From d823f1c1cdd96a0f81f78a4376aa46eb27a4afb5 Mon Sep 17 00:00:00 2001
From: "MATSUMOTO, Katsuyoshi" <github@katsyoshi.org>
Date: Sun, 11 Jan 2026 11:04:23 +0900
Subject: [PATCH 1/6] adjust symtab's section header

---
 lib/caotral/binary/elf/section_header.rb |  1 +
 lib/caotral/linker/writer.rb             | 13 +++++++++++--
 test/caotral/linker/writer_test.rb       |  2 +-
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/lib/caotral/binary/elf/section_header.rb b/lib/caotral/binary/elf/section_header.rb
index 3532d12..77f6aa4 100644
--- a/lib/caotral/binary/elf/section_header.rb
+++ b/lib/caotral/binary/elf/section_header.rb
@@ -46,6 +46,7 @@ def size = @size.pack("C*").unpack1("Q<")
         def type = SHT_BY_VALUE[@type.pack("C*").unpack1("L<")]
         def info = @info.pack("C*").unpack1("L<")
         def addr = @addr.pack("C*").unpack1("Q<")
+        def link = @link.pack("C*").unpack1("L<")
 
         private def bytes = [@name, @type, @flags, @addr, @offset, @size, @link, @info, @addralign, @entsize]
       end
diff --git a/lib/caotral/linker/writer.rb b/lib/caotral/linker/writer.rb
index ce74edf..1edcdf0 100644
--- a/lib/caotral/linker/writer.rb
+++ b/lib/caotral/linker/writer.rb
@@ -4,7 +4,7 @@ module Caotral
   class Linker
     class Writer
       include Caotral::Binary::ELF::Utils
-      ALLOW_SECTIONS = %w(.text .strtab .shstrtab).freeze
+      ALLOW_SECTIONS = %w(.text .strtab .shstrtab .symtab).freeze
       R_X86_64_PC32 = 2
       R_X86_64_PLT32 = 4
       ALLOW_RELOCATION_TYPES = [R_X86_64_PC32, R_X86_64_PLT32].freeze
@@ -73,14 +73,23 @@ def write
         write_sections = @elf_obj.sections.select { ALLOW_SECTIONS.include?(it.section_name) || it.section_name.nil? }
         shoffset = f.pos
         shstrndx = write_sections.index { it.section_name == ".shstrtab" }
+        strtabndx = write_sections.index { it.section_name == ".strtab" }
+
         shnum = write_sections.size
         @elf_obj.header.set!(shoffset:, shnum:, shstrndx:)
         names = @elf_obj.find_by_name(".shstrtab").body
 
         write_sections.each do |section|
+          header = section.header
           lookup_name = section.section_name
           name_offset = names.offset_of(lookup_name)
-          section.header.set!(name: name_offset) if name_offset
+          name, info, link, entsize = (name_offset.nil? ? 0 : name_offset), header.info, header.link, 0
+          if header.type == :symtab
+            info = section.body.size
+            link = strtabndx
+            entsize = header.entsize.nonzero? || 24
+          end
+          header.set!(name:, info:, link:, entsize:)
           f.write(section.header.build)
         end
 
diff --git a/test/caotral/linker/writer_test.rb b/test/caotral/linker/writer_test.rb
index 3b554fb..9c588dc 100644
--- a/test/caotral/linker/writer_test.rb
+++ b/test/caotral/linker/writer_test.rb
@@ -16,7 +16,7 @@ def test_write
     written_output = Caotral::Linker::Writer.write!(elf_obj: @elf_obj, output: "write.o", debug: false)
     read_written_elf = Caotral::Binary::ELF::Reader.read!(input: written_output, debug: false)
     assert_equal @elf_obj.header.shoffset, read_written_elf.header.shoffset
-    assert_equal 4, read_written_elf.sections.size
+    assert_equal 5, read_written_elf.sections.size
     assert_equal 0x401000, read_written_elf.header.entry
   end
 

From a145bf1f507e114eae035ea3de6ac45a9e526e8f Mon Sep 17 00:00:00 2001
From: "MATSUMOTO, Katsuyoshi" <github@katsyoshi.org>
Date: Sun, 11 Jan 2026 12:27:16 +0900
Subject: [PATCH 2/6] support rel/rela section in linker

---
 lib/caotral/binary/elf.rb    |  9 +++++++++
 lib/caotral/linker/writer.rb | 25 ++++++++++++++++++-------
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/lib/caotral/binary/elf.rb b/lib/caotral/binary/elf.rb
index 8a652b2..1145b54 100644
--- a/lib/caotral/binary/elf.rb
+++ b/lib/caotral/binary/elf.rb
@@ -24,6 +24,15 @@ def [](idx) = @sections[idx]
       def find_by_name(section_name) = @sections.find { section_name == it.section_name }
       def select_by_name(section_name) = @sections.select { section_name == it.section_name }
       def index(section_name) = @sections.index { section_name == it.section_name }
+      def select_by_names(section_names)
+        @sections.select do |section|
+          next true if section.section_name.nil?
+          section_names.any? do |name|
+            re = Regexp === name
+            re ? section.section_name.match(name) : section.section_name == name
+          end
+        end
+      end
     end
   end
 end
diff --git a/lib/caotral/linker/writer.rb b/lib/caotral/linker/writer.rb
index 1edcdf0..9eefea2 100644
--- a/lib/caotral/linker/writer.rb
+++ b/lib/caotral/linker/writer.rb
@@ -4,7 +4,7 @@ module Caotral
   class Linker
     class Writer
       include Caotral::Binary::ELF::Utils
-      ALLOW_SECTIONS = %w(.text .strtab .shstrtab .symtab).freeze
+      ALLOW_SECTIONS = [".text", ".strtab", ".shstrtab", ".symtab", /\.rela\./, /\.rel\./].freeze
       R_X86_64_PC32 = 2
       R_X86_64_PLT32 = 4
       ALLOW_RELOCATION_TYPES = [R_X86_64_PC32, R_X86_64_PLT32].freeze
@@ -15,6 +15,7 @@ def self.write!(elf_obj:, output:, entry: nil, debug: false)
       end
       def initialize(elf_obj:, output:, entry: nil, debug: false)
         @elf_obj, @output, @entry, @debug = elf_obj, output, entry, debug
+        @write_sections = @elf_obj.select_by_names(ALLOW_SECTIONS)
       end
       def write
         f = File.open(@output, "wb")
@@ -70,16 +71,15 @@ def write
         shstrtab_offset = f.pos
         f.write(shstrtab.body.names)
         shstrtab.header.set!(offset: shstrtab_offset, size: shstrtab.body.names.bytesize)
-        write_sections = @elf_obj.sections.select { ALLOW_SECTIONS.include?(it.section_name) || it.section_name.nil? }
         shoffset = f.pos
-        shstrndx = write_sections.index { it.section_name == ".shstrtab" }
-        strtabndx = write_sections.index { it.section_name == ".strtab" }
-
-        shnum = write_sections.size
+        shstrndx  = write_section_index(".shstrtab")
+        strtabndx = write_section_index(".strtab")
+        symtabndx = write_section_index(".symtab")
+        shnum = @write_sections.size
         @elf_obj.header.set!(shoffset:, shnum:, shstrndx:)
         names = @elf_obj.find_by_name(".shstrtab").body
 
-        write_sections.each do |section|
+        @write_sections.each do |section|
           header = section.header
           lookup_name = section.section_name
           name_offset = names.offset_of(lookup_name)
@@ -88,6 +88,9 @@ def write
             info = section.body.size
             link = strtabndx
             entsize = header.entsize.nonzero? || 24
+          elsif [:rel, :rela].include?(header.type)
+            link = symtabndx
+            info = ref_index(section)
           end
           header.set!(name:, info:, link:, entsize:)
           f.write(section.header.build)
@@ -99,6 +102,14 @@ def write
       ensure
         f.close if f
       end
+
+      private
+      def write_section_index(section_name) = @write_sections.index { it.section_name == section_name }
+      def ref_index(section)
+        section_name = section.section_name
+        ref = @elf_obj.select_by_names(section_name.split(".").filter { |sn| !sn.empty? && sn != "rel" && sn != "rela" }).first
+        write_section_index(ref.section_name)
+      end
     end
   end
 end

From 9c7236d7b9624630650bc4df48b6196f7fddbabc Mon Sep 17 00:00:00 2001
From: "MATSUMOTO, Katsuyoshi" <github@katsyoshi.org>
Date: Mon, 12 Jan 2026 15:27:58 +0900
Subject: [PATCH 3/6] set writer order for writer sections

---
 lib/caotral/linker/writer.rb       | 14 ++++++++++++--
 test/caotral/linker/writer_test.rb |  2 +-
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/lib/caotral/linker/writer.rb b/lib/caotral/linker/writer.rb
index 9eefea2..0ff76fe 100644
--- a/lib/caotral/linker/writer.rb
+++ b/lib/caotral/linker/writer.rb
@@ -4,7 +4,6 @@ module Caotral
   class Linker
     class Writer
       include Caotral::Binary::ELF::Utils
-      ALLOW_SECTIONS = [".text", ".strtab", ".shstrtab", ".symtab", /\.rela\./, /\.rel\./].freeze
       R_X86_64_PC32 = 2
       R_X86_64_PLT32 = 4
       ALLOW_RELOCATION_TYPES = [R_X86_64_PC32, R_X86_64_PLT32].freeze
@@ -15,7 +14,7 @@ def self.write!(elf_obj:, output:, entry: nil, debug: false)
       end
       def initialize(elf_obj:, output:, entry: nil, debug: false)
         @elf_obj, @output, @entry, @debug = elf_obj, output, entry, debug
-        @write_sections = @elf_obj.select_by_names(ALLOW_SECTIONS)
+        @write_sections = write_order_sections
       end
       def write
         f = File.open(@output, "wb")
@@ -59,6 +58,7 @@ def write
           end
           target.body = bytes
         end
+
         header.set!(entry: @entry || base_addr + text_offset)
         ph.set!(type:, offset: text_offset, vaddr:, paddr:, filesz:, memsz:, flags:, align:)
         text_section.header.set!(size: text_section.body.bytesize, addr: vaddr, offset: text_offset)
@@ -104,6 +104,16 @@ def write
       end
 
       private
+      def write_order_sections
+        write_order = []
+        write_order << @elf_obj.sections.find { |s| s.section_name.nil? }
+        write_order << @elf_obj.find_by_name(".text")
+        write_order << @elf_obj.find_by_name(".symtab")
+        write_order << @elf_obj.find_by_name(".strtab")
+        write_order.concat(@elf_obj.select_by_names([/\.rel\./, /\.rela\./]))
+        write_order << @elf_obj.find_by_name(".shstrtab")
+        write_order.compact
+      end
       def write_section_index(section_name) = @write_sections.index { it.section_name == section_name }
       def ref_index(section)
         section_name = section.section_name
diff --git a/test/caotral/linker/writer_test.rb b/test/caotral/linker/writer_test.rb
index 9c588dc..dfba324 100644
--- a/test/caotral/linker/writer_test.rb
+++ b/test/caotral/linker/writer_test.rb
@@ -16,7 +16,7 @@ def test_write
     written_output = Caotral::Linker::Writer.write!(elf_obj: @elf_obj, output: "write.o", debug: false)
     read_written_elf = Caotral::Binary::ELF::Reader.read!(input: written_output, debug: false)
     assert_equal @elf_obj.header.shoffset, read_written_elf.header.shoffset
-    assert_equal 5, read_written_elf.sections.size
+    assert_equal 6, read_written_elf.sections.size
     assert_equal 0x401000, read_written_elf.header.entry
   end
 

From f1fd105b0346fca13f0bec364f15700f98ca0986 Mon Sep 17 00:00:00 2001
From: "MATSUMOTO, Katsuyoshi" <github@katsyoshi.org>
Date: Mon, 12 Jan 2026 23:43:26 +0900
Subject: [PATCH 4/6] reject duplicate global symbols

---
 lib/caotral/binary/elf.rb                |  6 ++---
 lib/caotral/binary/elf/section/symtab.rb |  1 +
 lib/caotral/linker.rb                    |  3 +++
 lib/caotral/linker/builder.rb            | 30 ++++++++++++++++++++++++
 4 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 lib/caotral/linker/builder.rb

diff --git a/lib/caotral/binary/elf.rb b/lib/caotral/binary/elf.rb
index 1145b54..6486f0b 100644
--- a/lib/caotral/binary/elf.rb
+++ b/lib/caotral/binary/elf.rb
@@ -21,9 +21,9 @@ def initialize
       end
       def each(&block) = @sections.each(&block)
       def [](idx) = @sections[idx]
-      def find_by_name(section_name) = @sections.find { section_name == it.section_name }
-      def select_by_name(section_name) = @sections.select { section_name == it.section_name }
-      def index(section_name) = @sections.index { section_name == it.section_name }
+      def find_by_name(section_name) = @sections.find { |s| section_name == s.section_name }
+      def select_by_name(section_name) = @sections.select { |s| section_name == s.section_name }
+      def index(section_name) = @sections.index { |s| section_name == s.section_name }
       def select_by_names(section_names)
         @sections.select do |section|
           next true if section.section_name.nil?
diff --git a/lib/caotral/binary/elf/section/symtab.rb b/lib/caotral/binary/elf/section/symtab.rb
index f3882ce..06c7b0f 100644
--- a/lib/caotral/binary/elf/section/symtab.rb
+++ b/lib/caotral/binary/elf/section/symtab.rb
@@ -31,6 +31,7 @@ def set!(name: nil, info: nil, other: nil, shndx: nil, value: nil, size: nil)
 
           def name_offset = @name.pack("C*").unpack1("L<")
           def value = @value.pack("C*").unpack1("Q<")
+          def info = @info.pack("C*").unpack1("C")
 
           private def bytes = [@name, @info, @other, @shndx, @value, @size]
         end
diff --git a/lib/caotral/linker.rb b/lib/caotral/linker.rb
index 8abe92d..ea5764d 100644
--- a/lib/caotral/linker.rb
+++ b/lib/caotral/linker.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require_relative "binary/elf/reader"
+require_relative "linker/builder"
 require_relative "linker/writer"
 
 module Caotral
@@ -48,6 +49,8 @@ def gcc_libpath = @gcc_libpath ||= File.dirname(Dir.glob("/usr/lib/gcc/x86_64-*/
 
     def to_elf(input: @input, output: @output, debug: @debug)
       elf_obj = Caotral::Binary::ELF::Reader.new(input:, debug:).read
+      builder = Caotral::Linker::Builder.new(elf_obj:)
+      builder.resolve_symbols
       Caotral::Linker::Writer.new(elf_obj:, output:, debug:).write
     end
   end
diff --git a/lib/caotral/linker/builder.rb b/lib/caotral/linker/builder.rb
new file mode 100644
index 0000000..928353f
--- /dev/null
+++ b/lib/caotral/linker/builder.rb
@@ -0,0 +1,30 @@
+require "set"
+require "caotral/binary/elf"
+
+module Caotral
+  class Linker
+    class Builder
+      SYMTAB_BIND = { locals: 0, globals: 1, weaks: 2, }.freeze
+      BIND_BY_VALUE = SYMTAB_BIND.invert.freeze
+      attr_reader :symbols
+
+      def initialize(elf_obj:)
+        @elf_obj = elf_obj
+        @symbols = { locals: Set.new, globals: Set.new, weaks: Set.new }
+      end
+      def resolve_symbols
+        @elf_obj.find_by_name(".symtab").body.each do |symtab|
+          name = symtab.name_string
+          next if name.empty?
+          info = symtab.info
+          bind = BIND_BY_VALUE.fetch(info >> 4)
+          if bind == :globals && @symbols[bind].include?(name)
+            raise Caotral::Binary::ELF::Error,"cannot add into globals: #{name}"
+          end
+          @symbols[bind] << name
+        end
+        @symbols
+      end
+    end
+  end
+end

From 3a54b402629f496fe795fd09f46e7ad4c8d993e2 Mon Sep 17 00:00:00 2001
From: "MATSUMOTO, Katsuyoshi" <github@katsyoshi.org>
Date: Tue, 13 Jan 2026 09:59:39 +0900
Subject: [PATCH 5/6] fix search name condition

---
 lib/caotral/binary/elf.rb          | 10 +---------
 lib/caotral/linker/writer.rb       |  2 +-
 test/caotral/linker/writer_test.rb |  2 +-
 3 files changed, 3 insertions(+), 11 deletions(-)

diff --git a/lib/caotral/binary/elf.rb b/lib/caotral/binary/elf.rb
index 6486f0b..0836008 100644
--- a/lib/caotral/binary/elf.rb
+++ b/lib/caotral/binary/elf.rb
@@ -24,15 +24,7 @@ def [](idx) = @sections[idx]
       def find_by_name(section_name) = @sections.find { |s| section_name == s.section_name }
       def select_by_name(section_name) = @sections.select { |s| section_name == s.section_name }
       def index(section_name) = @sections.index { |s| section_name == s.section_name }
-      def select_by_names(section_names)
-        @sections.select do |section|
-          next true if section.section_name.nil?
-          section_names.any? do |name|
-            re = Regexp === name
-            re ? section.section_name.match(name) : section.section_name == name
-          end
-        end
-      end
+      def select_by_names(section_names) = @sections.select { |section| section_names.any? { |name| name === section.section_name.to_s } }
     end
   end
 end
diff --git a/lib/caotral/linker/writer.rb b/lib/caotral/linker/writer.rb
index 0ff76fe..01aad8b 100644
--- a/lib/caotral/linker/writer.rb
+++ b/lib/caotral/linker/writer.rb
@@ -117,7 +117,7 @@ def write_order_sections
       def write_section_index(section_name) = @write_sections.index { it.section_name == section_name }
       def ref_index(section)
         section_name = section.section_name
-        ref = @elf_obj.select_by_names(section_name.split(".").filter { |sn| !sn.empty? && sn != "rel" && sn != "rela" }).first
+        ref = @elf_obj.select_by_names(section_name.split(".").filter { |sn| !sn.empty? && sn != "rel" && sn != "rela" }.map { "." + it }).first
         write_section_index(ref.section_name)
       end
     end
diff --git a/test/caotral/linker/writer_test.rb b/test/caotral/linker/writer_test.rb
index dfba324..9c588dc 100644
--- a/test/caotral/linker/writer_test.rb
+++ b/test/caotral/linker/writer_test.rb
@@ -16,7 +16,7 @@ def test_write
     written_output = Caotral::Linker::Writer.write!(elf_obj: @elf_obj, output: "write.o", debug: false)
     read_written_elf = Caotral::Binary::ELF::Reader.read!(input: written_output, debug: false)
     assert_equal @elf_obj.header.shoffset, read_written_elf.header.shoffset
-    assert_equal 6, read_written_elf.sections.size
+    assert_equal 5, read_written_elf.sections.size
     assert_equal 0x401000, read_written_elf.header.entry
   end
 

From c2de6af929e89acc175832d463d6149f994a71c7 Mon Sep 17 00:00:00 2001
From: "MATSUMOTO, Katsuyoshi" <github@katsyoshi.org>
Date: Tue, 13 Jan 2026 20:56:59 +0900
Subject: [PATCH 6/6] update sigs

---
 sig/caotral/binary/elf.rbs                | 1 +
 sig/caotral/binary/elf/section/symtab.rbs | 1 +
 sig/caotral/linker/builder.rbs            | 9 +++++++++
 3 files changed, 11 insertions(+)
 create mode 100644 sig/caotral/linker/builder.rbs

diff --git a/sig/caotral/binary/elf.rbs b/sig/caotral/binary/elf.rbs
index e7bb7a8..df74cdc 100644
--- a/sig/caotral/binary/elf.rbs
+++ b/sig/caotral/binary/elf.rbs
@@ -10,4 +10,5 @@ class Caotral::Binary::ELF
   def find_by_name: (String | Symbol) -> Caotral::Binary::ELF::Section?
   def select_by_name: (String | Symbol) -> Array[Caotral::Binary::ELF::Section]
   def index: (String | Symbol) -> Integer?
+  def select_by_names: (Array[String | Regexp]) -> Array[Caotral::Binary::ELF::Section]
 end
diff --git a/sig/caotral/binary/elf/section/symtab.rbs b/sig/caotral/binary/elf/section/symtab.rbs
index a69110d..bddd938 100644
--- a/sig/caotral/binary/elf/section/symtab.rbs
+++ b/sig/caotral/binary/elf/section/symtab.rbs
@@ -12,5 +12,6 @@ class Caotral::Binary::ELF::Section::Symtab
   ) -> self
 
   def name_offset: () -> Integer
+  def info: () -> Integer
   def value: () -> Integer
 end
diff --git a/sig/caotral/linker/builder.rbs b/sig/caotral/linker/builder.rbs
new file mode 100644
index 0000000..0160a38
--- /dev/null
+++ b/sig/caotral/linker/builder.rbs
@@ -0,0 +1,9 @@
+class Caotral::Linker::Builder
+  @elf_obj: Caotral::Binary::ELF
+  @symbols: Hash[Symbol, Set[String]]
+
+  attr_reader symbols: Hash[Symbol, Set[String]]
+
+  def initialize: (elf_obj: Caotral::Binary::ELF) -> void
+  def resolve_symbols: () -> Hash[Symbol, Set[String]]
+end