Relocation is the process of connecting symbolic references with symbolic definitions. For example, when a program calls a function, the associated call instruction must transfer control to the proper destination address at execution. Relocatable files must have relocation entries which are necessary because they contain information that describes how to modify their section contents, thus allowing executable and shared object files to hold the right information for a process's program image.
Relocation Entries
typedef struct { Elf32_Addr r_offset; Elf32_Word r_info; } Elf32_Rel; typedef struct { Elf32_Addr r_offset; Elf32_Word r_info; Elf32_Sword r_addend; } Elf32_Rela; typedef struct { Elf64_Addr r_offset; Elf64_Xword r_info; } Elf64_Rel; typedef struct { Elf64_Addr r_offset; Elf64_Xword r_info; Elf64_Sxword r_addend; } Elf64_Rela;
#define ELF32_R_SYM(i) ((i)>>8) #define ELF32_R_TYPE(i) ((unsigned char)(i)) #define ELF32_R_INFO(s,t) (((s)<<8)+(unsigned char)(t)) #define ELF64_R_SYM(i) ((i)>>32) #define ELF64_R_TYPE(i) ((i)&0xffffffffL) #define ELF64_R_INFO(s,t) (((s)<<32)+((t)&0xffffffffL))
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.
A relocation section references two other sections: a symbol table and a section to modify. The section header's sh_info and sh_link members, described in Sections above, specify these relationships. Relocation entries for different object files have slightly different interpretations for the r_offset member.
Although the interpretation of r_offset changes for different object files to allow efficient access by the relevant programs, the relocation types' meanings stay the same.
The typical application of an ELF relocation is to determine the referenced symbol value, extract the addend (either from the field to be relocated or from the addend field contained in the relocation record, as appropriate for the type of relocation record), apply the expression implied by the relocation type to the symbol and addend, extract the desired part of the expression result, and place it in the field to be relocated.
If multiple consecutive relocation records are applied to the same relocation location (r_offset), they are composed instead of being applied independently, as described above. By consecutive, we mean that the relocation records are contiguous within a single relocation section. By composed, we mean that the standard application described above is modified as follows:
Note that a consequence of the above rules is that the location specified by a relocation type is relevant for the first element of a composed sequence (and then only for relocation records that do not contain an explicit addend field) and for the last element, where the location determines where the relocated value will be placed. For all other relocation operands in a composed sequence, the location specified is ignored.
An ABI processor supplement may specify individual relocation types that always stop a composition sequence, or always start a new one.
NOTE: This section requires processor-specific information. The ABI supplement for the desired processor describes the details.