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

Question

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?

Answer 1

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.

Answer 2

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