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rbelftools

Pure Ruby library for parsing and patching ELF files.

Introduction

An ELF parser implemented in pure Ruby. This work is inspired by pyelftools by Eli Bendersky.

The original motivation to create this gem is to be a dependency of pwntools-ruby. Since ELF parser is not an easy work, it should not be implemented directly in pwntools.

Now rbelftools is also used by the Homebrew project: https://github.com/Homebrew/brew/tree/master/Library/Homebrew/vendor/bundle/ruby/2.6.0/gems/elftools-1.1.3/lib

rbelftools's target is to create a nice ELF parsing library in Ruby. More features remain a work in progress.

Install

Available on RubyGems.org!

gem install elftools

Features

  • [x] Supports both big and little endian
  • [x] ELF parser
  • [x] ELF headers patcher

See example usage for more details.

Example Usage

Start from a file object

require 'elftools'

elf = ELFTools::ELFFile.new(File.open('spec/files/amd64.elf'))
#=> #<ELFTools::ELFFile:0x00560b147f8328 @elf_class=64, @endian=:little, @stream=#<File:spec/files/amd64>>

elf.machine
#=> 'Advanced Micro Devices X86-64 processor'

elf.build_id
#=> '73ab62cb7bc9959ce053c2b711322158708cdc07'

Sections

elf.section_by_name('.dynstr')
#=>
# #<ELFTools::Sections::StrTabSection:0x00560b148cef40
# @header=
#  {:sh_name=>86,
#   :sh_type=>3,
#   :sh_flags=>2,
#   :sh_addr=>4195224,
#   :sh_offset=>920,
#   :sh_size=>113,
#   :sh_link=>0,
#   :sh_info=>0,
#   :sh_addralign=>1,
#   :sh_entsize=>0},
# @name=".dynstr">

elf.sections.map(&:name).join(' ')
#=> " .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame .init_array .fini_array .jcr .dynamic .got .got.plt .data .bss .comment .shstrtab .symtab .strtab"

elf.section_by_name('.note.gnu.build-id').data
#=> "\x04\x00\x00\x00\x14\x00\x00\x00\x03\x00\x00\x00GNU\x00s\xABb\xCB{\xC9\x95\x9C\xE0S\xC2\xB7\x112!Xp\x8C\xDC\a"

Symbols

symtab_section = elf.section_by_name('.symtab')
symtab_section.num_symbols
#=> 75

symtab_section.symbol_by_name('puts@@GLIBC_2.2.5')
#=>
# #<ELFTools::Sections::Symbol:0x00560b14af67a0
#  @header={:st_name=>348, :st_info=>18, :st_other=>0, :st_shndx=>0, :st_value=>0, :st_size=>0},
#  @name="puts@@GLIBC_2.2.5">

symbols = symtab_section.symbols # Array of symbols
symbols.map(&:name).reject(&:empty?).first(5).join(' ')
#=> "crtstuff.c __JCR_LIST__ deregister_tm_clones register_tm_clones __do_global_dtors_aux"

What a symbol refers to and how it is linked are recorded in st_info and st_other, values are defined in ELFTools::Constants::STT, STB, and STV respectively.

main = symtab_section.symbol_by_name('main')
main.type == ELFTools::Constants::STT_FUNC
#=> true
main.bind == ELFTools::Constants::STB_GLOBAL
#=> true
main.visibility == ELFTools::Constants::STV_DEFAULT
#=> true

# Symbols to be resolved at runtime are not defined in any section.
symtab_section.symbol_by_name('puts@@GLIBC_2.2.5').section_index == ELFTools::Constants::SHN_UNDEF
#=> true

Each of those has a name. Several constants may define one value, so naming it takes the machine of the file, which names some of them for itself.

[main.type_name, main.bind_name, main.visibility_name]
#=> ["STT_FUNC", "STB_GLOBAL", "STV_DEFAULT"]

# 13 is a symbol type ARM and SPARC each define, and no one else names.
ELFTools::Constants::STT.mapping(ELFTools::Constants::EM_ARM, 13)
#=> "STT_ARM_TFUNC"
ELFTools::Constants::STT.mapping(ELFTools::Constants::EM_X86_64, 13)
#=> "<unknown>: 0xd"

The symbols a file is loaded by are recorded twice, and the tags are the copy the loader reads, so they answer even when the sections have been stripped away. Naming the symbol a relocation points at takes nothing but the tags.

dynamic = elf.dynamic
dynamic.symbols.map(&:name).reject(&:empty?).first(4).join(' ')
#=> "puts __stack_chk_fail printf __libc_start_main"
dynamic.symbol_by_name('printf').type_name
#=> "STT_FUNC"
dynamic.relocations.map { |rel| dynamic.symbol_at(rel.symbol_index).name }.first(3)
#=> ["__gmon_start__", "stdin", "puts"]

Nothing a file is loaded by records how large its symbol table is, because the loader looks a name up through a hash table and jumps straight to an index rather than ever enumerating it. num_symbols is therefore how far the hash table and the relocations reach between them, which is a lower bound, while symbol_at is exact for any index.

dynamic.num_symbols
#=> 9

Looking a name up is what the loader does through a hash table, rather than searching the symbol table for it, and symbol_by_name does the same wherever the file records one. A table only indexes the names a file exports, so a name it does not lead to, and a file that records no table at all, are searched for instead.

libc = ELFTools::ELFFile.new(File.open('spec/files/libc.so.6'))
# Reached through DT_HASH, without reading any of the other 2244 symbols.
libc.dynamic.symbol_by_name('malloc').type_name
#=> "STT_FUNC"

Segments

elf.segment_by_type(:note)
#=>
# #<ELFTools::Segments::NoteSegment:0x00555beaafe218
# @header=
#  {:p_type=>4,
#   :p_flags=>4,
#   :p_offset=>624,
#   :p_vaddr=>624,
#   :p_paddr=>624,
#   :p_filesz=>68,
#   :p_memsz=>68,
#   :p_align=>4}>

elf.segment_by_type(:interp).interp_name
#=> "/lib64/ld-linux-x86-64.so.2"

Relocations

elf = ELFTools::ELFFile.new(File.open('spec/files/amd64.elf'))
# Use relocation to get plt names.
rela_section = elf.sections_by_type(:rela).last
rela_section.name
#=> ".rela.plt"
relocations = rela_section.relocations
relocations.map { |r| '%x' % r.header.r_info }
#=> ["100000007", "200000007", "300000007", "400000007", "500000007", "700000007"]
symtab = elf.section_at(rela_section.header.sh_link) # get the symbol table section
relocations.map { |r| symtab.symbol_at(r.symbol_index).name }
#=> ["puts", "__stack_chk_fail", "printf", "__libc_start_main", "fgets", "scanf"]

Each architecture numbers relocation types on its own, so a type is named after the machine of the file it was read from.

relocations.map(&:type).uniq
#=> [7]
relocations.map(&:type_name).uniq
#=> ["R_X86_64_JUMP_SLOT"]

# The very same number means something else elsewhere.
ELFTools::Constants::R::ARM::R_ARM_THM_ABS5
#=> 7

A machine may also lay r_info out its own way. Knowing the machine is what tells those layouts apart, so such a file reads like any other.

mips = ELFTools::ELFFile.new(File.open('spec/files/mips64.o'))
relocation = mips.sections_by_type(:rela).first.relocations.first
# The 64-bit MIPS ABI packs three types and a second symbol index in there.
'%x' % relocation.header.r_info
#=> "800051807"
[relocation.symbol_index, relocation.type_name]
#=> [8, "R_MIPS_GPREL16"]

A relocation is recorded twice in a file that is loaded, and the tags are the copy the loader reads, so they answer even when the sections have been stripped away.

elf.dynamic.relocations.size
#=> 8
elf.dynamic.relocations.map(&:type_name).uniq
#=> ["R_X86_64_GLOB_DAT", "R_X86_64_COPY", "R_X86_64_JUMP_SLOT"]

Patch

Patch ELF is so easy!

All kinds of headers (i.e. Ehdr, Shdr, Phdr, etc.) can be patched. Patched slots will not be applied on the opened file. Invoke elf.save(filename) to save the patched ELF into filename.

elf = ELFTools::ELFFile.new(File.open('spec/files/amd64.elf'))
elf.machine
#=> "Advanced Micro Devices X86-64 processor"
elf.header.e_machine = 40
elf.machine
#=> "ARM"

interp_segment = elf.segment_by_type(:interp)
interp_segment.interp_name
#=> "/lib64/ld-linux-x86-64.so.2"
interp_segment.header.p_filesz
#=> 28
interp_segment.header.p_filesz = 20
interp_segment.interp_name
#=> "/lib64/ld-linux-x86"

# save the patched ELF
elf.save('elf.patched')

# in bash
# $ file elf.patched
# elf.patched: ELF 64-bit LSB executable, ARM, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86, for GNU...

Why rbelftools

  1. Fully documented
    Always important for an Open-Source project. Online document is here
  2. Fully tested
    Of course.
  3. Lazy loading on everything
    To use rbelftools, passing the stream object of an ELF file. rbelftools will read the stream object as least times as possible when parsing the file. Most information will not be fetched until you need it, which makes rbelftools efficient.
  4. To be a library
    rbelftools is designed to be a library for further usage. It will not add any too trivial features. For example, to check whether NX is disabled, rbelftools provides !elf.segment_by_type(:gnu_stack).executable? but not elf.nx?
  5. Section and segment parser
    Providing common sections and segments parser. For example, .symtab, .shstrtab .dynamic sections and INTERP, DYNAMIC segments, etc.

Development

git clone https://github.com/david942j/rbelftools
cd rbelftools
bundle
bundle exec rake

Constant tables are generated from binutils, regenerate them against the latest revision by

bundle exec rake gen:constants

Any comments or suggestions are welcome!

Cross Platform

rbelftools can be used and has been fully tested on all platforms include Linux, OSX, and Windows!

License

MIT License