implement a linker

.gitignore

@@ -4,6 +4,7 @@
 /coverage/
 /doc/
 /pkg/
+/AGENTS.md
 /spec/reports/
 /tmp/
 /Gemfile.lock
@@ -13,3 +14,5 @@
 /.gem_rbs_collection
 /node_modules
 /package*.json
+/*.md
+!/README.md

lib/caotral/linker.rb

@@ -12,6 +12,8 @@ def link(input: @input, output: @output, debug: @debug, shared: @shared) = IO.po
 
   def link_command(input: @input, output: @output, debug: @debug, shared: @shared)
     ld_path = []
+    return to_elf(input:, output:, debug:) if @linker == "self"
+
     if @shared
       ld_path << "--shared"
       ld_path << "#{libpath}/crti.o"
@@ -36,4 +38,9 @@ def link_command(input: @input, output: @output, debug: @debug, shared: @shared)
 
   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(input: @input, output: @output, debug: @debug)
+    elf_obj = Caotral::Linker::Reader.new(input:, debug:).read
+    Caotral::Linker::Writer.new(elf_obj:, output:, debug:).write
+  end
 end

lib/caotral/linker/elf.rb

@@ -0,0 +1,7 @@
+class Caotral::Linker::ELF
+  attr_reader :sections, :header
+  def initialize
+    @sections = Caotral::Linker::ELF::Sections.new
+    @header = Caotral::Linker::ELF::Header.new
+  end
+end

lib/caotral/linker/elf/header.rb

@@ -0,0 +1,60 @@
+class Caotral::Linker::ELF::Header
+  include Caotral::Assembler::ELF::Utils
+  IDENT = [0x7f, 0x45, 0x4c, 0x46, 0x02, 0x01, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00].freeze
+  ELF_FILE_TYPE = { NONE: 0, REL: 1, EXEC: 2, DYN: 3, CORE: 4 }.freeze
+
+  def initialize(endian: :little, type: :rel, arc: :amd64)
+    @ident = IDENT
+    @type = num2bytes(ELF_FILE_TYPE[elf(type)], 2)
+    @arch = arch(arc)
+    @version = num2bytes(1, 4)
+    @entry = num2bytes(0x00, 8)
+    @phoffset = num2bytes(0x00, 8)
+    @shoffset = num2bytes(0x00, 8)
+    @flags = num2bytes(0x00, 4)
+    @ehsize = num2bytes(0x40, 2)
+    @phsize = num2bytes(0x00, 2)
+    @phnum = num2bytes(0x00, 2)
+    @shentsize = num2bytes(0x40, 2)
+    @shnum = num2bytes(0x08, 2)
+    @shstrndx = num2bytes(0x07, 2)
+  end
+
+  def build = bytes.flatten.pack("C*")
+
+  def set!(entry: nil, phoffset: nil, shoffset: nil, shnum: nil, shstrndx: nil)
+    @entry = num2bytes(entry, 8) if check(entry, 8)
+    @phoffset = num2bytes(phoffset, 8) if check(phoffset, 8)
+    @shoffset = num2bytes(shoffset, 8) if check(shoffset, 8)
+    @shnum = num2bytes(shnum, 2) if check(shnum, 2)
+    @shstrndx = num2bytes(shstrndx, 2) if check(shstrndx, 2)
+  end
+
+  private
+
+  def bytes = [
+    @ident, @type, @arch, @version, @entry, @phoffset,
+    @shoffset, @flags, @ehsize, @phsize, @phnum, @shentsize,
+    @shnum, @shstrndx
+  ]
+
+  def arch(machine)
+    case machine.to_s
+    in "amd64" | "x86_64" | "x64"
+      [0x3e, 0x00]
+    end
+  end
+
+  def elf(type)
+    case type.to_s
+    in "relocatable" | "rel"
+      :REL
+    in "exe" | "ex" | "exec"
+      :EXEC
+    in "shared" | "share" | "dynamic" | "dyn"
+      :DYN
+    else
+      :NONE
+    end
+  end
+end

lib/caotral/linker/elf/section.rb

@@ -0,0 +1,13 @@
+class Caotral::Linker::ELF::Section
+  attr_accessor :name
+  attr_reader :section_name, :header, :body
+  def initialize(type:, options: {})
+    type_string = type.to_s.capitalize
+    type_string = type_string.upcase if type_string == "Bss"
+    # section_name is extra information about section type
+    @section_name = type_string.downcase
+    # name is used in section header string table in elf file
+    @name = section_name == "null" ? "" : "\0.#{section_name}"
+    @header, @body = nil, nil
+  end
+end

lib/caotral/linker/elf/section/shstrtab.rb

@@ -0,0 +1,6 @@
+class Caotral::Linker::ELF::Section::Shstrtab
+  include Caotral::Assembler::ELF::Utils
+  def initialize(**opts) = @name = []
+  def build = bytes.flatten.pack("C*")
+  def set!(name:) = (@name << name!(name); self)
+end

lib/caotral/linker/elf/section/strtab.rb

@@ -0,0 +1,5 @@
+class Caotral::Linker::ELF::Section::Strtab
+  include Caotral::Assembler::ELF::Utils
+  def initialize(names = "\0main\0", **opts) = @names = names
+  def build = @names.bytes.pack("C*")
+end

lib/caotral/linker/elf/section/symtab.rb

@@ -0,0 +1,12 @@
+class Caotral::Linker::ELF::Section::Symtab
+  include Caotral::Assembler::ELF::Utils
+  def initialize(**opts)
+    @entsize = []
+    @name = num2bytes(0, 4)
+    @info = num2bytes(0, 1)
+    @other = num2bytes(0, 1)
+    @shndx = num2bytes(0, 2)
+    @value = num2bytes(0, 8)
+    @size = num2bytes(0, 8)
+  end
+end

lib/caotral/linker/elf/section/text.rb

@@ -0,0 +1,4 @@
+class Caotral::Linker::ELF::Section::Text
+  def initialize = @bytes = []
+  def build = @bytes.flatten.pack("C*")
+end

lib/caotral/linker/elf/section_header.rb

@@ -0,0 +1,37 @@
+class Caotral::Linker::ELF::SectionHeader
+  include Caotral::Assembler::ELF::Utils
+  def initialize
+    @name = nil
+    @type = nil
+    @flags = nil
+    @addr = nil
+    @offset = nil
+    @size = nil
+    @link = nil
+    @info = nil
+    @addralign = nil
+    @entsize = nil
+  end
+
+  def build = bytes.flatten.pack("C*")
+
+  def set!(name: nil, type: nil, flags: nil, addr: nil,
+           offset: nil, size: nil, link: nil, info: nil,
+           addralign: nil, entsize: nil)
+    @name = num2bytes(name, 4) if check(name, 4)
+    @type = num2bytes(type, 4) if check(type, 4)
+    @flags = num2bytes(flags, 8)  if check(flags, 8)
+    @addr = num2bytes(addr, 8) if check(addr, 8)
+    @offset = num2bytes(offset, 8) if check(offset, 8)
+    @size = num2bytes(size, 8) if check(size, 8)
+    @link = num2bytes(link, 4) if check(link, 4)
+    @info = num2bytes(info, 4) if check(info, 4)
+    @addralign = num2bytes(addralign, 8) if check(addralign, 8)
+    @entsize = num2bytes(entsize, 8) if check(entsize, 8)
+    self
+  end
+
+  def null! = set!(name: 0, type: 0, flags: 0, addr: 0, offset: 0, size: 0, link: 0, info: 0, addralign: 0, entsize: 0)
+
+  private def bytes = [@name, @type, @flags, @addr, @offset, @size, @link, @info, @addralign, @entsize]
+end

lib/caotral/linker/elf/sections.rb

@@ -0,0 +1,18 @@
+class Caotral::Linker::ELF::Sections
+  include Enumerable
+
+  def initialize = @sections = []
+  def each(&block) = @sections.each(&block)
+  def add(section) = @sections << section
+  alias << add
+  def [](index)
+    case index
+    when Integer
+      @sections[index]
+    when String, Symbol
+      @sections.find { _1.section_name.to_s == index.to_s }
+    else
+      raise ArgumentError, "Invalid index type: #{index.class}"
+    end
+  end
+end

lib/caotral/linker/reader.rb

@@ -0,0 +1,29 @@
+class Caotral::Linker::Reader
+  attr_reader :sections
+  def self.read!(input:, debug: false, linker_options: [])
+    new(input:, debug:, linker_options:).read
+  end
+
+  def initialize(input:, debug: false, linker_options: [])
+    @input = decision(input)
+    @bin = @input.read
+    @context = Caotral::Linker::ELF.new
+  end
+
+  def read
+    @context
+  ensure
+    @input.close
+  end
+
+  private
+
+  def decision(input)
+    case input
+    when String, Pathname
+      File.open(File.expand_path(input.to_s), "rb")
+    else
+      raise ArgumentError, "wrong input type"
+    end
+  end
+end

lib/caotral/linker/writer.rb

@@ -0,0 +1,8 @@
+class Caotral::Linker::Writer
+  attr_reader :elf_obj, :output, :debug
+  def self.write!(elf_obj:, output:, debug: false)
+    new(elf_obj:, output:, debug:).write
+  end
+  def initialize(elf_obj:, output:, debug: false) = @elf_obj, @output, @debug = elf_obj, output, debug
+  def write = output
+end

sig/caotral/linker.rbs

@@ -0,0 +1,17 @@
+class Caotral::Linker
+  @input: String
+  @output: String
+  @linker: String
+  @options: Array[String]
+  @shared: bool
+  @debug: bool
+
+  def self.link!: (input: String, ?output: String, ?linker: String, ?debug: bool, ?shared: bool) -> void
+  def initialize: (input: String, ?output: String, ?linker: String, ?linker_options: Array[String], ?shared: bool, ?debug: bool) -> void
+  def link: (input: String, ?output: String, ?shared: bool, ?debug: bool) -> void
+
+  def link_command: (input: String, ?output: String, ?shared: bool, ?debug: bool) -> String
+  def libpath: () -> String
+  def gcc_libpath: () -> String
+  def to_elf: (input: String, ?output: String, ?debug: bool) -> void
+end

sig/caotral/linker/elf.rbs

@@ -0,0 +1,3 @@
+class Caotral::Linker::ELF
+  def initialize: () -> void
+end

sig/caotral/linker/reader.rbs

@@ -0,0 +1,5 @@
+class Caotral::Linker::Reader
+  def self.read!: (input: String, ?debug: bool, ?linker_options: Array[String]) -> Caotral::Linker::ELF
+  def initialize: (input: String, ?debug: bool, ?linker_options: Array[String]) -> void
+  def read: () -> Caotral::Linker::ELF
+end

sig/caotral/linker/writer.rbs

@@ -0,0 +1,5 @@
+class Caotral::Linker::Writer
+  def self.write!: (elf_obj: Caotral::Linker::ELF, output: String, ?debug: bool) -> String
+  def initialize: (elf_obj: Caotral::Linker::ELF, output: String, ?debug: bool) -> void
+  def write: () -> String
+end