Relocations
===========

BFD maintains relocations in much the same was as it maintains symbols; they are left alone until required, then read in en-mass and traslated into an internal form. There is a common routine bfd_perform_relocation which acts upon the canonical form to to the actual fixup. Note that relocations are maintained on a per section basis, whilst symbols are maintained on a per BFD basis. All a back end has to do to fit the BFD interface is to create as many struct reloc_cache_entry as there are relocations in a particuar section, and fill in the right bits:

typedef arelent
---------------

This is the structure of a relocation entry: .

     typedef enum bfd_reloc_status
     {
            	/* No errors detected */
       bfd_reloc_ok,

            	/* 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,

            	/* Unused */
       bfd_reloc_notsupported,

            	/* Unsupported relocation size requested.  */
       bfd_reloc_other,

            	/* The symbol to relocate against was undefined.*/
       bfd_reloc_undefined,

            	/* The relocation was performed, but may not be ok - presently
               generated only when linking i960 coff files with i960 b.out
               symbols. */
       bfd_reloc_dangerous
      }
      bfd_reloc_status_type;

     typedef struct reloc_cache_entry
     {
            	/* A pointer into the canonical table of pointers  */
       struct symbol_cache_entry **sym_ptr_ptr;

            	/* offset in section */
       rawdata_offset address;

            	/* addend for relocation value */
       bfd_vma addend;

            	/* Pointer to how to perform the required relocation */
       CONST struct reloc_howto_struct *howto;

     } arelent;

Description

Here is a description of each of the fields within a relent:

  * sym_ptr_ptr

     The symbol table pointer points to a pointer to the symbol associated with the relocation request. This would naturally be the pointer into the table returned by the back end's get_symtab action. See 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 whould 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. On 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 odd sized lumps. The designers of the a.out format chose not to use the data within the section for storing part of the offset; all the offset is kept within the reloc. Any thing 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 contains a pointer to foo, and the offsets would 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 struct which contains information on what to do with all 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.

reloc_howto_type
................

The reloc_howto_type is a structure which contains all the information that BFD needs to know to tie up a back end's data. .struct symbol_cache_entry;			/* Forward declaration */

     typedef CONST struct reloc_howto_struct
     {
            	/*  The type field has mainly a documetary use - the back end can
                to what it wants with it, though the normally the back end's
                external idea of what a reloc number would be would be stored
                in this field. For example, the a PC relative word relocation
                in a coff environment would have the type 023 - because that's
                what the outside world calls a R_PCRWORD reloc. */
       unsigned int type;

            	/*  The value the final relocation is shifted right by. This drops
                unwanted data from the relocation.  */
       unsigned int rightshift;

            	/*  The size of the item to be relocated - 0, is one byte, 1 is 2
                bytes, 3 is four bytes. */
       unsigned int size;

            	/*  Now obsolete */
       unsigned int bitsize;

            	/*  Notes that the relocation is relative to the location in the
                data section of the addend. The relocation function will
                subtract from the relocation value the address of the location
                being relocated. */
       boolean pc_relative;

            	/*  Now obsolete */
       unsigned int bitpos;

            	/*  Now obsolete */
       boolean absolute;

            	/* Causes the relocation routine to return an error if overflow
               is detected when relocating. */
       boolean complain_on_overflow;

            	/* 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 accomodated (eg, i960 callj
               instructions). */
       bfd_reloc_status_type EXFUN ((*special_function),
     					    (bfd *abfd,
     					     arelent *reloc_entry,
                                                 struct symbol_cache_entry *symbol,
                                                 PTR data,
                                                 asection *input_section));

            	/* The textual name of the relocation type. */
       char *name;

            	/* When performing a partial link, some formats must modify the
               relocations rather than the data - this flag signals this.*/
       boolean partial_inplace;

            	/* The src_mask is used to select what parts of the read in data
               are to be used in the relocation sum. Eg, if this was an 8 bit
               bit of data which we read and relocated, this would be
               0x000000ff. When we have relocs which have an addend, such as
               sun4 extended relocs, the value in the offset part of a
               relocating field is garbage so we never use it. In this case
               the mask would be 0x00000000. */
       bfd_word src_mask;

            	/* The dst_mask is what parts of the instruction are replaced
               into the instruction. In most cases src_mask == dst_mask,
               except in the above special case, where dst_mask would be
               0x000000ff, and src_mask would be 0x00000000.   */
       bfd_word dst_mask;

            	/* 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 (eg sun3 a.out).
               Some formats leave the displacement part of an instruction
               empty (eg m88k bcs), this flag signals the fact.*/
       boolean pcrel_offset;

     } reloc_howto_type;

the HOWTO macro
...............

Description

The HOWTO define is horrible and will go away.

     #define HOWTO(C, R,S,B, P, BI, ABS, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC) \
       {(unsigned)C,R,S,B, P, BI, ABS,O,SF,NAME,INPLACE,MASKSRC,MASKDST,PC}

Description

And will be replaced with the totally magic way. But for the moment, we are compatible, so do it this way..

     #define NEWHOWTO( FUNCTION, NAME,SIZE,REL,IN) HOWTO(0,0,SIZE,0,REL,0,false,false,FUNCTION, NAME,false,0,0,IN)

Description

Helper routine to turn a symbol into a relocation value.

     #define HOWTO_PREPARE(relocation, symbol)      \
       {                                            \
       if (symbol != (asymbol *)NULL) {             \
         if (symbol->section == &bfd_com_section) { \
           relocation = 0;                          \
         }                                          \
         else {                                     \
           relocation = symbol->value;              \
         }                                          \
       }                                            \
     }

reloc_chain
...........

Description

How relocs are tied together

     typedef unsigned char bfd_byte;

     typedef struct relent_chain {
       arelent relent;
       struct   relent_chain *next;
     } arelent_chain;

bfd_perform_relocation
......................

Synopsis

     bfd_reloc_status_type
     bfd_perform_relocation
        (bfd * abfd,
         arelent *reloc_entry,
         PTR data,
         asection *input_section,
         bfd *output_bfd);

Description

If an 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 (eg 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 howto manager
=================

When an application wants to create a relocation, but doesn't know what the target machine might call it, it can find out by using this bit of code.

bfd_reloc_code_type
...................

Description

The insides of a reloc code .

     typedef enum bfd_reloc_code_real
     {
            	/* 16 bits wide, simple reloc */
       BFD_RELOC_16,

            	/* 8 bits wide, but used to form an address like 0xffnn */
       BFD_RELOC_8_FFnn,

            	/* 8 bits wide, simple */
       BFD_RELOC_8,

            	/* 8 bits wide, pc relative */
       BFD_RELOC_8_PCREL,

            	/* The type of reloc used to build a contructor table - at the
               moment probably a 32 bit wide abs address, but the cpu can
               choose. */

       BFD_RELOC_CTOR
      } bfd_reloc_code_real_type;

bfd_reloc_type_lookup
=====================

Synopsis

     CONST struct reloc_howto_struct *
     bfd_reloc_type_lookup
        (CONST bfd_arch_info_type *arch, bfd_reloc_code_type code);

Description

This routine returns a pointer to a howto struct which when invoked, will perform the supplied relocation on data from the architecture noted.

bfd_default_reloc_type_lookup
.............................

Synopsis

     CONST struct reloc_howto_struct *bfd_default_reloc_type_lookup
        (CONST struct bfd_arch_info *,
         bfd_reloc_code_type  code);

Description

Provides a default relocation lookuperer for any architectue

bfd_generic_relax_section
.........................

Synopsis

     boolean bfd_generic_relax_section
        (bfd *abfd,
         asection *section,
         asymbol **symbols);

Description

Provides default handling for relaxing for back ends which don't do relaxing - ie does nothing

bfd_generic_get_relocated_section_contents
..........................................

Synopsis

     bfd_byte *
     bfd_generic_get_relocated_section_contents(bfd *abfd,
         struct bfd_seclet_struct  *seclet)

Description

Provides default handling of relocation effort for back ends which can't be bothered to do it efficiently.