Relocation addend in ELF files – Elf64_Rel vs Elf64_Rela?

ELF files contain two structures to handle relocations:

Elf64_Rel:

typedef struct {
    Elf64_Addr r_offset;
    Uint64_t
    r_info;
} Elf64_Rel;

and Elf64_Rela:

typedef struct {
    Elf64_Addr r_offset;
    uint64_t
    r_info;
    int64_t
    r_addend;
} Elf64_Rela;

I want to take peek at the relocation entries but I’m not sure which one to use. The manual pages are quite cryptic about this. Is there a specific usage for each?

Answer

This depends on the target. Most targets use only one of the two forms. The Relocation chapter in the System V Application Binary Interface (the ELF specification) says this:

As specified previously, only Elf32_Rela and Elf64_Rela entries contain an explicit addend. Entries of type Elf32_Rel and Elf64_Rel store an implicit addend in the location to be modified. Depending on the processor architecture, one form or the other might be necessary or more convenient. Consequently, an implementation for a particular machine may use one form exclusively or either form depending on context.

The ELF format is self-describing in the sense that it shows whether REL or RELA relocations are used (SHT_REL or SHT_RELA for section type; DT_REL, DT_RELSZ, DT_RELENT or DT_RELA, DT_RELASZ, DT_RELAENT in the dynamic section). But relocation-processing itself is very target-specific.