Refactoring linker

[katsyoshi]

Summary

This PR refactors linker write flow and adds PIE dynamic-link scaffolding around PLT relocations.

• Refactor Writer phases (section write, relocation patching, dynamic patching, program-header patching, section-header finalization).
• Keep Builder section order as the source of truth and remove implicit writer-side reorder assumptions.
• Add .plt, .got.plt, and .rela.plt scaffolding and wire them through Builder/Writer.
• Patch .rela.plt relocation info/offsets against final .dynsym/.got.plt layout.
• Add explicit unsupported handling for GOTPCREL-family relocation types for now.

Main changes

Linker/ELF internals

• lib/caotral/linker/writer.rb
  • Phase-oriented write flow cleanup.
  • .plt/.got.plt section writes + header updates.
  • .rela.plt symbol remap to dynamic symbol indexes before write.
  • sh_link/sh_info/sh_entsize handling for relocation sections adjusted for dynamic relocations.
• lib/caotral/linker/builder.rb
  • PLT relocation path wiring for PIE (.rela.plt, .got.plt, .dynsym/.dynstr integration).
• lib/caotral/binary/elf/section/rel.rb
  • Relocation type table updates used by linker paths.
• lib/caotral/binary/elf/section/dynamic.rb
  • Dynamic tag helpers for PLT-related tags.
• Reader/header/program-header helpers updated to support relocation/dynamic validation paths.

Test/fixtures

• Added fixtures:
  • sample/C/plt-call.c
  • sample/C/gotpcrel-call.c
• Added tests:
  • test/caotral/linker/plt-call_test.rb (PLT/rela.plt and dynamic-tag consistency)
  • test/caotral/linker/gotpc-call_test.rb (unsupported GOTPCREL path)
• Updated existing linker tests for refactor compatibility.

Validation run

• ruby test/caotral/linker/multi-link_test.rb
• ruby test/caotral/linker/pie-object_test.rb
• ruby test/caotral/linker/plt-call_test.rb
• ruby test/caotral/linker/gotpc-call_test.rb

Follow-up

• Finalize PLT dynamic tags for self linker #37 finalize PLT dynamic-tag consistency closure.
• Decide and implement R_X86_64_GOTPCREL handling #38 decide/implement full GOTPCREL handling path.
• Add Reader-based validation for PLT relocations and dynamic tags #39 expand Reader-based validation helpers.

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[chatgpt-codex-connector]

Preserve call-site relocation for undefined PLT32 symbols

When handling undefined R_X86_64_PLT32 relocations, this branch appends a .rela.plt record and immediately nexts, so the original text relocation path is skipped and no call-site patching happens here. For executable/PIE outputs that call external symbols (like sample/C/plt-call.c), .rela.text is later removed, leaving the call displacement at its placeholder value and producing a runtime jump to the wrong address.

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[chatgpt-codex-connector]

Remap .rela.plt symbol indexes after sorting .symtab

This lookup treats rel.sym as a stable .symtab index, but .rela.plt entries are created before symtab_section.body.sort_by! and are never reindexed afterward (only rel_sections get remapped). In multi-object links, sorting shifts symbol positions, so .rela.plt can resolve against the wrong symbol here or fail with cannot find symbol ... in .dynsym even though the undefined symbol exists.

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