = self.readULEB()
    return self.buf.read(size).decode('utf-8')

  def readLimits(self):
    flags = self.readByte()
    initial = self.readULEB()
    maximum = 0
    if flags & LIMITS_HAS_MAX:
      maximum = self.readULEB()
    return Limits(flags, initial, maximum)

  def seek(self, offset):
    return self.buf.seek(offset)

  def tell(self):
    return self.buf.tell()

  def skip(self, count):
    self.buf.seek(count, os.SEEK_CUR)

  def sections(self):
    """Generator that lazily returns sections from the wasm file."""
    offset = HEADER_SIZE
    while offset < self.size:
      self.seek(offset)
      section_type = SecType(self.readByte())
      section_size = self.readULEB()
      section_offset = self.buf.tell()
      yield Section(section_type, section_size, section_offset)
      offset = section_offset + section_size


def parse_dylink_section(wasm_file):
  module = Module(wasm_file)

  dylink_section = next(module.sections())
  assert dylink_section.type == SecType.CUSTOM
  module.seek(dylink_section.offset)
  # section name
  section_name = module.readString()
  needed = []

  if section_name == 'dylink':
    mem_size = module.readULEB()
    mem_align = module.readULEB()
    table_size = module.readULEB()
    table_align = module.readULEB()

    needed_count = module.readULEB()
    while needed_count:
      libname = module.readString()
      needed.append(libname)
      needed_count -= 1
  elif section_name == 'dylink.0':
    section_end = dylink_section.offset + dylink_section.size
    while module.tell() < section_end:
      subsection_type = module.readULEB()
      subsection_size = module.readULEB()
      end = module.tell() + subsection_size
      if subsection_type == DylinkType.MEM_INFO:
        mem_size = module.readULEB()
        mem_align = module.readULEB()
        table_size = module.readULEB()
        table_align = module.readULEB()
      elif subsection_type == DylinkType.NEEDED:
        needed_count = module.readULEB()
        while needed_count:
          libname = module.readString()
          needed.append(libname)
          needed_count -= 1
      else:
        print(f'unknown subsection: {subsection_type}')
        # ignore unknown subsections
        module.skip(subsection_size)
      assert(module.tell() == end)
  else:
    utils.exit_with_error('error parsing shared library')

  return Dylink(mem_size, mem_align, table_size, table_align, needed)


def get_exports(wasm_file):
  module = Module(wasm_file)
  export_section = next((s for s in module.sections() if s.type == SecType.EXPORT), None)

  module.seek(export_section.offset)
  num_exports = module.readULEB()
  exports = []
  for i in range(num_exports):
    name = module.readString()
    kind = ExternType(module.readByte())
    index = module.readULEB()
    exports.append(Export(name, kind, index))

  return exports


def get_imports(wasm_file):
  module = Module(wasm_file)
  import_section = next((s for s in module.sections() if s.type == SecType.IMPORT), None)
  if not import_section:
    return []

  module.seek(import_section.offset)
  num_imports = module.readULEB()
  imports = []
  for i in range(num_imports):
    mod = module.readString()
    field = module.readString()
    kind = ExternType(module.readByte())
    imports.append(Import(kind, mod, field))
    if kind == ExternType.FUNC:
      module.readULEB()  # sig
    elif kind == ExternType.GLOBAL:
      module.readSLEB()  # global type
      module.readByte()  # mutable
    elif kind == ExternType.MEMORY:
      module.readLimits()  # limits
    elif kind == ExternType.TABLE:
      module.readSLEB()  # table type
      module.readLimits()  # limits
    else:
      assert False

  return imports