Relocation

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 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 are these data.

Relocation entries can have the following structure, defined in sys/elf.h:

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;

The elements of this structure are:

r_offset

This member gives the location at which to apply the relocation action. Different object files have slightly different interpretations for this member.

For a relocatable file, the value indicates a section offset. The relocation section itself describes how to modify another section in the file. Relocation offsets designate a storage unit within the second section.

For an executable or shared object, the value indicates the virtual address of the storage unit affected by the relocation. This information makes the relocation entries more useful for the runtime linker.

Although the interpretation of the member changes for different object files to allow efficient access by the relevant programs, the meanings of the relocation types stay the same.

r_info

This member gives both the symbol table index, with respect to which the relocation must be made, and the type of relocation to apply. For example, a call instruction's relocation entry holds the symbol table index of the function being called. If the index is STN_UNDEF, the undefined symbol index, the relocation uses 0 as the symbol value.

Relocation types are processor-specific. A relocation entry's relocation type or symbol table index is the result of applying ELF32_R_TYPE or ELF32_R_SYM, respectively, to the entry's r_info member:

#define ELF32_R_SYM(info)             ((info)>>8)
#define ELF32_R_TYPE(info)            ((unsigned char)(info))
#define ELF32_R_INFO(sym, type)       (((sym)<<8)+(unsigned char)(type))

#define ELF64_R_SYM(info)             ((info)>>32)
#define ELF64_R_TYPE(info)            ((Elf64_Word)(info))
#define ELF64_R_INFO(sym, type)       (((Elf64_Xword)(sym)<<32)+ \ 
                                        (Elf64_Xword)(type))

For Elf64_Rel and Elf64_Rela structures, the r_info field is further broken down into an 8–bit type identifier and a 24–bit type dependent data field:

#define ELF64_R_TYPE_DATA(info)       (((Elf64_Xword)(info)<<32)>>40)
#define ELF64_R_TYPE_ID(info)         (((Elf64_Xword)(info)<<56)>>56)
#define ELF64_R_TYPE_INFO(data, type) (((Elf64_Xword)(data)<<8)+ \ 
                                        (Elf64_Xword)(type))

r_addend

This member specifies a constant addend used to compute the value to be stored into the relocatable field.

Rela entries contain an explicit addend. Entries of type Rel store an implicit addend in the location to be modified. 32–bit and 64–bit SPARC use only Elf32_Rela and Elf64_Rela relocation entries respectively. Thus, the r_addend member serves as the relocation addend. IA uses only Elf32_Rel relocation entries. The field to be relocated holds the addend. In all cases, the addend and the computed result use the same byte order.

A relocation section can reference two other sections: a symbol table, identified by the sh_info section header entry, and a section to modify, identified by the sh_link section header entry. “Sections” specifies these relationships. An sh_link entry is required when a relocation section exists in a relocatable object, but is optional for executables and shared objects. The relocation offset is sufficient to perform the relocation.

Relocation Types (Processor-Specific)

Relocation entries describe how to alter instruction and data fields in the following figures. Bit numbers appear in the lower box corners.

On the SPARC platform, relocation entries apply to bytes (byte8), half-words (half16), or words (the others).

On 64–bit SPARC, relocations also apply to extended-words (xword64):

On IA, relocation entries apply to words (word32):

word32 specifies a 32–bit field occupying 4 bytes with an arbitrary byte alignment. These values use the same byte order as other word values in the IA architecture:

In all cases, the r_offset value designates the offset or virtual address of the first byte of the affected storage unit. The relocation type specifies which bits to change and how to calculate their values.

Calculations for the following relocation types assume the actions are transforming a relocatable file into either an executable or a shared object file. Conceptually, the link-editor merges one or more relocatable files to form the output. The link-editor first decides how to combine and locate the input files. Then it updates the symbol values. Finally the link-editor performs the relocation. Relocations applied to executable or shared object files are similar and accomplish the same result. Descriptions in the tables in this section use the following notation:

A

The addend used to compute the value of the relocatable field.

B

The base address at which a shared object is loaded into memory during execution. Generally, a shared object file is built with a 0 base virtual address, but the execution address is different. See “Program Header”.

G

The offset into the global offset table at which the address of the relocation entry's symbol resides during execution. See “Global Offset Table (Processor-Specific)”.

GOT

The address of the global offset table. See “Global Offset Table (Processor-Specific)”.

L

The section offset or address of the procedure linkage table entry for a symbol. See “Procedure Linkage Table (Processor-Specific)”.

P

The section offset or address of the storage unit being relocated, computed using r_offset.

S

The value of the symbol whose index resides in the relocation entry.

SPARC: Relocation Types

Field names in the following table tell whether the relocation type checks for overflow. A calculated relocation value can be larger than the intended field, and a relocation type can verify (V) the value fits or truncate (T) the result. As an example, V-simm13 means that the computed value can not have significant, nonzero bits outside the simm13 field.

Table 7-27 SPARC: ELF Relocation Types

Name	Value	Field	Calculation
R_SPARC_NONE	0	None	None
R_SPARC_8	1	V-byte8	S + A
R_SPARC_16	2	V-half16	S + A
R_SPARC_32	3	V-word32	S + A
R_SPARC_DISP8	4	V-byte8	S + A - P
R_SPARC_DISP16	5	V-half16	S + A - P
R_SPARC_DISP32	6	V-disp32	S + A - P
R_SPARC_WDISP30	7	V-disp30	(S + A - P) >> 2
R_SPARC_WDISP22	8	V-disp22	(S + A - P) >> 2
R_SPARC_HI22	9	T-imm22	(S + A) >> 10
R_SPARC_22	10	V-imm22	S + A
R_SPARC_13	11	V-simm13	S + A
R_SPARC_LO10	12	T-simm13	(S + A) & 0x3ff
R_SPARC_GOT10	13	T-simm13	G & 0x3ff
R_SPARC_GOT13	14	V-simm13	G
R_SPARC_GOT22	15	T-simm22	G >> 10
R_SPARC_PC10	16	T-simm13	(S + A - P) & 0x3ff
R_SPARC_PC22	17	V-disp22	(S + A - P) >> 10
R_SPARC_WPLT30	18	V-disp30	(L + A - P) >> 2
R_SPARC_COPY	19	None	None
R_SPARC_GLOB_DAT	20	V-word32	S + A
R_SPARC_JMP_SLOT	21	None	See R_SPARC_JMP_SLOT,
R_SPARC_RELATIVE	22	V-word32	B + A
R_SPARC_UA32	23	V-word32	S + A
R_SPARC_PLT32	24	V-word32	L + A
R_SPARC_HIPLT22	25	T-imm22	(L + A) >> 10
R_SPARC_LOPLT10	26	T-simm13	(L + A) & 0x3ff
R_SPARC_PCPLT32	27	V-word32	L + A - P
R_SPARC_PCPLT22	28	V-disp22	(L + A - P) >> 10
R_SPARC_PCPLT10	29	V-simm13	(L + A - P) & 0x3ff
R_SPARC_10	30	V-simm10	S + A
R_SPARC_11	31	V-simm11	S + A
R_SPARC_OLO10	33	V-simm13	((S + A) & 0x3ff) + O
R_SPARC_HH22	34	V-imm22	(S + A) >> 42
R_SPARC_HM10	35	T-simm13	((S + A) >> 32) & 0x3ff
R_SPARC_LM22	36	T-imm22	(S + A) >> 10
R_SPARC_PC_HH22	37	V-imm22	(S + A - P) >> 42
R_SPARC_PC_HM10	38	T-simm13	((S + A - P) >> 32) & 0x3ff
R_SPARC_PC_LM22	39	T-imm22	(S + A - P) >> 10
R_SPARC_WDISP16	40	V-d2/disp14	(S + A - P) >> 2
R_SPARC_WDISP19	41	V-disp19	(S + A - P) >> 2
R_SPARC_7	43	V-imm7	S + A
R_SPARC_5	44	V-imm5	S + A
R_SPARC_6	45	V-imm6	S + A
R_SPARC_HIX22	48	V-imm22	((S + A) ^ 0xffffffffffffffff) >> 10
R_SPARC_LOX10	49	T-simm13	((S + A) & 0x3ff) | 0x1c00
R_SPARC_H44	50	V-imm22	(S + A) >> 22
R_SPARC_M44	51	T-imm10	((S + A) >> 12) & 0x3ff
R_SPARC_L44	52	T-imm13	(S + A) & 0xfff
R_SPARC_REGISTER	53	V-word32	S + A
R_SPARC_UA16	55	V-half16	S + A

Some relocation types have semantics beyond simple calculation:

R_SPARC_GOT10

Resembles R_SPARC_LO10, except that it refers to the address of the symbol's global offset table entry. Additionally, R_SPARC_GOT10 instructs the link-editor to create a global offset table.

R_SPARC_GOT13

Resembles R_SPARC_13, except that it refers to the address of the symbol's global offset table entry. Additionally, R_SPARC_GOT13 instructs the link-editor to create a global offset table.

R_SPARC_GOT22

Resembles R_SPARC_22, except that it refers to the address of the symbol's global offset table entry. Additionally, R_SPARC_GOT22 instructs the link-editor to create a global offset table.

R_SPARC_WPLT30

Resembles R_SPARC_WDISP30, except that it refers to the address of the symbol's procedure linkage table entry. Additionally, R_SPARC_WPLT30 instructs the link-editor to create a procedure linkage table.

R_SPARC_COPY

Created by the link-editor for dynamic executables to preserve a read-only text segment. Its offset member refers to a location in a writable segment. The symbol table index specifies a symbol that should exist both in the current object file and in a shared object. During execution, the runtime linker copies data associated with the shared object's symbol to the location specified by the offset. See “Copy Relocations”.

R_SPARC_GLOB_DAT

Resembles R_SPARC_32, except that it sets a global offset table entry to the address of the specified symbol. The special relocation type enables you to determine the correspondence between symbols and global offset table entries.

R_SPARC_JMP_SLOT

Created by the link-editor for dynamic objects to provide lazy binding. Its offset member gives the location of a procedure linkage table entry. The runtime linker modifies the procedure linkage table entry to transfer control to the designated symbol address.

R_SPARC_RELATIVE

Created by the link-editor for dynamic objects. Its offset member gives the location within a shared object that contains a value representing a relative address. The runtime linker computes the corresponding virtual address by adding the virtual address at which the shared object is loaded to the relative address. Relocation entries for this type must specify 0 for the symbol table index.

R_SPARC_UA32

Resembles R_SPARC_32, except that it refers to an unaligned word. The word to be relocated must be treated as four separate bytes with arbitrary alignment, not as a word aligned according to the architecture requirements.

R_SPARC_OLO10

Resembles R_SPARC_LO10, except that an extra offset is added to make full use of the 13-bit signed immediate field.

R_SPARC_LM22

Resembles R_SPARC_HI22, except that it truncates rather than validates.

R_SPARC_PC_LM22

Resembles R_SPARC_PC22, except that it truncates rather than validates.

R_SPARC_HIX22

Used with R_SPARC_LOX10 for executables that will be confined to the uppermost 4 gigabytes of the 64–bit address space. Similar to R_SPARC_HI22, but supplies ones complement of linked value.

R_SPARC_LOX10

Used with R_SPARC_HIX22. Similar to R_SPARC_LO10, but always sets bits 10 through 12 of the linked value.

R_SPARC_L44

Used with the R_SPARC_H44 and R_SPARC_M44 relocation types to generate a 44-bit absolute addressing model.

R_SPARC_REGISTER

Used to initialize a register symbol. Its offset member contains the register number to be initialized. There must be a corresponding register symbol for this register of type SHN_ABS.

64-bit SPARC: Relocation Types

The relocations listed in the following table extend, or alter, those define for 32–bit SPARC. See “SPARC: Relocation Types”.

Table 7-28 64-bit SPARC: ELF Relocation Types

Name	Value	Field	Calculation
R_SPARC_HI22	9	V-imm22	(S + A) >> 10
R_SPARC_GLOB_DAT	20	V-xword64	S + A
R_SPARC_RELATIVE	22	V-xword64	B + A
R_SPARC_64	32	V-xword64	S + A
R_SPARC_DISP64	46	V-xword64	S + A - P
R_SPARC_PLT64	47	V-xword64	L + A
R_SPARC_REGISTER	53	V-xword64	S + A
R_SPARC_UA64	54	V-xword64	S + A

IA: Relocation Types

The relocations listed in the following table are defined for 32–bit IA.

Table 7-29 IA: ELF Relocation Types

Name	Value	Field	Calculation
R_386_NONE	0	none	none
R_386_32	1	word32	S + A
R_386_PC32	2	word32	S + A - P
R_386_GOT32	3	word32	G + A
R_386_PLT32	4	word32	L + A - P
R_386_COPY	5	none	none
R_386_GLOB_DAT	6	word32	S
R_386_JMP_SLOT	7	word32	S
R_386_RELATIVE	8	word32	B + A
R_386_GOTOFF	9	word32	S + A - GOT
R_386_GOTPC	10	word32	GOT + A - P
R_386_32PLT	11	word32	L + A

Some relocation types have semantics beyond simple calculation:

R_386_GOT32

Computes the distance from the base of the global offset table to the symbol's global offset table entry. It also instructs the link-editor to create a global offset table.

R_386_PLT32

Computes the address of the symbol's procedure linkage table entry and instructs the link-editor to create a procedure linkage table.

R_386_COPY

Created by the link-editor for dynamic executables to preserve a read-only text segment. Its offset member refers to a location in a writable segment. The symbol table index specifies a symbol that should exist both in the current object file and in a shared object. During execution, the runtime linker copies data associated with the shared object's symbol to the location specified by the offset. See “Copy Relocations”.

R_386_GLOB_DAT

Used to set a global offset table entry to the address of the specified symbol. The special relocation type enable you to determine the correspondence between symbols and global offset table entries.

R_386_JMP_SLOT

Created by the link-editor for dynamic objects to provide lazy binding. Its offset member gives the location of a procedure linkage table entry. The runtime linker modifies the procedure linkage table entry to transfer control to the designated symbol address.

R_386_RELATIVE

Created by the link-editor for dynamic objects. Its offset member gives the location within a shared object that contains a value representing a relative address. The runtime linker computes the corresponding virtual address by adding the virtual address at which the shared object is loaded to the relative address. Relocation entries for this type must specify 0 for the symbol table index.

R_386_GOTOFF

Computes the difference between a symbol's value and the address of the global offset table. It also instructs the link-editor to create the global offset table.

R_386_GOTPC

Resembles R_386_PC32, except that it uses the address of the global offset table in its calculation. The symbol referenced in this relocation normally is _GLOBAL_OFFSET_TABLE_, which also instructs the link-editor to create the global offset table.