examples/dwarf_location_info.py

   1 #-------------------------------------------------------------------------------
   2 # elftools example: dwarf_location_info.py
   3 #
   4 # Examine DIE entries which have either location list values or location
   5 # expression values and decode that information.
   6 #
   7 # Location information can either be completely contained within a DIE
   8 # (using 'DW_FORM_exprloc' in DWARFv4 or 'DW_FORM_block1' in earlier
   9 # versions) or be a reference to a location list contained within
  10 # the .debug_loc section (using 'DW_FORM_sec_offset' in DWARFv4 or
  11 # 'DW_FORM_data4' / 'DW_FORM_data8' in earlier versions).
  12 #
  13 # The LocationParser object parses the DIE attributes and handles both
  14 # formats.
  15 #
  16 # The directory 'test/testfiles_for_location_info' contains test files with
  17 # location information represented in both DWARFv4 and DWARFv2 forms.
  18 #
  19 # Eli Bendersky (eliben@gmail.com)
  20 # This code is in the public domain
  21 #-------------------------------------------------------------------------------
  22 from __future__ import print_function
  23 import sys
  24
  25 # If pyelftools is not installed, the example can also run from the root or
  26 # examples/ dir of the source distribution.
  27 sys.path[0:0] = ['.', '..']
  28
  29 from elftools.elf.elffile import ELFFile
  30 from elftools.dwarf.descriptions import (
  31     describe_DWARF_expr, set_global_machine_arch)
  32 from elftools.dwarf.locationlists import (
  33     LocationEntry, LocationExpr, LocationParser)
  34
  35 def process_file(filename):
  36     print('Processing file:', filename)
  37     with open(filename, 'rb') as f:
  38         elffile = ELFFile(f)
  39
  40         if not elffile.has_dwarf_info():
  41             print('  file has no DWARF info')
  42             return
  43
  44         # get_dwarf_info returns a DWARFInfo context object, which is the
  45         # starting point for all DWARF-based processing in pyelftools.
  46         dwarfinfo = elffile.get_dwarf_info()
  47
  48         # The location lists are extracted by DWARFInfo from the .debug_loc
  49         # section, and returned here as a LocationLists object.
  50         location_lists = dwarfinfo.location_lists()
  51
  52         # This is required for the descriptions module to correctly decode
  53         # register names contained in DWARF expressions.
  54         set_global_machine_arch(elffile.get_machine_arch())
  55
  56         # Create a LocationParser object that parses the DIE attributes and
  57         # creates objects representing the actual location information.
  58         loc_parser = LocationParser(location_lists)
  59
  60         for CU in dwarfinfo.iter_CUs():
  61             # DWARFInfo allows to iterate over the compile units contained in
  62             # the .debug_info section. CU is a CompileUnit object, with some
  63             # computed attributes (such as its offset in the section) and
  64             # a header which conforms to the DWARF standard. The access to
  65             # header elements is, as usual, via item-lookup.
  66             print('  Found a compile unit at offset %s, length %s' % (
  67                 CU.cu_offset, CU['unit_length']))
  68
  69             # A CU provides a simple API to iterate over all the DIEs in it.
  70             for DIE in CU.iter_DIEs():
  71                 # Go over all attributes of the DIE. Each attribute is an
  72                 # AttributeValue object (from elftools.dwarf.die), which we
  73                 # can examine.
  74                 for attr in DIE.attributes.values():
  75                     # Check if this attribute contains location information
  76                     if loc_parser.attribute_has_location(attr, CU['version']):
  77                         print('   DIE %s. attr %s.' % (DIE.tag, attr.name))
  78                         loc = loc_parser.parse_from_attribute(attr,
  79                                                               CU['version'])
  80                         # We either get a list (in case the attribute is a
  81                         # reference to the .debug_loc section) or a LocationExpr
  82                         # object (in case the attribute itself contains location
  83                         # information).
  84                         if isinstance(loc, LocationExpr):
  85                             print('      %s' % (
  86                                 describe_DWARF_expr(loc.loc_expr,
  87                                                     dwarfinfo.structs, CU.cu_offset)))
  88                         elif isinstance(loc, list):
  89                             print(show_loclist(loc,
  90                                                dwarfinfo,
  91                                                '      ', CU.cu_offset))
  92
  93 def show_loclist(loclist, dwarfinfo, indent, cu_offset):
  94     """ Display a location list nicely, decoding the DWARF expressions
  95         contained within.
  96     """
  97     d = []
  98     for loc_entity in loclist:
  99         if isinstance(loc_entity, LocationEntry):
 100             d.append('%s <<%s>>' % (
 101                 loc_entity,
 102                 describe_DWARF_expr(loc_entity.loc_expr, dwarfinfo.structs, cu_offset)))
 103         else:
 104             d.append(str(loc_entity))
 105     return '\n'.join(indent + s for s in d)
 106
 107 if __name__ == '__main__':
 108     if sys.argv[1] == '--test':
 109         for filename in sys.argv[2:]:
 110             process_file(filename)