Skip to content

Support dt needed in linker #49

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
katsyoshi merged 10 commits into from
Apr 15, 2026
Merged

Support dt needed in linker #49

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
10 commits
Select commit Hold shift + click to select a range
b803795
add needed option for linker
katsyoshi Apr 2, 2026
6bc6e71
add needed in dynamic section
katsyoshi Apr 4, 2026
1849b94
exclude RELTEXT without .rel.text
katsyoshi Apr 4, 2026
7f6e988
support DT_NEEDED and _start entry in self linker
katsyoshi Apr 4, 2026
d2c0653
prepare plt entries for dynamic self linking
katsyoshi Apr 5, 2026
4a4b56d
prepare and patch plt entries for dynamic self linking
katsyoshi Apr 5, 2026
49ad62f
initialize plt and got.plt entries
katsyoshi Apr 5, 2026
09e52e8
prepare writer-side text relocation rewriting
katsyoshi Apr 11, 2026
ead9a88
support rodata and rewrite text relocations in writer
katsyoshi Apr 12, 2026
75d81c1
rewrite text relocations by target section in writer
katsyoshi Apr 14, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
4 changes: 3 additions & 1 deletion lib/caotral/binary/elf/reader.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -97,7 +97,9 @@ def read
shndx = sym_bin[6, 2].unpack1("S<")
value = sym_bin[8, 8].unpack1("Q<")
size = sym_bin[16, 8].unpack1("Q<")
Caotral::Binary::ELF::Section::Symtab.new.set!(name:, info:, other:, shndx:, value:, size:)
name_string = @context.sections[section.header.link]&.body&.lookup(name).to_s

Caotral::Binary::ELF::Section::Symtab.new.set!(name:, info:, other:, shndx:, value:, size:, name_string:)
end
when :rel, :rela
rela = type == :rela
Expand Down
3 changes: 3 additions & 0 deletions lib/caotral/binary/elf/section/dynamic.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -1,4 +1,5 @@
require "caotral/binary/elf/utils"

module Caotral
module Binary
class ELF
Expand All @@ -7,6 +8,7 @@ class Dynamic
include Caotral::Binary::ELF::Utils
TAG_TYPES = {
NULL: 0,
NEEDED: 1,
PLTRELSZ: 2,
PLTGOT: 3,
HASH: 4,
Expand Down Expand Up @@ -46,6 +48,7 @@ def jmp_rel? = tag == TAG_TYPES[:JMPREL]
def plt_rel? = tag == TAG_TYPES[:PLTREL]
def plt_rel_size? = tag == TAG_TYPES[:PLTRELSZ]
def plt_got? = tag == TAG_TYPES[:PLTGOT]
def needed? = tag == TAG_TYPES[:NEEDED]

private def bytes = [@tag, @un]
end
Expand Down
3 changes: 2 additions & 1 deletion lib/caotral/binary/elf/section/symtab.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -19,13 +19,14 @@ def initialize(**opts)
end
def build = bytes.flatten.pack("C*")

def set!(name: nil, info: nil, other: nil, shndx: nil, value: nil, size: nil)
def set!(name: nil, info: nil, other: nil, shndx: nil, value: nil, size: nil, name_string: nil)
@name = num2bytes(name, 4) if check(name, 4)
@info = num2bytes(info, 1) if check(info, 1)
@other = num2bytes(other, 1) if check(other, 1)
@shndx = num2bytes(shndx, 2) if check(shndx, 2)
@value = num2bytes(value, 8) if check(value, 8)
@size = num2bytes(size, 8) if check(size, 8)
@name_string = name_string if name_string
self
end

Expand Down
14 changes: 8 additions & 6 deletions lib/caotral/linker.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -5,13 +5,14 @@

module Caotral
class Linker
def self.link!(inputs:, output: "a.out", linker: "mold", debug: false, shared: false, executable: true, pie: false)
new(inputs:, output:, linker:, debug:, shared:, executable:, pie:).link
def self.link!(inputs:, output: "a.out", linker: "mold", debug: false, shared: false, executable: true, pie: false, needed: [])
new(inputs:, output:, linker:, debug:, shared:, executable:, pie:, needed:).link
end

def initialize(inputs:, output: "a.out", linker: "mold", linker_options: [], executable: true, shared: false, pie: false, debug: false)
def initialize(inputs:, output: "a.out", linker: "mold", linker_options: [], executable: true, shared: false, pie: false, debug: false, needed: [])
@inputs, @output, @linker = inputs, output, linker
@options = linker_options
@needed = needed
@executable, @debug, @shared, @pie = executable, debug, shared, pie
end

Expand Down Expand Up @@ -49,12 +50,13 @@ def link_command(inputs: @inputs, output: @output)
def libpath = @libpath ||= File.dirname(Dir.glob("/usr/lib*/**/crti.o").last)
def gcc_libpath = @gcc_libpath ||= File.dirname(Dir.glob("/usr/lib/gcc/x86_64-*/*/crtbegin.o").last)

def to_elf(inputs: @inputs, output: @output, debug: @debug, shared: @shared, executable: @executable, pie: @pie)
def to_elf(inputs: @inputs, output: @output, debug: @debug, shared: @shared, executable: @executable, pie: @pie, needed: @needed)
elf_objs = inputs.map { |input| Caotral::Binary::ELF::Reader.new(input:, debug:).read }
builder = Caotral::Linker::Builder.new(elf_objs:, debug:, shared:, executable:, pie:)
builder = Caotral::Linker::Builder.new(elf_objs:, debug:, shared:, executable:, pie:, needed:)
builder.resolve_symbols
elf_obj = builder.build
Caotral::Linker::Writer.new(elf_obj:, output:, debug:, shared:, executable:, pie:).write
metadata = builder.linker_metadata
Caotral::Linker::Writer.new(elf_obj:, output:, metadata:, debug:, shared:, executable:, pie:).write
File.chmod(0755, output) if executable
output
end
Expand Down
120 changes: 69 additions & 51 deletions lib/caotral/linker/builder.rb
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -14,6 +14,7 @@ class Builder
GENERATED_SECTION_NAMES = [".text", ".data", ".strtab", ".symtab", ".shstrtab", /\.rela?\./, ".dynstr", ".dynsym", ".dynamic", ".interp", ".rela.dyn", ".plt", ".got.plt"].freeze
SHT = Caotral::Binary::ELF::SectionHeader::SHT
SHF = Caotral::Binary::ELF::SectionHeader::SHF
SectionDynamic = Caotral::Binary::ELF::Section::Dynamic
UNSUPPORTED_REL_TYPES = [
REL_TYPES[:AMD64_GOTPCREL],
REL_TYPES[:AMD64_GOTPCRELX],
Expand All @@ -25,13 +26,14 @@ class Builder
DYNAMIC_TAGS[:JMPREL],
].freeze

attr_reader :symbols
attr_reader :symbols, :linker_metadata

def initialize(elf_objs:, executable: true, debug: false, shared: false, pie: false)
def initialize(elf_objs:, executable: true, debug: false, shared: false, pie: false, needed: [])
@elf_objs = elf_objs
@executable, @debug, @shared, @pie = executable, debug, shared, pie
@symbols = { locals: Set.new, globals: Set.new, weaks: Set.new }
@executable, @debug, @shared, @pie, @needed = executable, debug, shared, pie, needed
@symbols = { locals: Set.new, globals: Set.new, weaks: Set.new, }.freeze
_mode!
@linker_metadata = {}
end

def build
Expand All @@ -48,20 +50,26 @@ def build
section_name: ".text",
header: Caotral::Binary::ELF::SectionHeader.new
)
rodata_section = Caotral::Binary::ELF::Section.new(
body: String.new,
section_name: ".rodata",
header: Caotral::Binary::ELF::SectionHeader.new
.set!(type: SHT[:progbits], flags: SHF[:ALLOC], addralign: 16)
)
data_section = Caotral::Binary::ELF::Section.new(
body: String.new,
section_name: ".data",
header: Caotral::Binary::ELF::SectionHeader.new
.set!(type: SHT[:progbits], flags: SHF[:ALLOC] | SHF[:WRITE], addralign: 8)
)
plt_section = Caotral::Binary::ELF::Section.new(
body: [0, 0].pack("Q<Q<"),
body: [[0xff, 0x35, 0, 0, 0, 0, 0xff, 0x25, 0, 0, 0, 0, 0x0f, 0x1f, 0x40, 0]],
section_name: ".plt",
header: Caotral::Binary::ELF::SectionHeader.new
.set!(type: SHT[:progbits], flags: SHF[:ALLOC] | SHF[:EXECINSTR], addralign: 16)
)
got_plt_section = Caotral::Binary::ELF::Section.new(
body: [0].pack("Q<"),
body: [[0] * 8, [0] * 8, [0] * 8],
section_name: ".got.plt",
header: Caotral::Binary::ELF::SectionHeader.new
.set!(type: SHT[:progbits], flags: SHF[:ALLOC] | SHF[:WRITE], addralign: 8)
Expand Down Expand Up @@ -106,16 +114,22 @@ def build
start_len = start_bytes.length
sections = []
rel_sections = []
rel_texts = []
elf.header = elf_obj.header.dup
strtab_names = []
text_offsets = {}
rodata_offsets = {}
data_offsets = {}
got_plt_offsets = {}
text_offset = 0
rodata_offset = 0
data_offset = 0
got_plt_offset = 8
got_plt_offset = 24
sym_by_elf = Hash.new { |h, k| h[k] = [] }
dynstr, dynsym = build_shared_dynamic_sections if dynamic?

start_len = 0 if @elf_objs.any? { |elf| elf.find_by_name(".symtab").body.find { |sym| sym.name_string == "_start" } }
Copy link
Copy Markdown

@chatgpt-codex-connector chatgpt-codex-connector bot Apr 14, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

P2 Badge Require defined _start before enabling custom entry mode

This _start probe matches by name only, so an undefined _start reference (shndx == 0) in any input object will disable the normal main startup path. That can produce an executable without a valid startup routine even though no _start definition was provided; restrict the detection to symbols that are actually defined.

Useful? React with 👍 / 👎.


@elf_objs.each do |elf_obj|
text = elf_obj.find_by_name(".text")
unless text.nil?
Expand All @@ -124,6 +138,12 @@ def build
size = text.body.bytesize
text_offset += size
end
rodata = elf_obj.find_by_name(".rodata")
unless rodata.nil?
rodata_section.body << rodata.body
rodata_offsets[elf_obj.object_id] = rodata_offset
rodata_offset += rodata.body.bytesize
end
data = elf_obj.find_by_name(".data")
unless data.nil?
data_section.body << data.body
Expand All @@ -138,13 +158,15 @@ def build
base_index = symtab_section.body.size
text_index = elf_obj.sections.index(text) unless text.nil?
data_index = elf_obj.sections.index(data) unless data.nil?
rodata_index = elf_obj.sections.index(rodata) unless rodata.nil?

symtab.body.each_with_index do |st, index|
sym = Caotral::Binary::ELF::Section::Symtab.new
name, info, other, shndx, value, size = st.build.unpack("L<CCS<Q<Q<")
sym_by_elf[elf_obj] << sym
value += text_offsets.fetch(elf_obj.object_id, 0) if shndx == text_index
value += data_offsets.fetch(elf_obj.object_id, 0) if shndx == data_index
value += rodata_offsets.fetch(elf_obj.object_id, 0) if shndx == rodata_index
sym.set!(name:, info:, other:, shndx:, value:, size:)
sym.name_string = strtab.body.lookup(name) unless strtab.nil?
symtab_section.body << sym
Expand All @@ -157,6 +179,7 @@ def build
header: Caotral::Binary::ELF::SectionHeader.new
)
section.body.each do |rel|
sym = base_index.nil? ? rel.sym : base_index + rel.sym
if rel.type == REL_TYPES[:AMD64_64]
base_offset = case section.section_name.to_s
when /\.rela?\.text/
Expand All @@ -177,16 +200,18 @@ def build
addend = sym_addr - base_addr
rela_dyn_section.body << Caotral::Binary::ELF::Section::Rel.new.set!(offset:, info: (0 << 32) | REL_TYPES[:AMD64_RELATIVE], addend:)
next
elsif (undefined = symtab.body[rel.sym]&.shndx.to_i == 0) && rel.type == REL_TYPES[:AMD64_PLT32]
elsif (undefined = symtab.body[rel.sym]&.shndx.to_i == 0) && ALLOW_RELOCATION_TYPES.include?(rel.type)
sym = base_index.to_i + rel.sym
first_insertion = got_plt_offsets[sym].nil?
got_plt_offsets[sym] ||= got_plt_offset.tap { got_plt_offset += 8 }
if dynamic? && undefined && first_insertion
got_plt_section.body << [0].pack("Q<")
got_plt_body = [0]*8
got_plt_section.body << got_plt_body
rps = Caotral::Binary::ELF::Section::Rel.new.set!(
offset: got_plt_offsets[sym],
info: ((sym) << 32) | REL_TYPES[:AMD64_JUMP_SLOT]
)

name = symtab_section.body[sym].name_string
dynstr_index = dynstr.body.offset_of(name)
if dynstr_index.nil?
Expand All @@ -197,15 +222,16 @@ def build
)
end
rela_plt_section.body << rps
next
pltx = [0xff, 0x25, 0, 0, 0, 0, 0x68, 0, 0, 0, 0, 0xe9, 0, 0, 0, 0]
plt_section.body << pltx
end
elsif UNSUPPORTED_REL_TYPES.include?(rel.type)
raise Caotral::Binary::ELF::Error, "unsupported relocation type: #{rel.type_name}"
end
offset = rel.offset + text_offsets.fetch(elf_obj.object_id, 0)
offset += start_len if section.section_name.to_s.start_with?(".rela.text", ".rel.text")
addend = rel.addend? ? rel.addend : nil
new_rel = Caotral::Binary::ELF::Section::Rel.new(addend: rel.addend?)
sym = base_index.nil? ? rel.sym : base_index + rel.sym
info = (sym << 32) | rel.type
new_rel.set!(offset:, info:, addend:)
rel_section.body << new_rel
Expand All @@ -214,15 +240,19 @@ def build
end
rel_sections += rels
end

strtab_section.body.names = strtab_names.to_a.sort.join("\0") + "\0"
sections << null_section

main_sym = symtab_section.body.find { |sym| sym.name_string == "main" }
raise Caotral::Binary::ELF::Error, "main function not found" if @executable && main_sym.nil?
main_offset = main_sym.nil? ? 0 : main_sym.value + start_len
start_bytes[1, 4] = num2bytes((main_offset - 5), 4) if @executable
text_section.body.prepend(start_bytes.pack("C*"))
_start = symtab_section.body.find { |sym| sym.name_string == "_start" }
entry_sym = _start || symtab_section.body.find { |sym| sym.name_string == "main" }

raise Caotral::Binary::ELF::Error, "main or _start function not found" if @executable && entry_sym.nil?
if _start.nil?
main_offset = entry_sym.nil? ? 0 : entry_sym.value + start_len
start_bytes[1, 4] = num2bytes((main_offset - 5), 4) if @executable
text_section.body.prepend(start_bytes.pack("C*"))
end
Comment on lines +251 to +255
Copy link
Copy Markdown

@chatgpt-codex-connector chatgpt-codex-connector bot Apr 14, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

P1 Badge Set entrypoint to resolved _start address

When _start exists, this branch skips inserting the generated startup stub, but no replacement entry address is propagated; the writer still defaults e_entry to the start of .text. If _start is not at offset 0 in merged .text (for example, when its object is not first in inputs), the process starts at the wrong instructions and can crash immediately.

Useful? React with 👍 / 👎.


text_section.header.set!(
type: 1,
Expand All @@ -235,6 +265,7 @@ def build

sections << text_section
strtab_section.header.set!(type: 3, flags: 0, addralign: 1, entsize: 0)
sections << rodata_section
sections << data_section
if dynamic?
sections << plt_section
Expand Down Expand Up @@ -264,6 +295,7 @@ def build

sections += build_pie_sections if @pie
if dynamic?
@needed.each { |lib| dynstr.body.names += lib + "\0" if dynstr.body.offset_of(lib).nil? }
sections << dynstr
sections << dynsym
hash_section = build_hash_section
Expand Down Expand Up @@ -293,7 +325,7 @@ def build
hash.chain[i] = num2bytes(0, 4)
end
hash_section.body = hash
dynamic_section = build_dynamic_section
dynamic_section = build_dynamic_section(dynstr:)
if rela_plt_section.body.size == 0 && dynamic?
bodies = dynamic_section.body.reject { |ent| REJECT_DYNAMIC_TAGS.include?(ent.tag) }
dynamic_section.body = bodies
Expand Down Expand Up @@ -358,29 +390,14 @@ def build
addralign: 8,
entsize: rel_entsize(rel_section)
)
end

rel_sections.each do |rel|
target = sections[rel.header.info]
bytes = target.body.dup
symtab_body = symtab_section.body
rel.body.each do |entry|
next unless ALLOW_RELOCATION_TYPES.include?(entry.type)
sym = symtab_body[entry.sym]
next if sym.nil? || sym.shndx == 0
target_addr = target == text_section ? vaddr : target.header.addr
sym_addr = sym.shndx >= 0xff00 ? sym.value : sections[sym.shndx].then { |st| st.header.addr + sym.value }
sym_offset = entry.offset + start_len
sym_addend = entry.addend? ? entry.addend : bytes[sym_offset, 4].unpack1("l<")
value = sym_addr + sym_addend - (target_addr + sym_offset)
bytes[sym_offset, 4] = [value].pack("l<")
end
target.body = bytes
rel_texts << rel_section if rel_section.section_name.to_s.start_with?(".rela.text", ".rel.text")
end

sections = sections.reject { |section| RELOCATION_SECTION_NAMES.any? { |name| name === section.section_name.to_s } } if @executable
sections.each { |section| elf.sections << section }

@linker_metadata[:got_plt_offsets] = got_plt_offsets
@linker_metadata[:pending_text_relocations] = rel_texts
elf
end

Expand Down Expand Up @@ -444,24 +461,25 @@ def build_hash_section
hash_section
end

def build_dynamic_section
def build_dynamic_section(dynstr:)
tag_types = Caotral::Binary::ELF::Section::Dynamic::TAG_TYPES
dynamic_section = Caotral::Binary::ELF::Section.new(
body: [
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:PLTRELSZ]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:PLTGOT]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:HASH]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:RELA]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:RELASZ]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:RELAENT], un: 24),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:STRTAB]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:STRSZ]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:SYMTAB]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:SYMENT], un: 24),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:PLTREL]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:TEXTREL]),
Caotral::Binary::ELF::Section::Dynamic.new.set!(tag: tag_types[:JMPREL]),
Caotral::Binary::ELF::Section::Dynamic.new
*@needed.map { |lib| SectionDynamic.new.set!(tag: tag_types[:NEEDED], un: dynstr.body.offset_of(lib)) },
SectionDynamic.new.set!(tag: tag_types[:PLTRELSZ]),
SectionDynamic.new.set!(tag: tag_types[:PLTGOT]),
SectionDynamic.new.set!(tag: tag_types[:HASH]),
SectionDynamic.new.set!(tag: tag_types[:RELA]),
SectionDynamic.new.set!(tag: tag_types[:RELASZ]),
SectionDynamic.new.set!(tag: tag_types[:RELAENT], un: 24),
SectionDynamic.new.set!(tag: tag_types[:STRTAB]),
SectionDynamic.new.set!(tag: tag_types[:STRSZ]),
SectionDynamic.new.set!(tag: tag_types[:SYMTAB]),
SectionDynamic.new.set!(tag: tag_types[:SYMENT], un: 24),
SectionDynamic.new.set!(tag: tag_types[:PLTREL]),
SectionDynamic.new.set!(tag: tag_types[:TEXTREL]),
SectionDynamic.new.set!(tag: tag_types[:JMPREL]),
SectionDynamic.new
].compact,
section_name: ".dynamic",
header: Caotral::Binary::ELF::SectionHeader.new
Expand Down
Loading
Loading