CODEWITHSUNDEEP
cassemblyelfportable-executablegot
Neetika Bishwas
Neetika Bishwas

Reputation: 61

What is the difference between Import Address Table and Global Offset Table?

I tried googling them separately, but one thing stuck out the most. Is it just that IAT is for PE files and GoT is for ELF?

Upvotes: 0

Views: 1209

Answers (2)

Martin Rosenau
Martin Rosenau

Reputation: 18523

Is it just that IAT is for PE files and GoT is for ELF?

This is the main difference.

Another difference is that the GOT in ELF files may also contain entries describing symbols defined in the ELF file itself. This is typical for 32-bit shared libraries.

IATs in PE files contain only entries that describe symbols defined in other DLL files.

Upvotes: 4

Mahonri Moriancumer
Mahonri Moriancumer

Reputation: 6013

On Linux, the nm command can be used to see all symbols that must be imported from shared libraries. I compiled a small program called "main". Using the nm command, I can see the offsets of objects. Symbols prefixed with a 'T' are functions can be found in the "main" code, along with logical offset (Global Offset Table). Symbols prefixed with a 'U' are undefined, and must be imported (Import Address Table). 

$ nm main
0000000100000d70 T _AddAlbum
                 U ___error
                 U ___stderrp
0000000100000000 T __mh_execute_header
                 U _fprintf
0000000100000e40 T _main
                 U _malloc
                 U _printf
                 U _realloc
                 U _strerror
                 U dyld_stub_binder

For more detail, use the objdump with the -x option: 

$ objdump -x main

main:   file format Mach-O 64-bit x86-64

Sections:
Idx Name          Size      Address          Type
  0 __text        0000018a 0000000100000d70 TEXT 
  1 __stubs       00000024 0000000100000efa TEXT 
  2 __stub_helper 0000004c 0000000100000f20 TEXT 
  3 __cstring     00000042 0000000100000f6c DATA 
  4 __unwind_info 00000048 0000000100000fb0 DATA 
  5 __nl_symbol_ptr 00000008 0000000100001000 DATA 
  6 __got         00000010 0000000100001008 DATA 
  7 __la_symbol_ptr 00000030 0000000100001018 DATA 

SYMBOL TABLE:
0000000100000d70 g     F __TEXT,__text  _AddAlbum
0000000100000000 g     F __TEXT,__text  __mh_execute_header
0000000100000e40 g     F __TEXT,__text  _main
0000000000000000         *UND*  ___error
0000000000000000         *UND*  ___stderrp
0000000000000000         *UND*  _fprintf
0000000000000000         *UND*  _malloc
0000000000000000         *UND*  _printf
0000000000000000         *UND*  _realloc
0000000000000000         *UND*  _strerror
0000000000000000         *UND*  dyld_stub_binder
Mach header
      magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
MH_MAGIC_64  X86_64        ALL LIB64     EXECUTE    15       1296   NOUNDEFS DYLDLINK TWOLEVEL PIE
Load command 0
      cmd LC_SEGMENT_64
  cmdsize 72
  segname __PAGEZERO
   vmaddr 0x0000000000000000
   vmsize 0x0000000100000000
  fileoff 0
 filesize 0
  maxprot ---
 initprot ---
   nsects 0
    flags (none)
Load command 1
      cmd LC_SEGMENT_64
  cmdsize 472
  segname __TEXT
   vmaddr 0x0000000100000000
   vmsize 0x0000000000001000
  fileoff 0
 filesize 4096
  maxprot r-x
 initprot r-x
   nsects 5
    flags (none)
Section
  sectname __text
   segname __TEXT
      addr 0x0000000100000d70
      size 0x000000000000018a
    offset 3440
     align 2^4 (16)
    reloff 0
    nreloc 0
      type S_REGULAR
attributes PURE_INSTRUCTIONS SOME_INSTRUCTIONS
 reserved1 0
 reserved2 0
Section
  sectname __stubs
   segname __TEXT
      addr 0x0000000100000efa
      size 0x0000000000000024
    offset 3834
     align 2^1 (2)
    reloff 0
    nreloc 0
      type S_SYMBOL_STUBS
attributes PURE_INSTRUCTIONS SOME_INSTRUCTIONS
 reserved1 0 (index into indirect symbol table)
 reserved2 6 (size of stubs)
Section
  sectname __stub_helper
   segname __TEXT
      addr 0x0000000100000f20
      size 0x000000000000004c
    offset 3872
     align 2^2 (4)
    reloff 0
    nreloc 0
      type S_REGULAR
attributes PURE_INSTRUCTIONS SOME_INSTRUCTIONS
 reserved1 0
 reserved2 0
Section
  sectname __cstring
   segname __TEXT
      addr 0x0000000100000f6c
      size 0x0000000000000042
    offset 3948
     align 2^0 (1)
    reloff 0
    nreloc 0
      type S_CSTRING_LITERALS
attributes (none)
 reserved1 0
 reserved2 0
Section
  sectname __unwind_info
   segname __TEXT
      addr 0x0000000100000fb0
      size 0x0000000000000048
    offset 4016
     align 2^2 (4)
    reloff 0
    nreloc 0
      type S_REGULAR
attributes (none)
 reserved1 0
 reserved2 0
Load command 2
      cmd LC_SEGMENT_64
  cmdsize 312
  segname __DATA
   vmaddr 0x0000000100001000
   vmsize 0x0000000000001000
  fileoff 4096
 filesize 4096
  maxprot rw-
 initprot rw-
   nsects 3
    flags (none)
Section
  sectname __nl_symbol_ptr
   segname __DATA
      addr 0x0000000100001000
      size 0x0000000000000008
    offset 4096
     align 2^3 (8)
    reloff 0
    nreloc 0
      type S_NON_LAZY_SYMBOL_POINTERS
attributes (none)
 reserved1 6 (index into indirect symbol table)
 reserved2 0
Section
  sectname __got
   segname __DATA
      addr 0x0000000100001008
      size 0x0000000000000010
    offset 4104
     align 2^3 (8)
    reloff 0
    nreloc 0
      type S_NON_LAZY_SYMBOL_POINTERS
attributes (none)
 reserved1 7 (index into indirect symbol table)
 reserved2 0
Section
  sectname __la_symbol_ptr
   segname __DATA
      addr 0x0000000100001018
      size 0x0000000000000030
    offset 4120
     align 2^3 (8)
    reloff 0
    nreloc 0
      type S_LAZY_SYMBOL_POINTERS
attributes (none)
 reserved1 9 (index into indirect symbol table)
 reserved2 0
Load command 3
      cmd LC_SEGMENT_64
  cmdsize 72
  segname __LINKEDIT
   vmaddr 0x0000000100002000
   vmsize 0x0000000000001000
  fileoff 8192
 filesize 572
  maxprot r--
 initprot r--
   nsects 0
    flags (none)
Load command 4
            cmd LC_DYLD_INFO_ONLY
        cmdsize 48
     rebase_off 8192
    rebase_size 8
       bind_off 8200
      bind_size 40
  weak_bind_off 0
 weak_bind_size 0
  lazy_bind_off 8240
 lazy_bind_size 96
     export_off 8336
    export_size 64
Load command 5
     cmd LC_SYMTAB
 cmdsize 24
  symoff 8408
   nsyms 11
  stroff 8644
 strsize 120
Load command 6
            cmd LC_DYSYMTAB
        cmdsize 80
      ilocalsym 0
      nlocalsym 0
     iextdefsym 0
     nextdefsym 3
      iundefsym 3
      nundefsym 8
         tocoff 0
           ntoc 0
      modtaboff 0
        nmodtab 0
   extrefsymoff 0
    nextrefsyms 0
 indirectsymoff 8584
  nindirectsyms 15
      extreloff 0
        nextrel 0
      locreloff 0
        nlocrel 0
Load command 7
          cmd LC_LOAD_DYLINKER
      cmdsize 32
         name /usr/lib/dyld (offset 12)
Load command 8
     cmd LC_UUID
 cmdsize 24
    uuid 55558635-5195-3609-B115-706912ED1DA7
Load command 9
       cmd LC_BUILD_VERSION
   cmdsize 32
  platform macos
       sdk 10.14
     minos 10.14
    ntools 1
      tool ld
   version 520.0
Load command 10
      cmd LC_SOURCE_VERSION
  cmdsize 16
  version 0.0
Load command 11
       cmd LC_MAIN
   cmdsize 24
  entryoff 3648
 stacksize 0
Load command 12
          cmd LC_LOAD_DYLIB
      cmdsize 56
         name /usr/lib/libSystem.B.dylib (offset 24)
   time stamp 2 Wed Dec 31 17:00:02 1969
      current version 1281.0.0
compatibility version 1.0.0
Load command 13
      cmd LC_FUNCTION_STARTS
  cmdsize 16
  dataoff 8400
 datasize 8
Load command 14
      cmd LC_DATA_IN_CODE
  cmdsize 16
  dataoff 8408
 datasize 0

Upvotes: -1

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