blob: fa0d92c7ef23507da1a8ba395edb5deb298da03f [file] [log] [blame]
# Copyright 2014 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""ELF parsing related helper functions/classes."""
from __future__ import print_function
import cStringIO
import os
from chromite.scripts import lddtree
from elftools.elf import elffile
from elftools.elf import enums
from elftools.common import utils
# Reverse dict() from numeric values to strings used to lookup st_shndx.
SH_TYPE_VALUES = dict((value, name)
for name, value in enums.ENUM_SH_TYPE.iteritems())
def ParseELFSymbols(elf):
"""Parses list of symbols in an ELF file.
elf: An elffile.ELFFile instance.
A 2-tuple of (imported, exported) symbols. |imported| is a set of strings
of undefined symbols. |exported| is a dict where the keys are defined
symbols and the values are 3-tuples (st_info_bind, st_size, st_shndx) with
the details of the corresponding exported symbol. Note that for imported
symbols this information is always ('STB_GLOBAL', 0, 'SHN_UNDEF') and thus
not included in the result.
imp = set()
exp = dict()
if elf.header.e_type not in ('ET_DYN', 'ET_EXEC'):
return imp, exp
for segment in elf.iter_segments():
if segment.header.p_type != 'PT_DYNAMIC':
# Find strtab and symtab virtual addresses.
strtab_ptr = None
symtab_ptr = None
symbol_size = elf.structs.Elf_Sym.sizeof()
for tag in segment.iter_tags():
if tag.entry.d_tag == 'DT_SYMTAB':
symtab_ptr = tag.entry.d_ptr
if tag.entry.d_tag == 'DT_STRTAB':
strtab_ptr = tag.entry.d_ptr
if tag.entry.d_tag == 'DT_SYMENT':
assert symbol_size == tag.entry.d_val
stringtable = segment._get_stringtable() # pylint: disable=W0212
symtab_offset = next(elf.address_offsets(symtab_ptr))
# Assume that symtab ends right before strtab.
# This is the same assumption that glibc makes in dl-addr.c.
# The first symbol is always local undefined, unnamed so we ignore it.
for i in range(1, (strtab_ptr - symtab_ptr) / symbol_size):
symbol_offset = symtab_offset + (i * symbol_size)
symbol = utils.struct_parse(elf.structs.Elf_Sym,,
if symbol['st_info']['bind'] == 'STB_LOCAL':
# Ignore local symbols.
symbol_name = stringtable.get_string(symbol.st_name)
if symbol['st_shndx'] == 'SHN_UNDEF':
if symbol['st_info']['bind'] == 'STB_GLOBAL':
# Global undefined --> required symbols.
# We ignore weak undefined symbols.
elif symbol['st_other']['visibility'] == 'STV_DEFAULT':
# Exported symbols must have default visibility.
st_shndx = SH_TYPE_VALUES.get(symbol['st_shndx'], symbol['st_shndx'])
exp[symbol_name] = (symbol['st_info']['bind'], symbol['st_size'],
return imp, exp
def ParseELF(root, rel_path, ldpaths=None, parse_symbols=True):
"""Parse the ELF file.
Loads and parses the passed elf file.
root: Path to the directory where the rootfs is mounted.
rel_path: The path to the parsing file relative to root.
ldpaths: The dict() with the ld path information. See lddtree.LoadLdpaths()
for details.
parse_symbols: Whether the result includes the dynamic symbols 'imp_sym' and
'exp_sym' sections. Disabling it reduces the time for large files with
many symbols.
If the passed file isn't a supported ELF file, returns None. Otherwise,
returns a dict() with information about the parsed ELF.
# Ensure root has a trailing / so removing the root prefix also removes any
# / from the beginning of the path.
root = root.rstrip('/') + '/'
with open(os.path.join(root, rel_path), 'rb') as f:
if != '\x7fELF':
# Ignore non-ELF files. This check is done to speedup the process.
# Continue reading and cache the whole file to speedup seeks.
stream = cStringIO.StringIO(
elf = elffile.ELFFile(stream)
except elffile.ELFError:
# Ignore unsupported ELF files.
if elf.header.e_type == 'ET_REL':
# Don't parse relocatable ELF files (mostly kernel modules).
return {
'type': elf.header.e_type,
'realpath': rel_path,
if ldpaths is None:
ldpaths = lddtree.LoadLdpaths(root)
result = lddtree.ParseELF(os.path.join(root, rel_path), root=root,
# Convert files to relative paths.
for libdef in result['libs'].values():
for path in ('realpath', 'path'):
if not libdef[path] is None and libdef[path].startswith(root):
libdef[path] = libdef[path][len(root):]
for path in ('interp', 'realpath'):
if not result[path] is None and result[path].startswith(root):
result[path] = result[path][len(root):]
result['type'] = elf.header.e_type
result['sections'] = dict((str(, sec['sh_size'])
for sec in elf.iter_sections())
result['segments'] = set(seg['p_type'] for seg in elf.iter_segments())
# Some libraries (notably, the libc, which you can execute as a normal
# binary) have the interp set. We use the file extension in those cases
# because exec files shouldn't have a .so extension.
result['is_lib'] = ((result['interp'] is None or rel_path[-3:] == '.so') and
elf.header.e_type == 'ET_DYN')
if parse_symbols:
result['imp_sym'], result['exp_sym'] = ParseELFSymbols(elf)
return result