Relocations

A relocation is an instruction to the linker/loader: "patch this location with the address of this symbol (plus offset)."

Types (x86-64):

• R_X86_64_64: 64-bit absolute address.
• R_X86_64_PC32: 32-bit signed PC-relative offset.
• R_X86_64_GOT32, R_X86_64_GOTPCREL: GOT entry / PC-rel ref to GOT.
• R_X86_64_PLT32: PC-rel reference to PLT.
• R_X86_64_RELATIVE: image base + addend (no symbol).
• R_X86_64_GLOB_DAT: address of symbol → GOT[i].
• R_X86_64_JUMP_SLOT: address of symbol → PLT[i].
• R_X86_64_TPOFF / DTPOFF: TLS offsets.

A relocation entry:

struct Rela {
    uint64_t r_offset;        // where to patch
    uint64_t r_info;          // symbol index + reloc type
    int64_t  r_addend;        // value to add
};

For static linking:

• Relocations resolve at link time.
• Linker patches the executable.
• Stripped from final binary.

For dynamic linking:

• Some relocations resolve at link time (within .so).
• Others (GLOB_DAT, JUMP_SLOT) resolve at load time / call time.

Position-independent code:

• Use only PC-relative or GOT-indirect references.
• Loader can place anywhere; no patching of code (only GOT).
• Required for shared libraries; common for executables (PIE for ASLR).

Example:

mov rax, [rip + foo@GOTPCREL]    ; load GOT entry for foo
mov rbx, [rax]                    ; deref

vs non-PIC absolute:

mov rax, foo                      ; absolute address

Toolchain flags:

• -fPIC: position-independent code.
• -pie: position-independent executable.
• -Wl,-z,now: full RELRO.
• -Wl,-z,relro: standard RELRO.

ASLR (Address Space Layout Randomization):

• Each load picks random base for executable + libraries + stack + heap.
• Defends against ROP / address-guessing attacks.
• Requires PIE binaries.