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namelut.py
#------------------------------------------------------------------------------- # elftools: dwarf/namelut.py # # DWARF pubtypes/pubnames section decoding (.debug_pubtypes, .debug_pubnames) # # Vijay Ramasami (rvijayc@gmail.com) # This code is in the public domain #------------------------------------------------------------------------------- import os import collections from collections import OrderedDict from ..common.utils import struct_parse from ..common.py3compat import Mapping from bisect import bisect_right import math from ..construct import CString, Struct, If NameLUTEntry = collections.namedtuple('NameLUTEntry', 'cu_ofs die_ofs') class NameLUT(Mapping): """ A "Name LUT" holds any of the tables specified by .debug_pubtypes or .debug_pubnames sections. This is basically a dictionary where the key is the symbol name (either a public variable, function or a type), and the value is the tuple (cu_offset, die_offset) corresponding to the variable. The die_offset is an absolute offset (meaning, it can be used to search the CU by iterating until a match is obtained). An ordered dictionary is used to preserve the CU order (i.e, items are stored on a per-CU basis (as it was originally in the .debug_* section). Usage: The NameLUT walks and talks like a dictionary and hence it can be used as such. Some examples below: # get the pubnames (a NameLUT from DWARF info). pubnames = dwarf_info.get_pubnames() # lookup a variable. entry1 = pubnames["var_name1"] entry2 = pubnames.get("var_name2", default=<default_var>) print(entry2.cu_ofs) ... # iterate over items. for (name, entry) in pubnames.items(): # do stuff with name, entry.cu_ofs, entry.die_ofs # iterate over items on a per-CU basis. import itertools for cu_ofs, item_list in itertools.groupby(pubnames.items(), key = lambda x: x[1].cu_ofs): # items are now grouped by cu_ofs. # item_list is an iterator yeilding NameLUTEntry'ies belonging # to cu_ofs. # We can parse the CU at cu_offset and use the parsed CU results # to parse the pubname DIEs in the CU listed by item_list. for item in item_list: # work with item which is part of the CU with cu_ofs. """ def __init__(self, stream, size, structs): self._stream = stream self._size = size self._structs = structs # entries are lazily loaded on demand. self._entries = None # CU headers (for readelf). self._cu_headers = None def get_entries(self): """ Returns the parsed NameLUT entries. The returned object is a dictionary with the symbol name as the key and NameLUTEntry(cu_ofs, die_ofs) as the value. This is useful when dealing with very large ELF files with millions of entries. The returned entries can be pickled to a file and restored by calling set_entries on subsequent loads. """ if self._entries is None: self._entries, self._cu_headers = self._get_entries() return self._entries def set_entries(self, entries, cu_headers): """ Set the NameLUT entries from an external source. The input is a dictionary with the symbol name as the key and NameLUTEntry(cu_ofs, die_ofs) as the value. This option is useful when dealing with very large ELF files with millions of entries. The entries can be parsed once and pickled to a file and can be restored via this function on subsequent loads. """ self._entries = entries self._cu_headers = cu_headers def __len__(self): """ Returns the number of entries in the NameLUT. """ if self._entries is None: self._entries, self._cu_headers = self._get_entries() return len(self._entries) def __getitem__(self, name): """ Returns a namedtuple - NameLUTEntry(cu_ofs, die_ofs) - that corresponds to the given symbol name. """ if self._entries is None: self._entries, self._cu_headers = self._get_entries() return self._entries.get(name) def __iter__(self): """ Returns an iterator to the NameLUT dictionary. """ if self._entries is None: self._entries, self._cu_headers = self._get_entries() return iter(self._entries) def items(self): """ Returns the NameLUT dictionary items. """ if self._entries is None: self._entries, self._cu_headers = self._get_entries() return self._entries.items() def get(self, name, default=None): """ Returns NameLUTEntry(cu_ofs, die_ofs) for the provided symbol name or None if the symbol does not exist in the corresponding section. """ if self._entries is None: self._entries, self._cu_headers = self._get_entries() return self._entries.get(name, default) def get_cu_headers(self): """ Returns all CU headers. Mainly required for readelf. """ if self._cu_headers is None: self._entries, self._cu_headers = self._get_entries() return self._cu_headers def _get_entries(self): """ Parse the (name, cu_ofs, die_ofs) information from this section and store as a dictionary. """ self._stream.seek(0) entries = OrderedDict() cu_headers = [] offset = 0 # According to 6.1.1. of DWARFv4, each set of names is terminated by # an offset field containing zero (and no following string). Because # of sequential parsing, every next entry may be that terminator. # So, field "name" is conditional. entry_struct = Struct("Dwarf_offset_name_pair", self._structs.Dwarf_offset('die_ofs'), If(lambda ctx: ctx['die_ofs'], CString('name'))) # each run of this loop will fetch one CU worth of entries. while offset < self._size: # read the header for this CU. namelut_hdr = struct_parse(self._structs.Dwarf_nameLUT_header, self._stream, offset) cu_headers.append(namelut_hdr) # compute the next offset. offset = (offset + namelut_hdr.unit_length + self._structs.initial_length_field_size()) # before inner loop, latch data that will be used in the inner # loop to avoid attribute access and other computation. hdr_cu_ofs = namelut_hdr.debug_info_offset # while die_ofs of the entry is non-zero (which indicates the end) ... while True: entry = struct_parse(entry_struct, self._stream) # if it is zero, this is the terminating record. if entry.die_ofs == 0: break # add this entry to the look-up dictionary. entries[entry.name.decode('utf-8')] = NameLUTEntry( cu_ofs = hdr_cu_ofs, die_ofs = hdr_cu_ofs + entry.die_ofs) # return the entries parsed so far. return (entries, cu_headers)
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