freebsd/gnu/usr.bin/ld/PORTING

$Id$

This document describes some of the machine dependent parts in ld(1) and rtld(?)
Most of the machine dependencies are a result of different ways in which
relocation information is conveyed in an architecture's object files.
Especially RISC architectures are likely candidates to have deviated from the
traditional relocation record structure due a tendency to use odd sized
"bitfields" to denote immediate operands within their fixed size instructions.

Also, there may be slight differences in the model used for Position
Independent Code generation by the compiler.

Lastly, both ld and rtld must fiddle with actual machine instructions to setup
a transfer vector to accommodate PIC code and dynamic linking.


Machine dependent macros and data structures.

typedef struct jmpslot { ... } jmpslot_t;

	The Procedure Linkage Table (PLT) is an array of these structures.
	The structure contains room for a control transfer instruction
	and a relocation index. md_make_jmpslot() and md_fix_jmpslot()
	are responsible for filling these in.

JMPSLOT_NEEDS_RELOC()

	Macro indicating whether or not a jmpslot entry needs a run-time
	relocation when ld has already resolved the symbolic reference
	(eg. when `-Bsymbolic' was given). Usually the case if the control
	transfer instruction is PC relative or something.

RELOC_STATICS_THROUGH_GOT_P(r)

	Predicate taking a `struct relocation_info *' as an argument. It
	decides whether variables with file scope are addressed relative to
	the start Global Offset Table (1) or an entry in GOT must be
	allocated (0). The compiler has a say in this.


Some other random macros:

BAD_MID(ex)

	Tells whether the machine ID in an input file header is acceptable.

N_SET_FLAG(ex,f)

	Set flags F in a.out header. Must account for possible non-NetBSD
	headers; QMAGIC is still a documented ld output format.

N_IS_DYNAMIC(ex)

	Return true if this appears to be a dynamically linked object.

#define relocation_info		reloc_info_<machine>

	Define (possibly machine dependent) relocation record format.
	Should convert to a typedef someday for greater opacity.

md_got_reloc(r)

	Adjustment to be applied to the relocation value of references
	to "_GLOBAL_OFFSET_TABLE". It's here because of Sun's sparc as(1),
	(it's is a *bug*), and could have been `#ifdef SUN_COMPAT' if I
	had not let it slip into NetBSD's gas for compatibility.

md_get_rt_segment_addend(r,a)

	Another SunOS bug workaround.


The macros below have defaults defined in ld.h for the traditional relocation
structure format; each takes a `struct relocation_info *' argument (see
ld.h for more detailed comments):

RELOC_ADDRESS(r)		- the address at which to relocate
RELOC_EXTERN_P(r)		- relocation for external symbol
RELOC_TYPE(r)			- segment (text/data/bss), non-external relocs
RELOC_SYMBOL(r)			- symbol index, external relocs
RELOC_MEMORY_SUB_P(r)		- relocation involves something to subtract
RELOC_MEMORY_ADD_P(r)		- relocation involves something to add
RELOC_ADD_EXTRA(r)		- <disused> (moved into MD files)
RELOC_PCREL_P(r)		- relocation is PC relative
RELOC_VALUE_RIGHTSHIFT(r)	- <disused>
RELOC_TARGET_SIZE(r)		- size (in bytes) of relocated value
RELOC_TARGET_BITPOS(r)		- <disused> (moved into MD files)
RELOC_TARGET_BITSIZE(r)		- <disused> (moved into MD files)
RELOC_JMPTAB_P(r)		- relocation is for a PLT entry
RELOC_BASEREL_P(r)		- relocation is for a GOT entry
RELOC_RELATIVE_P(r)		- relocation is load address relative
RELOC_COPY_P(r)			- relocation involves an initialization
RELOC_INIT_SEGMENT_RELOC(r)	- initialize a relocation record pertaining to
				  a segment; traditional archs can use bzero().
RELOC_LAZY_P(r)			- (run-time) resolution can be lazy.
CHECK_GOT_RELOC(r)		- consistency check on relocations involving
				  the "_GLOBAL_OFFSET_TABLE" symbol



Things which are currently defined as routines in <machine>/md.c:


md_init_header(struct exec *hp, int magic, int flags)

	Initializes the output file header. Straightforward, unless
	multiple OS'es are supported.


md_swap*_exec_hdr(struct exec *)

	Input/output a.out header in target byte-order.


md_swap*_reloc(struct relocation_info *, n)

	Input/output N relocation records in target byte-order.


md_get_addend(struct relocation_info *rp, char *addr)

	Return a relocation addend. Traditionally found in the object file
	at address ADDR. The relocation record determines the data width
	in bytes (using RELOC_TARGET_SIZE()).

md_relocate(struct relocation_info *rp, long reloc, char *addr,
	    int relocatable_output)

	Perform a relocation at the given address, usually by entering
	the specified value RELOC into the object file. Some architectures
	may store the relocation in RP when RELOCATABLE_OUTPUT is set.
	Again, the byte size of the relocation value is determined from
	RP through RELOC_TARGET_SIZE().

md_make_reloc(struct relocation_info *rp, int type)

	Construct the machine dependent part of a relocation record used
	for run-time relocation. Sets RP's type field or one or more
	bitfields according to TYPE which is ld's internal relocation
	representation of the type of (run-time) relocation. TYPE is a
	combination of the following bits:

	RELTYPE_EXTERN		- relocation is for unresolved symbol
	RELTYPE_JMPSLOT		- relocation is for a PLT entry
	RELTYPE_BASEREL		- <not used>
	RELTYPE_RELATIVE	- relocation is load address relative
	RELTYPE_COPY		- relocation is an initalization

md_make_jmpreloc(struct relocation_info *rp, *r, int type)

	Set up a run-time relocation record pertaining to a jmpslot.
	This usually sets a bit or a relocation type dedicated to jmpslot
	relocations. R is the relocation record to be updated (ie. the
	run-time relocation record), while RP points at the relocation
	record from the object file on behalf of which we allocated a
	PLT entry. RP may not be needed.

md_make_gotreloc(struct relocation_info *rp, *r, int type)

	Set up a run-time relocation record pertaining to a GOT entry.
	This usually sets a bit or a relocation type dedicated to GOT
	relocations. R is the relocation record to be updated (ie. the
	run-time relocation record), while RP points at the relocation
	record from the object file on behalf of which we allocated a
	GOT entry.

md_make_cpyreloc(struct relocation_info *rp, *r)

	Mark the relocation record as pertaining to a location that needs
	run-time initializing from some shared object source.
	R and RP same as above.

md_make_jmpslot(jmpslot_t *sp, long offset, long index)

	Construct a jmpslot, ie. fill in the machine opcodes that comprise
	a transfer to slot 0 of the PLT. OFFSET is the location of SP
	relative to the start of the PLT (ie. (int)sp - (int)&PLT[0] ).
	INDEX is the entry in the run-time relocation record table which
	determines what symbol this jmpslot is supposed to resolve.

md_fix_jmpslot(jmpslot_t *sp, long offset, long addr)

	Fix up a jmpslot so that it will transfer directly to ADDR
	in stead of to PLT[0]. OFFSET has the same meaning as in
	md_make_jmpslot(). This function is called by binder() after
	it has resolved a symbol into a (run-time) address.

md_set_breakpoint(long where, long *savep)

	Set a breakpoint. Used when run under a debugger. The breakpoint
	instruction is to be set at location WHERE. The original contents
	of *WHERE is to be saved in *SAVEP.