extract_metadata.py

# Copyright 2022 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.

import logging
from dataclasses import dataclass

from . import webassembly
from .settings import settings
from .utils import exit_with_error
from .webassembly import AtomicOpCode, MemoryOpCode, OpCode

logger = logging.getLogger('extract_metadata')


def skip_function_header(module):
  num_local_decls = module.read_uleb()
  while num_local_decls:
    local_count = module.read_uleb()  # noqa
    local_type = module.read_type()  # noqa
    num_local_decls -= 1


def is_orig_main_wrapper(module, function):
  module.get_types()
  module.get_function_types()
  module.seek(function.offset)
  skip_function_header(module)
  end = function.offset + function.size
  while module.tell() != end:
    opcode = module.read_byte()
    try:
      opcode = OpCode(opcode)
    except ValueError:
      return False
    match opcode:
      case OpCode.CALL:
        callee = module.read_uleb()
        callee_type = module.get_function_type(callee)
        if len(callee_type.params) != 0:
          return False
      case OpCode.LOCAL_GET | OpCode.LOCAL_SET:
        module.read_uleb()  # local index
      case OpCode.END | OpCode.RETURN:
        pass
      case _:
        # Any other opcodes and we assume this not a simple wrapper
        return False

  assert opcode == OpCode.END
  return True


def get_const_expr_value(expr):
  assert len(expr) == 2
  assert expr[1][0] == OpCode.END
  opcode, immediates = expr[0]
  match opcode:
    case OpCode.I32_CONST | OpCode.I64_CONST:
      assert len(immediates) == 1
      return immediates[0]
    case OpCode.GLOBAL_GET:
      return 0
    case _:
      exit_with_error('unexpected opcode in const expr: %s', opcode)


def get_global_value(globl):
  return get_const_expr_value(globl.init)


def parse_function_for_memory_inits(module, func_index, offset_map):
  """Very limited function parser that uses `memory.init` instructions
  to derive segment offset.

  When segments are passive they don't have an offset but (at least with
  llvm-generated code) are loaded during the start function
  (`__wasm_init_memory`) using `memory.init` instructions.

  Here we parse the `__wasm_init_memory` function and make many assumptions
  about its layout.  For example, we assume the first argument to `memory.init`
  is either an `i32.const` or the result of an `i32.add`.
  """
  segments = module.get_segments()
  func = module.get_function(func_index)
  module.seek(func.offset)
  skip_function_header(module)
  end = func.offset + func.size
  const_values = []
  call_targets = []
  while module.tell() != end:
    opcode = OpCode(module.read_byte())
    match opcode:
      case OpCode.END | OpCode.NOP | OpCode.DROP | OpCode.I32_ADD | OpCode.I64_ADD:
        pass
      case OpCode.BLOCK:
        module.read_type()
      case OpCode.I32_CONST | OpCode.I64_CONST:
        const_values.append(module.read_sleb())
      case OpCode.GLOBAL_SET | OpCode.BR | OpCode.GLOBAL_GET | OpCode.LOCAL_SET | OpCode.LOCAL_GET | OpCode.LOCAL_TEE:
        module.read_uleb()
      case OpCode.CALL:
        call_targets.append(module.read_uleb())
      case OpCode.MEMORY_PREFIX:
        opcode = MemoryOpCode(module.read_byte())
        match opcode:
          case MemoryOpCode.MEMORY_INIT:
            segment_idx = module.read_uleb()
            segment = segments[segment_idx]
            offset = to_unsigned(const_values[-3])
            offset_map[segment] = offset
            memory = module.read_uleb()
            assert memory == 0
          case MemoryOpCode.MEMORY_FILL:
            memory = module.read_uleb() # noqa
            assert memory == 0
          case MemoryOpCode.MEMORY_DROP:
            segment = module.read_uleb() # noqa
          case _:
            assert False, "unknown: %s" % opcode
      case OpCode.ATOMIC_PREFIX:
        opcode = AtomicOpCode(module.read_byte())
        if opcode in (AtomicOpCode.ATOMIC_I32_RMW_CMPXCHG, AtomicOpCode.ATOMIC_I32_STORE,
                      AtomicOpCode.ATOMIC_NOTIFY, AtomicOpCode.ATOMIC_WAIT32,
                      AtomicOpCode.ATOMIC_WAIT64):
          module.read_uleb()
          module.read_uleb()
        else:
          assert False, "unknown: %s" % opcode
      case OpCode.BR_TABLE:
        count = module.read_uleb()
        for _ in range(count):
          depth = module.read_uleb() # noqa
        default = module.read_uleb() # noqa
      case _:
        assert False, "unknown: %s" % opcode

  # Recursion is safe here because the layout of the wasm-ld-generated
  # start function has a specific structure and has at most on level
  # of call stack depth.
  for t in call_targets:
    parse_function_for_memory_inits(module, t, offset_map)


@webassembly.memoize
def get_passive_segment_offsets(module):
  start_func_index = module.get_start()
  assert start_func_index is not None
  offset_map = {}
  parse_function_for_memory_inits(module, start_func_index, offset_map)
  return offset_map


def to_unsigned(val):
  if val < 0:
    return val & ((2 ** 32) - 1)
  else:
    return val


def find_segment_with_address(module, address):
  segments = module.get_segments()
  active = [s for s in segments if s.init]

  for seg in active:
    offset = to_unsigned(get_const_expr_value(seg.init))
    if offset is None:
      continue
    if address >= offset and address < offset + seg.size:
      return (seg, address - offset)

  passive = [s for s in segments if not s.init]
  if passive:
    offset_map = get_passive_segment_offsets(module)
    for seg, offset in offset_map.items():
      if address >= offset and address < offset + seg.size:
        return (seg, address - offset)

  raise AssertionError('unable to find segment for address: %s' % address)


def data_to_string(data):
  data = data.decode('utf8')
  # We have at least one test (test/core/test_utf8.c) that uses a double
  # backslash in the C++ source code, in order to represent a single backslash.
  # This is because these strings historically were written and read back via
  # JSON and a single slash is interpreted as an escape char there.
  # Technically this escaping is no longer needed and could be removed
  # but in order to maintain compatibility we strip out the double
  # slashes here.
  data = data.replace('\\\\', '\\')
  return data


def get_section_strings(module, export_map, section_name):
  start_name = f'__start_{section_name}'
  stop_name = f'__stop_{section_name}'
  if start_name not in export_map or stop_name not in export_map:
    logger.debug(f'no start/stop symbols found for section: {section_name}')
    return {}

  start = export_map[start_name]
  end = export_map[stop_name]
  start_global = module.get_global(start.index)
  end_global = module.get_global(end.index)
  start_addr = to_unsigned(get_global_value(start_global))
  end_addr = to_unsigned(get_global_value(end_global))

  seg = find_segment_with_address(module, start_addr)
  if not seg:
    exit_with_error(f'unable to find segment starting at __start_{section_name}: {start_addr}')
  seg, seg_offset = seg

  asm_strings = {}
  str_start = seg_offset
  data = module.read_at(seg.offset, seg.size)
  size = end_addr - start_addr
  end = seg_offset + size
  while str_start < end:
    str_end = data.find(b'\0', str_start)
    asm_strings[start_addr - seg_offset + str_start] = data_to_string(data[str_start:str_end])
    str_start = str_end + 1
  return asm_strings


def get_main_reads_params(module, export_map):
  if settings.STANDALONE_WASM:
    return True

  main = export_map.get('main') or export_map.get('__main_argc_argv')
  if not main or main.kind != webassembly.ExternType.FUNC:
    return False

  main_func = module.get_function(main.index)
  if is_orig_main_wrapper(module, main_func):
    # If main is simple wrapper function then we know that __original_main
    # doesn't read arguments.
    return False

  # By default assume params are read
  return True


def get_function_exports(module):
  rtn = {}
  for e in module.get_exports():
    if e.kind == webassembly.ExternType.FUNC:
      rtn[e.name] = module.get_function_type(e.index)
  return rtn


def get_other_exports(module):
  rtn = []
  for e in module.get_exports():
    if e.kind == webassembly.ExternType.GLOBAL:
      rtn.append((e, module.get_global(e.index)))
    elif e.kind != webassembly.ExternType.FUNC:
      rtn.append((e, None))
  return rtn


def read_module_imports(module, metadata):
  em_js_funcs = metadata.em_js_funcs
  types = module.get_types()

  imports = metadata.imports = []
  invoke_funcs = metadata.invoke_funcs = []
  em_js_func_types = metadata.em_js_func_types = {}

  for i in module.get_imports():
    if i.kind == webassembly.ExternType.FUNC:
      if i.field.startswith('invoke_'):
        invoke_funcs.append(i.field)
      else:
        if i.field in em_js_funcs:
          em_js_func_types[i.field] = types[i.type]
        imports.append(i.field)
    elif i.kind in (webassembly.ExternType.GLOBAL, webassembly.ExternType.TAG):
      imports.append(i.field)


def update_metadata(filename, metadata):
  with webassembly.Module(filename) as module:
    metadata.function_exports = get_function_exports(module)
    metadata.other_exports = get_other_exports(module)
    metadata.all_exports = [e.name.removeprefix('__em_js__') for e in module.get_exports()]
    read_module_imports(module, metadata)


def get_string_at(module, address):
  seg, offset = find_segment_with_address(module, address)
  data = module.read_at(seg.offset, seg.size)
  str_end = data.find(b'\0', offset)
  return data_to_string(data[offset:str_end])


@dataclass(init=False)
class Metadata:
  imports: list[str]
  export: list[str]
  em_asm_consts: dict[int, str]
  js_deps: list[str]
  em_js_funcs: dict[str, str]
  em_js_func_types: dict[str, webassembly.FuncType]
  features: list[str]
  invoke_funcs: list[str]
  main_reads_params: bool
  function_exports: dict[str, webassembly.FuncType]
  other_exports: list[webassembly.Export]
  all_exports: list[str]


def extract_metadata(filename):
  em_js_funcs = {}

  with webassembly.Module(filename) as module:
    exports = module.get_exports()

    export_map = {e.name: e for e in exports}
    for e in exports:
      if e.kind == webassembly.ExternType.GLOBAL and e.name.startswith('__em_js__'):
        name = e.name.removeprefix('__em_js__')
        globl = module.get_global(e.index)
        string_address = to_unsigned(get_global_value(globl))
        em_js_funcs[name] = get_string_at(module, string_address)

    features = module.get_target_features()
    features = [f'--enable-{feature}' for feature, used in features.items() if used == webassembly.TargetFeaturePrefix.USED]
    features = [f.replace('--enable-atomics', '--enable-threads') for f in features]
    features = [f.replace('--enable-simd128', '--enable-simd') for f in features]
    features = [f.replace('--enable-nontrapping-fptoint', '--enable-nontrapping-float-to-int') for f in features]

    # If main does not read its parameters, it will just be a stub that
    # calls __original_main (which has no parameters).
    metadata = Metadata()
    metadata.function_exports = get_function_exports(module)
    metadata.other_exports = get_other_exports(module)
    metadata.all_exports = [e.name.removeprefix('__em_js__') for e in exports]
    metadata.em_asm_consts = get_section_strings(module, export_map, 'em_asm')
    metadata.js_deps = [d for d in get_section_strings(module, export_map, 'em_lib_deps').values() if d]
    metadata.em_js_funcs = em_js_funcs
    metadata.features = features
    metadata.main_reads_params = get_main_reads_params(module, export_map)

    read_module_imports(module, metadata)

    # print("Metadata parsed: " + pprint.pformat(metadata))
    return metadata