Nooblink

Write a simplified (💩) linker from more or less scratch

Supported ELF Low endian 64 bits

Dev steps ☕

• Decode header, sections & symbols
• Display symbols defined and undefined
• Handle multiple object files
• 👉 Display relocation info
• Describe layout of the result file
• Resolve symbol references
• Relocate entries

Components logical layering

graph TD;
    subgraph level3
        session
    end
    subgraph level2
        vocabulary
    end
    subgraph level1
        raw
    end
    subgraph level0
        io
        utility
    end
    raw-->utility
    vocabulary-->raw
    vocabulary-->utility
    session-->io
    session-->vocabulary

Todo 📝

• (WIP) Introduce an object file type to wrap over vocabulary types as they get vivifed from decoding the header
• Write a Context component that keep state of current linking session, this should be passed around or installed, so that client can find out what offset to apply to object files for example...
• ObjectFile should rely on an AddressLoader facility that knows how to fix address based on whether they need offset correction or not
• Y no UT ?
• Lot of redundant identical objects in ObjectFile, the containers should hold shared ptr instead

Bug 🐛

Timeline ⌛

• 04/07 - Fix symbol names For section type symbols, we have to (unless I'm wrong) go back to the section header definition using the symbol.sectionHeaderIndex(), then use classic way to get the name for that section... The string table linked to via sh_link from the symtab section header is useless for those.

So the symbol

{
  "name": ".debug_str",
  "section": "Symtab",
  "symbol": {
    "binding": "Local",
    "nameIndex": 0,
    "sectionHeaderIndex": 11,
    "size": 0,
    "type": "Section",
    "value": "0x00000000",
    "visibility": "Default"
  }
}

tracks back to

{
  "index": 11,
  "name": ".debug_str",
  "section": {
    "addr": 0,
    "addrAlign": 1,
    "entrySize": 1,
    "flags": [
      "isMerge",
      "isStrings"
    ],
    "info": 0,
    "link": 0,
    "nameIndex": 112,
    "offset": 674,
    "size": 101,
    "type": "Progbits"
  }
}

• 02/25 - Fix interpretation of section index in symbols, confirm (eyeball) vs readelf

{
  "name": "__gmon_start__",
  "section": "Dynsym",
  "symbol": {
    "binding": "Weak",
    "nameIndex": 1,
    "sectionHeaderIndex": "Undef",
    "size": 0,
    "type": "NoType",
    "value": "0x00000000",
    "visibility": "Default"
}
61: 0000000000000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__

{
  "name": "cairo_clip_preserve",
  "section": "Dynsym",
  "symbol": {
    "binding": "Global",
    "nameIndex": 1421,
    "sectionHeaderIndex": 11,
    "size": 38,
    "type": "Func",
    "value": "0x000236b0",
    "visibility": "Default"
}
404: 00000000000236b0    38 FUNC    GLOBAL DEFAULT   11 cairo_clip_preserve

• 02/11 - Decode symbol names

    {
      "content": {
        "binding": "Global",
        "nameIndex": 13238,
        "sectionHeaderIndex": "0",
        "size": 0,
        "type": 2,
        "value": "0x00000000",
        "visibility": "Default"
      },
      "name": "xcb_render_create_picture"
    }

• 02/04 - Decode symbol table entries, w/o decoded symbol names

• 01/21 - Decode name of section header from the string table

   {
    "ELFHeader": {
      "abi": "None",
      "abiVersion": 0,
      "addressClass": "64bits",
      "architecture": "Advanced Micro Devices X86-64 processor",
      "endianness": "LowEndian",
      "executionAddress": "0x00015860",
      "flags": "0x0000",
      "headerSize": 64,
      "headerVersion": 1,
      "objectFileType": "Shared",
      "objectFileVersion": 1,
      "padding": "       ",
      "programTableAddress": "0x00000040",
      "programTableCount": 7,
      "programTableSize": 56,
      "sectionNameIndex": 25,
      "sectionTableAddress": "0x001175d8",
      "sectionTableCount": 26,
      "sectionTableSize": 64
    },
    "sectionHeaders": [
      {
        "content": {
          "addr": 0,
          "addrAlign": 0,
          "entrySize": 0,
          "flags": "[]",
          "info": 0,
          "link": 0,
          "nameIndex": 0,
          "offset": 0,
          "size": 0,
          "type": "Null"
        },
        "name": ""
      },
      { 
        "...": "..."
      },
      {
        "content": {
          "addr": 0,
          "addrAlign": 1,
          "entrySize": 0,
          "flags": "[]",
          "info": 0,
          "link": 0,
          "nameIndex": 1,
          "offset": 1144064,
          "size": 211,
          "type": "Strtab"
        },
        "name": ".shstrtab"
      }
    ]
  }

• 01/14 - Extract all section headers from ELF header

  === Elf header ===
  {"Abi":"None","AbiVersion":0,"AddressClass":"64bits","Architecture":"Advanced Micro Devices X86-64 processor","Endianness":"LowEndian","ExecutionAddress":"0x00015860","Flags":"0x0000","HeaderSize":64,"HeaderVersion":1,"ObjectFileType":"Shared","ObjectFileVersion":1,"Padding":"       ","ProgramTableAddress":"0x00000040","ProgramTableCount":7,"ProgramTableSize":56,"SectionNameIndex":25,"SectionTableAddress":"0x001175d8","SectionTableCount":26,"SectionTableSize":64}
  === Section table header ===
  {"addr":0,"addrAlign":0,"entrySize":0,"flags":"[]","info":0,"link":0,"nameIndex":0,"offset":0,"size":0,"type":"Null"}
  === Section table header ===
  {"addr":456,"addrAlign":8,"entrySize":4,"flags":"[]","info":0,"link":456,"nameIndex":15,"offset":456,"size":7352,"type":"Hash"}
  === Section table header ===
  {"addr":7808,"addrAlign":8,"entrySize":0,"flags":"[]","info":0,"link":7808,"nameIndex":11,"offset":7808,"size":5184,"type":"Unknown (0x6ffffff6)"}
  === Section table header ===
  {"addr":12992,"addrAlign":8,"entrySize":24,"flags":"[]","info":2,"link":12992,"nameIndex":21,"offset":12992,"size":19560,"type":"Dynsym"}
  ...
  

• 01/08 - ELF header decoded, can search for section header entry of a particular type

  {"Abi":"None","AbiVersion":0,"AddressClass":"64bits","Architecture":"Advanced Micro Devices X86-64 processor","Endianness":"LowEndian","ExecutionAddress":"0x00015860","Flags":"0x0000","HeaderSize":64,"HeaderVersion":1,"ObjectFileType":"Shared","ObjectFileVersion":1,"Padding":"       ","ProgramTableAddress":"0x00000040","ProgramTableCount":7,"ProgramTableSize":56,"SectionNameIndex":25,"SectionTableAddress":"0x001175d8","SectionTableCount":26,"SectionTableSize":64}
  Found 'Strtab' entry at: 0x1089fe6d8
  Found 'Strtab' entry at: 0x1089fec18
  

Note

PLT (Procedure Linkage Table)

There is no PLT in .O files as this is created by the linker. If an object file needs PLT , it will create a special relocation entries, which the linker will use to populate the PLT. What does it do , when do you need it , draw me a PLT , what uses it

GOT (Global Offset Table)

There is no GOT in .O files as this is created by the linker. Usually the address is held in a register for efficient access.

Strings

Two fundamental entities require names in an object file: sections, and symbols. For symbols, the symbol (SymTab, DynSym) section header point to the strings table via the link field. For sections, the object file directly point to the string table via the section header string index field

Reference

1. https://refspecs.linuxbase.org/elf/elf.pdf
2. https://www.sco.com/developers/gabi/2000-07-17/contents.html
3. https://en.m.wikipedia.org/wiki/Executable_and_Linkable_Format
4. https://uclibc.org/docs/elf-64-gen.pdf
5. https://lwn.net/Articles/276782/