What is the difference between .got and .got.plt section?

Question

What is the difference between .got and .got.plt section in ELF format?

Spiegeleisen

According to: acsu.buffalo.edu/~charngda/elf.html .got entries are never resolved lazily, but .got.plt entries can be resolved lazily. – Chanellechaney

I think .got is for relocations regarding global 'variables' while .got.plt is a auxiliary section to act together with .plt when resolving procedures absolute addresses. – Squirrel

Answer 1

My previous comment turns to be right:

I think .got is for relocations regarding global 'variables' while .got.plt is a auxiliary section to act together with .plt when resolving procedures absolute addresses.

The example below makes things a bit clear.

These are the relocations for my 32 bits i686-linux /lib/libm.so

Relocation section '.rel.dyn' at offset 0x32b8 contains 8 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00025030  00000008 R_386_RELATIVE   
00024fd8  00005706 R_386_GLOB_DAT    00025034   _LIB_VERSION
00024fdc  00000406 R_386_GLOB_DAT    00000000   __gmon_start__
00024fe0  00000506 R_386_GLOB_DAT    00000000   _Jv_RegisterClasses
00024fe4  00000806 R_386_GLOB_DAT    00000000   _rtld_global_ro
00024fe8  00000906 R_386_GLOB_DAT    00000000   stderr
00024fec  00013006 R_386_GLOB_DAT    0002507c   signgam
00024ff0  00000e06 R_386_GLOB_DAT    00000000   __cxa_finalize

Relocation section '.rel.plt' at offset 0x32f8 contains 12 entries:
 Offset     Info    Type            Sym.Value  Sym. Name
00025000  00000107 R_386_JUMP_SLOT   00000000   fputs
00025004  00000207 R_386_JUMP_SLOT   00000000   __errno_location
00025008  00000307 R_386_JUMP_SLOT   00000000   sprintf
0002500c  00000407 R_386_JUMP_SLOT   00000000   __gmon_start__
00025010  00000607 R_386_JUMP_SLOT   00000000   strtod
00025014  00000707 R_386_JUMP_SLOT   00000000   __assert_fail
00025018  00000a07 R_386_JUMP_SLOT   00000000   strlen
0002501c  00000b07 R_386_JUMP_SLOT   00000000   strtof
00025020  00000c07 R_386_JUMP_SLOT   00000000   fwrite
00025024  00000d07 R_386_JUMP_SLOT   00000000   strtold
00025028  00005e07 R_386_JUMP_SLOT   00005970   matherr
0002502c  00000e07 R_386_JUMP_SLOT   00000000   __cxa_finalize

Look that as you noted there are two relocation sections, namely .rel.dyn and .rel.plt. You can see that all relocations for .rel.plt are of type R_386_JUMP_SLOT which means that they are branch relocations on the other hand almost all relocations in .rel.dyn are R_386_GLOB_DAT which means relocation for global variables.

Another subtle difference exist between .symtab and .dynsym. While the first contain references for all symbols used during static link editing the later contain only those symbols needed for dynamic linking. Thus, the relocations mentioned above refer only to .dynsym section.

Squirrel

IIRC, a .got entry is also be needed for functions that have their address taken; compilers usually choose to require early binding for them so they can load the final address from the GOT, instead of just taking the address of the PLT stub as the function pointer. – Messiah