bfd: typedef arelent
2.10.1 typedef arelent
----------------------
This is the structure of a relocation entry:
typedef enum bfd_reloc_status
{
/* No errors detected. Note - the value 2 is used so that it
will not be mistaken for the boolean TRUE or FALSE values. */
bfd_reloc_ok = 2,
/* The relocation was performed, but there was an overflow. */
bfd_reloc_overflow,
/* The address to relocate was not within the section supplied. */
bfd_reloc_outofrange,
/* Used by special functions. */
bfd_reloc_continue,
/* Unsupported relocation size requested. */
bfd_reloc_notsupported,
/* Unused. */
bfd_reloc_other,
/* The symbol to relocate against was undefined. */
bfd_reloc_undefined,
/* The relocation was performed, but may not be ok. If this type is
returned, the error_message argument to bfd_perform_relocation
will be set. */
bfd_reloc_dangerous
}
bfd_reloc_status_type;
typedef const struct reloc_howto_struct reloc_howto_type;
typedef struct reloc_cache_entry
{
/* A pointer into the canonical table of pointers. */
struct bfd_symbol **sym_ptr_ptr;
/* offset in section. */
bfd_size_type address;
/* addend for relocation value. */
bfd_vma addend;
/* Pointer to how to perform the required relocation. */
reloc_howto_type *howto;
}
arelent;
*Description*
Here is a description of each of the fields within an 'arelent':
* 'sym_ptr_ptr'
The symbol table pointer points to a pointer to the symbol associated
with the relocation request. It is the pointer into the table returned
by the back end's 'canonicalize_symtab' action. 
Symbols. The
symbol is referenced through a pointer to a pointer so that tools like
the linker can fix up all the symbols of the same name by modifying only
one pointer. The relocation routine looks in the symbol and uses the
base of the section the symbol is attached to and the value of the
symbol as the initial relocation offset. If the symbol pointer is zero,
then the section provided is looked up.
* 'address'
The 'address' field gives the offset in bytes from the base of the
section data which owns the relocation record to the first byte of
relocatable information. The actual data relocated will be relative to
this point; for example, a relocation type which modifies the bottom two
bytes of a four byte word would not touch the first byte pointed to in a
big endian world.
* 'addend'
The 'addend' is a value provided by the back end to be added (!) to
the relocation offset. Its interpretation is dependent upon the howto.
For example, on the 68k the code:
char foo[];
main()
{
return foo[0x12345678];
}
Could be compiled into:
linkw fp,#-4
moveb @#12345678,d0
extbl d0
unlk fp
rts
This could create a reloc pointing to 'foo', but leave the offset in
the data, something like:
RELOCATION RECORDS FOR [.text]:
offset type value
00000006 32 _foo
00000000 4e56 fffc ; linkw fp,#-4
00000004 1039 1234 5678 ; moveb @#12345678,d0
0000000a 49c0 ; extbl d0
0000000c 4e5e ; unlk fp
0000000e 4e75 ; rts
Using coff and an 88k, some instructions don't have enough space in
them to represent the full address range, and pointers have to be loaded
in two parts. So you'd get something like:
or.u r13,r0,hi16(_foo+0x12345678)
ld.b r2,r13,lo16(_foo+0x12345678)
jmp r1
This should create two relocs, both pointing to '_foo', and with
0x12340000 in their addend field. The data would consist of:
RELOCATION RECORDS FOR [.text]:
offset type value
00000002 HVRT16 _foo+0x12340000
00000006 LVRT16 _foo+0x12340000
00000000 5da05678 ; or.u r13,r0,0x5678
00000004 1c4d5678 ; ld.b r2,r13,0x5678
00000008 f400c001 ; jmp r1
The relocation routine digs out the value from the data, adds it to
the addend to get the original offset, and then adds the value of
'_foo'. Note that all 32 bits have to be kept around somewhere, to cope
with carry from bit 15 to bit 16.
One further example is the sparc and the a.out format. The sparc has
a similar problem to the 88k, in that some instructions don't have room
for an entire offset, but on the sparc the parts are created in odd
sized lumps. The designers of the a.out format chose to not use the
data within the section for storing part of the offset; all the offset
is kept within the reloc. Anything in the data should be ignored.
save %sp,-112,%sp
sethi %hi(_foo+0x12345678),%g2
ldsb [%g2+%lo(_foo+0x12345678)],%i0
ret
restore
Both relocs contain a pointer to 'foo', and the offsets contain junk.
RELOCATION RECORDS FOR [.text]:
offset type value
00000004 HI22 _foo+0x12345678
00000008 LO10 _foo+0x12345678
00000000 9de3bf90 ; save %sp,-112,%sp
00000004 05000000 ; sethi %hi(_foo+0),%g2
00000008 f048a000 ; ldsb [%g2+%lo(_foo+0)],%i0
0000000c 81c7e008 ; ret
00000010 81e80000 ; restore
* 'howto'
The 'howto' field can be imagined as a relocation instruction. It is
a pointer to a structure which contains information on what to do with
all of the other information in the reloc record and data section. A
back end would normally have a relocation instruction set and turn
relocations into pointers to the correct structure on input - but it
would be possible to create each howto field on demand.
2.10.1.1 'enum complain_overflow'
.................................
Indicates what sort of overflow checking should be done when performing
a relocation.
enum complain_overflow
{
/* Do not complain on overflow. */
complain_overflow_dont,
/* Complain if the value overflows when considered as a signed
number one bit larger than the field. ie. A bitfield of N bits
is allowed to represent -2**n to 2**n-1. */
complain_overflow_bitfield,
/* Complain if the value overflows when considered as a signed
number. */
complain_overflow_signed,
/* Complain if the value overflows when considered as an
unsigned number. */
complain_overflow_unsigned
};
2.10.1.2 'reloc_howto_type'
...........................
The 'reloc_howto_type' is a structure which contains all the information
that libbfd needs to know to tie up a back end's data.
struct reloc_howto_struct
{
/* The type field has mainly a documentary use - the back end can
do what it wants with it, though normally the back end's idea of
an external reloc number is stored in this field. */
unsigned int type;
/* The encoded size of the item to be relocated. This is *not* a
power-of-two measure. Use bfd_get_reloc_size to find the size
of the item in bytes. */
unsigned int size:3;
/* The number of bits in the field to be relocated. This is used
when doing overflow checking. */
unsigned int bitsize:7;
/* The value the final relocation is shifted right by. This drops
unwanted data from the relocation. */
unsigned int rightshift:6;
/* The bit position of the reloc value in the destination.
The relocated value is left shifted by this amount. */
unsigned int bitpos:6;
/* What type of overflow error should be checked for when
relocating. */
ENUM_BITFIELD (complain_overflow) complain_on_overflow:2;
/* The relocation value should be negated before applying. */
unsigned int negate:1;
/* The relocation is relative to the item being relocated. */
unsigned int pc_relative:1;
/* Some formats record a relocation addend in the section contents
rather than with the relocation. For ELF formats this is the
distinction between USE_REL and USE_RELA (though the code checks
for USE_REL == 1/0). The value of this field is TRUE if the
addend is recorded with the section contents; when performing a
partial link (ld -r) the section contents (the data) will be
modified. The value of this field is FALSE if addends are
recorded with the relocation (in arelent.addend); when performing
a partial link the relocation will be modified.
All relocations for all ELF USE_RELA targets should set this field
to FALSE (values of TRUE should be looked on with suspicion).
However, the converse is not true: not all relocations of all ELF
USE_REL targets set this field to TRUE. Why this is so is peculiar
to each particular target. For relocs that aren't used in partial
links (e.g. GOT stuff) it doesn't matter what this is set to. */
unsigned int partial_inplace:1;
/* When some formats create PC relative instructions, they leave
the value of the pc of the place being relocated in the offset
slot of the instruction, so that a PC relative relocation can
be made just by adding in an ordinary offset (e.g., sun3 a.out).
Some formats leave the displacement part of an instruction
empty (e.g., ELF); this flag signals the fact. */
unsigned int pcrel_offset:1;
/* src_mask selects the part of the instruction (or data) to be used
in the relocation sum. If the target relocations don't have an
addend in the reloc, eg. ELF USE_REL, src_mask will normally equal
dst_mask to extract the addend from the section contents. If
relocations do have an addend in the reloc, eg. ELF USE_RELA, this
field should normally be zero. Non-zero values for ELF USE_RELA
targets should be viewed with suspicion as normally the value in
the dst_mask part of the section contents should be ignored. */
bfd_vma src_mask;
/* dst_mask selects which parts of the instruction (or data) are
replaced with a relocated value. */
bfd_vma dst_mask;
/* If this field is non null, then the supplied function is
called rather than the normal function. This allows really
strange relocation methods to be accommodated. */
bfd_reloc_status_type (*special_function)
(bfd *, arelent *, struct bfd_symbol *, void *, asection *,
bfd *, char **);
/* The textual name of the relocation type. */
char *name;
};
2.10.1.3 'The HOWTO Macro'
..........................
*Description*
The HOWTO macro fills in a reloc_howto_type (a typedef for const struct
reloc_howto_struct).
#define HOWTO(type, right, size, bits, pcrel, left, ovf, func, name, \
inplace, src_mask, dst_mask, pcrel_off) \
{ (unsigned) type, size < 0 ? -size : size, bits, right, left, ovf, \
size < 0, pcrel, inplace, pcrel_off, src_mask, dst_mask, func, name }
*Description*
This is used to fill in an empty howto entry in an array.
#define EMPTY_HOWTO(C) \
HOWTO ((C), 0, 0, 0, FALSE, 0, complain_overflow_dont, NULL, \
NULL, FALSE, 0, 0, FALSE)
2.10.1.4 'bfd_get_reloc_size'
.............................
*Synopsis*
unsigned int bfd_get_reloc_size (reloc_howto_type *);
*Description*
For a reloc_howto_type that operates on a fixed number of bytes, this
returns the number of bytes operated on.
2.10.1.5 'arelent_chain'
........................
*Description*
How relocs are tied together in an 'asection':
typedef struct relent_chain
{
arelent relent;
struct relent_chain *next;
}
arelent_chain;
2.10.1.6 'bfd_check_overflow'
.............................
*Synopsis*
bfd_reloc_status_type bfd_check_overflow
(enum complain_overflow how,
unsigned int bitsize,
unsigned int rightshift,
unsigned int addrsize,
bfd_vma relocation);
*Description*
Perform overflow checking on RELOCATION which has BITSIZE significant
bits and will be shifted right by RIGHTSHIFT bits, on a machine with
addresses containing ADDRSIZE significant bits. The result is either of
'bfd_reloc_ok' or 'bfd_reloc_overflow'.
2.10.1.7 'bfd_reloc_offset_in_range'
....................................
*Synopsis*
bfd_boolean bfd_reloc_offset_in_range
(reloc_howto_type *howto,
bfd *abfd,
asection *section,
bfd_size_type offset);
*Description*
Returns TRUE if the reloc described by HOWTO can be applied at OFFSET
octets in SECTION.
2.10.1.8 'bfd_perform_relocation'
.................................
*Synopsis*
bfd_reloc_status_type bfd_perform_relocation
(bfd *abfd,
arelent *reloc_entry,
void *data,
asection *input_section,
bfd *output_bfd,
char **error_message);
*Description*
If OUTPUT_BFD is supplied to this function, the generated image will be
relocatable; the relocations are copied to the output file after they
have been changed to reflect the new state of the world. There are two
ways of reflecting the results of partial linkage in an output file: by
modifying the output data in place, and by modifying the relocation
record. Some native formats (e.g., basic a.out and basic coff) have no
way of specifying an addend in the relocation type, so the addend has to
go in the output data. This is no big deal since in these formats the
output data slot will always be big enough for the addend. Complex
reloc types with addends were invented to solve just this problem. The
ERROR_MESSAGE argument is set to an error message if this return
'bfd_reloc_dangerous'.
2.10.1.9 'bfd_install_relocation'
.................................
*Synopsis*
bfd_reloc_status_type bfd_install_relocation
(bfd *abfd,
arelent *reloc_entry,
void *data, bfd_vma data_start,
asection *input_section,
char **error_message);
*Description*
This looks remarkably like 'bfd_perform_relocation', except it does not
expect that the section contents have been filled in. I.e., it's
suitable for use when creating, rather than applying a relocation.
For now, this function should be considered reserved for the
assembler.
Info Catalog
bfd: Relocations
bfd: Relocations
bfd: howto manager