elftools/common/construct_utils.py

   1 #-------------------------------------------------------------------------------
   2 # elftools: common/construct_utils.py
   3 #
   4 # Some complementary construct utilities
   5 #
   6 # Eli Bendersky (eliben@gmail.com)
   7 # This code is in the public domain
   8 #-------------------------------------------------------------------------------
   9 from ..construct import (
  10     Subconstruct, ConstructError, ArrayError, Adapter, Field, RepeatUntil,
  11     Rename, SizeofError, Construct
  12     )
  13
  14
  15 class RepeatUntilExcluding(Subconstruct):
  16     """ A version of construct's RepeatUntil that doesn't include the last
  17         element (which casued the repeat to exit) in the return value.
  18
  19         Only parsing is currently implemented.
  20
  21         P.S. removed some code duplication
  22     """
  23     __slots__ = ["predicate"]
  24     def __init__(self, predicate, subcon):
  25         Subconstruct.__init__(self, subcon)
  26         self.predicate = predicate
  27         self._clear_flag(self.FLAG_COPY_CONTEXT)
  28         self._set_flag(self.FLAG_DYNAMIC)
  29     def _parse(self, stream, context):
  30         obj = []
  31         try:
  32             context_for_subcon = context
  33             if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
  34                 context_for_subcon = context.__copy__()
  35
  36             while True:
  37                 subobj = self.subcon._parse(stream, context_for_subcon)
  38                 if self.predicate(subobj, context):
  39                     break
  40                 obj.append(subobj)
  41         except ConstructError as ex:
  42             raise ArrayError("missing terminator", ex)
  43         return obj
  44     def _build(self, obj, stream, context):
  45         raise NotImplementedError('no building')
  46     def _sizeof(self, context):
  47         raise SizeofError("can't calculate size")
  48
  49
  50 def _LEB128_reader():
  51     """ Read LEB128 variable-length data from the stream. The data is terminated
  52         by a byte with 0 in its highest bit.
  53     """
  54     return RepeatUntil(
  55                 lambda obj, ctx: ord(obj) < 0x80,
  56                 Field(None, 1))
  57
  58
  59 class _ULEB128Adapter(Adapter):
  60     """ An adapter for ULEB128, given a sequence of bytes in a sub-construct.
  61     """
  62     def _decode(self, obj, context):
  63         value = 0
  64         for b in reversed(obj):
  65             value = (value << 7) + (ord(b) & 0x7F)
  66         return value
  67
  68
  69 class _SLEB128Adapter(Adapter):
  70     """ An adapter for SLEB128, given a sequence of bytes in a sub-construct.
  71     """
  72     def _decode(self, obj, context):
  73         value = 0
  74         for b in reversed(obj):
  75             value = (value << 7) + (ord(b) & 0x7F)
  76         if ord(obj[-1]) & 0x40:
  77             # negative -> sign extend
  78             value |= - (1 << (7 * len(obj)))
  79         return value
  80
  81
  82 def ULEB128(name):
  83     """ A construct creator for ULEB128 encoding.
  84     """
  85     return Rename(name, _ULEB128Adapter(_LEB128_reader()))
  86
  87
  88 def SLEB128(name):
  89     """ A construct creator for SLEB128 encoding.
  90     """
  91     return Rename(name, _SLEB128Adapter(_LEB128_reader()))
  92
  93 class StreamOffset(Construct):
  94     """
  95     Captures the current stream offset
  96
  97     Parameters:
  98     * name - the name of the value
  99
 100     Example:
 101     StreamOffset("item_offset")
 102     """
 103     __slots__ = []
 104     def __init__(self, name):
 105         Construct.__init__(self, name)
 106         self._set_flag(self.FLAG_DYNAMIC)
 107     def _parse(self, stream, context):
 108         return stream.tell()
 109     def _build(self, obj, stream, context):
 110         context[self.name] = stream.tell()
 111     def _sizeof(self, context):
 112         return 0