Relocation Types (Processor-Specific)

On SPARC, relocation entries describe how to alter the following instruction and data fields (bit numbers appear in the lower box corners):

On x86, relocation entries describe how to alter the following instruction and data fields (bit numbers appear in the lower box corners):

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 x86 architecture):

Calculations below 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. It first decides how to combine and locate the input files, then updates the symbol values, and finally performs the relocation. Relocations applied to executable or shared object files are similar and accomplish the same result. Descriptions below use the following notation:

A

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

B

means 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" for more information about the base address.

G

means 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)" for more information.

GOT

means the address of the global offset table. See "Global Offset Table (Processor-Specific)" for more information.

L

means the place (section offset or address) of the procedure linkage table entry for a symbol. A procedure linkage table entry redirects a function call to the proper destination. The link-editor builds the initial procedure linkage table, and the runtime linker modifies the entries during execution. See "Procedure Linkage Table (Processor-Specific)" for more information.

P

means the place (section offset or address) of the storage unit being relocated (computed using r_offset).

S

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

SPARC relocation entries apply to bytes (byte8), half-words (half16), or words (the others). x86 relocation entries apply to words. In any case, 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.

SPARC uses only Elf32_Rela relocation entries with explicit addends. Thus the r_addend member serves as the relocation addend. x86 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.

SPARC: Relocation Types

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Note -

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

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Table 7-20 SPARC: 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-simm12	(L + A - P) & 0x3ff
R_SPARC_10	30	V_simm10	S + A
R_SPARC_11	31	V_simm11	S + A
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

Some relocation types have semantics beyond simple calculation:

R_SPARC_GOT10

This relocation type resembles R_SPARC_LO10, except it refers to the address of the symbol's global offset table entry and additionally instructs the link-editor to build a global offset table.

R_SPARC_GOT13

This relocation type resembles R_SPARC_13, except it refers to the address of the symbol's global offset table entry and additionally instructs the link-editor to build a global offset table.

R_SPARC_GOT22

This relocation type resembles R_SPARC_22, except it refers to the address of the symbol's global offset table entry and additionally instructs the link-editor to build a global offset table.

R_SPARC_WPLT30

This relocation type resembles R_SPARC_WDISP30, except it refers to the address of the symbol's procedure linkage table entry and additionally instructs the link-editor to build a procedure linkage table.

R_SPARC_COPY

The link-editor creates this relocation type for dynamic linking. 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" for more details.

R_SPARC_GLOB_DAT

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

R_SPARC_JMP_SLOT

The link-editor creates this relocation type for dynamic linking. 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

The link-editor creates this relocation type for dynamic linking. 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

This relocation type resembles R_SPARC_32, except it refers to an unaligned word. That is, 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.

x86: Relocation Types

Table 7-21 x86: 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

This relocation type computes the distance from the base of the global offset table to the symbol's global offset table entry. It also tells the link-editor to build a global offset table.

R_386_PLT32

This relocation type computes the address of the symbol's procedure linkage table entry and tells the link-editor to build a procedure linkage table.

R_386_COPY

The link-editor creates this relocation type for dynamic linking. 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

This relocation type is used to set a global offset table entry to the address of the specified symbol. The special relocation type lets one determine the correspondence between symbols and global offset table entries.

R_386_JMP_SLOT

The link-editor creates this relocation type for dynamic linking. 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

The link-editor creates this relocation type for dynamic linking. 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

This relocation type computes the difference between a symbol's value and the address of the global offset table. It also tells the link-editor to build the global offset table.

R_386_GOTPC

This relocation type resembles R_386_PC32, except 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 tells the link-editor to build the global offset table.