examples/dwarf_location_lists.py

   1 #-------------------------------------------------------------------------------
   2 # elftools example: dwarf_location_lists.py
   3 #
   4 # Examine DIE entries which have location list values, and decode these
   5 # location lists.
   6 #
   7 # Eli Bendersky (eliben@gmail.com)
   8 # This code is in the public domain
   9 #-------------------------------------------------------------------------------
  10 from __future__ import print_function
  11 import sys
  12
  13 # If elftools is not installed, maybe we're running from the root or examples
  14 # dir of the source distribution
  15 try:
  16     import elftools
  17 except ImportError:
  18     sys.path.extend(['.', '..'])
  19
  20 from elftools.common.py3compat import itervalues
  21 from elftools.elf.elffile import ELFFile
  22 from elftools.dwarf.descriptions import (
  23     describe_DWARF_expr, set_global_machine_arch)
  24 from elftools.dwarf.locationlists import LocationEntry
  25
  26
  27 def process_file(filename):
  28     print('Processing file:', filename)
  29     with open(filename, 'rb') as f:
  30         elffile = ELFFile(f)
  31
  32         if not elffile.has_dwarf_info():
  33             print('  file has no DWARF info')
  34             return
  35
  36         # get_dwarf_info returns a DWARFInfo context object, which is the
  37         # starting point for all DWARF-based processing in pyelftools.
  38         dwarfinfo = elffile.get_dwarf_info()
  39
  40         # The location lists are extracted by DWARFInfo from the .debug_loc
  41         # section, and returned here as a LocationLists object.
  42         location_lists = dwarfinfo.location_lists()
  43
  44         # This is required for the descriptions module to correctly decode
  45         # register names contained in DWARF expressions.
  46         set_global_machine_arch(elffile.get_machine_arch())
  47
  48         for CU in dwarfinfo.iter_CUs():
  49             # DWARFInfo allows to iterate over the compile units contained in
  50             # the .debug_info section. CU is a CompileUnit object, with some
  51             # computed attributes (such as its offset in the section) and
  52             # a header which conforms to the DWARF standard. The access to
  53             # header elements is, as usual, via item-lookup.
  54             print('  Found a compile unit at offset %s, length %s' % (
  55                 CU.cu_offset, CU['unit_length']))
  56
  57             # A CU provides a simple API to iterate over all the DIEs in it.
  58             for DIE in CU.iter_DIEs():
  59                 # Go over all attributes of the DIE. Each attribute is an
  60                 # AttributeValue object (from elftools.dwarf.die), which we
  61                 # can examine.
  62                 for attr in itervalues(DIE.attributes):
  63                     if attribute_has_location_list(attr):
  64                         # This is a location list. Its value is an offset into
  65                         # the .debug_loc section, so we can use the location
  66                         # lists object to decode it.
  67                         loclist = location_lists.get_location_list_at_offset(
  68                             attr.value)
  69
  70                         print('   DIE %s. attr %s.\n%s' % (
  71                             DIE.tag,
  72                             attr.name,
  73                             show_loclist(loclist, dwarfinfo, indent='      ')))
  74
  75
  76 def show_loclist(loclist, dwarfinfo, indent):
  77     """ Display a location list nicely, decoding the DWARF expressions
  78         contained within.
  79     """
  80     d = []
  81     for loc_entity in loclist:
  82         if isinstance(loc_entity, LocationEntry):
  83             d.append('%s <<%s>>' % (
  84                 loc_entity,
  85                 describe_DWARF_expr(loc_entity.loc_expr, dwarfinfo.structs)))
  86         else:
  87             d.append(str(loc_entity))
  88     return '\n'.join(indent + s for s in d)
  89
  90
  91 def attribute_has_location_list(attr):
  92     """ Only some attributes can have location list values, if they have the
  93         required DW_FORM (loclistptr "class" in DWARF spec v3)
  94     """
  95     if (attr.name in (  'DW_AT_location', 'DW_AT_string_length',
  96                         'DW_AT_const_value', 'DW_AT_return_addr',
  97                         'DW_AT_data_member_location', 'DW_AT_frame_base',
  98                         'DW_AT_segment', 'DW_AT_static_link',
  99                         'DW_AT_use_location', 'DW_AT_vtable_elem_location')):
 100         if attr.form in ('DW_FORM_data4', 'DW_FORM_data8'):
 101             return True
 102     return False
 103
 104
 105 if __name__ == '__main__':
 106     for filename in sys.argv[1:]:
 107         process_file(filename)
 108
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