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 pyelftools is not installed, the example can also run from the root or
  14 # examples/ dir of the source distribution.
  15 sys.path[0:0] = ['.', '..']
  16
  17
  18 from elftools.common.py3compat import itervalues
  19 from elftools.elf.elffile import ELFFile
  20 from elftools.dwarf.descriptions import (
  21     describe_DWARF_expr, set_global_machine_arch)
  22 from elftools.dwarf.locationlists import LocationEntry
  23
  24
  25 def process_file(filename):
  26     print('Processing file:', filename)
  27     with open(filename, 'rb') as f:
  28         elffile = ELFFile(f)
  29
  30         if not elffile.has_dwarf_info():
  31             print('  file has no DWARF info')
  32             return
  33
  34         # get_dwarf_info returns a DWARFInfo context object, which is the
  35         # starting point for all DWARF-based processing in pyelftools.
  36         dwarfinfo = elffile.get_dwarf_info()
  37
  38         # The location lists are extracted by DWARFInfo from the .debug_loc
  39         # section, and returned here as a LocationLists object.
  40         location_lists = dwarfinfo.location_lists()
  41
  42         # This is required for the descriptions module to correctly decode
  43         # register names contained in DWARF expressions.
  44         set_global_machine_arch(elffile.get_machine_arch())
  45
  46         for CU in dwarfinfo.iter_CUs():
  47             # DWARFInfo allows to iterate over the compile units contained in
  48             # the .debug_info section. CU is a CompileUnit object, with some
  49             # computed attributes (such as its offset in the section) and
  50             # a header which conforms to the DWARF standard. The access to
  51             # header elements is, as usual, via item-lookup.
  52             print('  Found a compile unit at offset %s, length %s' % (
  53                 CU.cu_offset, CU['unit_length']))
  54
  55             # A CU provides a simple API to iterate over all the DIEs in it.
  56             for DIE in CU.iter_DIEs():
  57                 # Go over all attributes of the DIE. Each attribute is an
  58                 # AttributeValue object (from elftools.dwarf.die), which we
  59                 # can examine.
  60                 for attr in itervalues(DIE.attributes):
  61                     if attribute_has_location_list(attr):
  62                         # This is a location list. Its value is an offset into
  63                         # the .debug_loc section, so we can use the location
  64                         # lists object to decode it.
  65                         loclist = location_lists.get_location_list_at_offset(
  66                             attr.value)
  67
  68                         print('   DIE %s. attr %s.\n%s' % (
  69                             DIE.tag,
  70                             attr.name,
  71                             show_loclist(loclist, dwarfinfo, indent='      ')))
  72
  73
  74 def show_loclist(loclist, dwarfinfo, indent):
  75     """ Display a location list nicely, decoding the DWARF expressions
  76         contained within.
  77     """
  78     d = []
  79     for loc_entity in loclist:
  80         if isinstance(loc_entity, LocationEntry):
  81             d.append('%s <<%s>>' % (
  82                 loc_entity,
  83                 describe_DWARF_expr(loc_entity.loc_expr, dwarfinfo.structs)))
  84         else:
  85             d.append(str(loc_entity))
  86     return '\n'.join(indent + s for s in d)
  87
  88
  89 def attribute_has_location_list(attr):
  90     """ Only some attributes can have location list values, if they have the
  91         required DW_FORM (loclistptr "class" in DWARF spec v3)
  92     """
  93     if (attr.name in (  'DW_AT_location', 'DW_AT_string_length',
  94                         'DW_AT_const_value', 'DW_AT_return_addr',
  95                         'DW_AT_data_member_location', 'DW_AT_frame_base',
  96                         'DW_AT_segment', 'DW_AT_static_link',
  97                         'DW_AT_use_location', 'DW_AT_vtable_elem_location')):
  98         if attr.form in ('DW_FORM_data4', 'DW_FORM_data8'):
  99             return True
 100     return False
 101
 102
 103 if __name__ == '__main__':
 104     for filename in sys.argv[1:]:
 105         process_file(filename)
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