Ciro Santilli
OurBigBook.comSection type:
sh_type == SHT_RELA.Common name: "relocation section".
.rela.text holds relocation data which says how the address should be modified when the final executable is linked. This points to bytes of the text area that must be modified when linking happens to point to the correct memory locations.Basically, it translates the object text containing the placeholder 0x0 address:
to the actual executable code containing the final 0x6000d8:
a: 48 be 00 00 00 00 00 movabs $0x0,%rsi
11: 00 00 004000ba: 48 be d8 00 60 00 00 movabs $0x6000d8,%rsi
4000c1: 00 00 00It was pointed to by
sh_info = 6 of the .symtab section.readelf -r hello_world.o outputs:
Relocation section '.rela.text' at offset 0x3b0 contains 1 entries:
Offset Info Type Sym. Value Sym. Name + Addend
00000000000c 000200000001 R_X86_64_64 0000000000000000 .data + 0The section does not exist in the executable.
The actual bytes are:
00000370 0c 00 00 00 00 00 00 00 01 00 00 00 02 00 00 00 |................|
00000380 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|The
struct represented is:
typedef struct {
Elf64_Addr r_offset;
Elf64_Xword r_info;
Elf64_Sxword r_addend;
} Elf64_Rela;So:
- 370 0:
r_offset= 0xC: address into the.textwhose address this relocation will modify - 370 8:
r_info= 0x200000001. Contains 2 fields:ELF64_R_TYPE= 0x1: meaning depends on the exact architecture.ELF64_R_SYM= 0x2: index of the section to which the address points, so.datawhich is at index 2.
The AMD64 ABI says that type1is calledR_X86_64_64and that it represents the operationS + Awhere:S: the value of the symbol on the object file, here0because we point to the00 00 00 00 00 00 00 00ofmovabs $0x0,%rsiA: the addend, present in fieldr_added
This address is added to the section on which the relocation operates.This relocation operation acts on a total 8 bytes. - 380 0:
r_addend= 0
So in our example we conclude that the new address will be:
S + A = .data + 0, and thus the first thing in the data section.