/*doc* @section 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 @code{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 @code{struct reloc_cache_entry} as there are relocations in a particuar section, and fill in the right bits: @menu * typedef arelent:: * reloc handling functions:: @end menu */ #include "sysdep.h" #include "bfd.h" #include "libbfd.h" /*doc *node typedef arelent, Relocations, reloc handling functions, Relocations @section typedef arelent */ /*proto* bfd_perform_relocation The relocation routine returns as a status an enumerated type: *+++ $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 relocaction 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_enum_type; *--- */ /*proto* *+++ $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; if sym is null this is the section $ struct sec *section; Pointer to how to perform the required relocation $ struct reloc_howto_struct *howto; $} arelent; *--- */ /*doc* @table @code @item sym_ptr_ptr The symbol table pointer points to a pointer to the symbol ascociated with the relocation request. This would naturaly be the pointer into the table returned by the back end's get_symtab action. @xref{Symbols}. The symbol is referenced through a pointer to a pointer so that tools like the linker can fixup 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 attatched 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. @item 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. @item addend The addend is a value provided by the back end to be added (!) to the relocation offset. It's 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 *- @item section The section field is only used when the symbol pointer field is null. It supplies the section into which the data should be relocated. The field's main use comes from assemblers which do most of the symbol fixups themselves; an assembler may take an internal reference to a label, but since it knows where the label is, it can turn the relocation request from a symbol lookup into a section relative relocation - the relocation emitted has no symbol, just a section to relocate against. I'm not sure what it means when both a symbol pointer an a section pointer are present. Some formats use this sort of mechanism to describe PIC relocations, but bfd can't to that sort of thing yet. @item 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. @end table */ /*proto* reloc_howto_type The @code{reloc_howto_type} is a structure which contains all the information that bfd needs to know to tie up a back end's data. *+++ $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_enum_type (*special_function)(); 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; *--- */ /*proto* HOWTO 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} *- */ /*proto* reloc_chain *+ typedef unsigned char bfd_byte; typedef struct relent_chain { arelent relent; struct relent_chain *next; } arelent_chain; *- */ /*proto* 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. *; PROTO(bfd_reloc_status_enum_type, bfd_perform_relocation, (bfd * abfd, arelent *reloc_entry, PTR data, asection *input_section, bfd *output_bfd)); */ bfd_reloc_status_enum_type DEFUN(bfd_perform_relocation,(abfd, reloc_entry, data, input_section, output_bfd), bfd *abfd AND arelent *reloc_entry AND PTR data AND asection *input_section AND bfd *output_bfd) { bfd_vma relocation; bfd_reloc_status_enum_type flag = bfd_reloc_ok; bfd_vma addr = reloc_entry->address ; bfd_vma output_base = 0; reloc_howto_type *howto = reloc_entry->howto; asection *reloc_target_output_section; asection *reloc_target_input_section; asymbol *symbol; if (reloc_entry->sym_ptr_ptr) { symbol = *( reloc_entry->sym_ptr_ptr); if ((symbol->flags & BSF_UNDEFINED) && output_bfd == (bfd *)NULL) { flag = bfd_reloc_undefined; } } else { symbol = (asymbol*)NULL; } if (howto->special_function){ bfd_reloc_status_enum_type cont; cont = howto->special_function(abfd, reloc_entry, symbol, data, input_section); if (cont != bfd_reloc_continue) return cont; } /* Work out which section the relocation is targetted at and the initial relocation command value. */ if (symbol != (asymbol *)NULL){ if (symbol->flags & BSF_FORT_COMM) { relocation = 0; } else { relocation = symbol->value; } if (symbol->section != (asection *)NULL) { reloc_target_input_section = symbol->section; } else { reloc_target_input_section = (asection *)NULL; } } else if (reloc_entry->section != (asection *)NULL) { relocation = 0; reloc_target_input_section = reloc_entry->section; } else { relocation = 0; reloc_target_input_section = (asection *)NULL; } if (reloc_target_input_section != (asection *)NULL) { reloc_target_output_section = reloc_target_input_section->output_section; if (output_bfd && howto->partial_inplace==false) { output_base = 0; } else { output_base = reloc_target_output_section->vma; } relocation += output_base + reloc_target_input_section->output_offset; } relocation += reloc_entry->addend ; if(reloc_entry->address > (bfd_vma)(input_section->size)) { return bfd_reloc_outofrange; } if (howto->pc_relative == true) { /* Anything which started out as pc relative should end up that way too. There are two ways we can see a pcrel instruction. Sometimes the pcrel displacement has been partially calculated, it includes the distance from the start of the section to the instruction in it (eg sun3), and sometimes the field is totally blank - eg m88kbcs. */ relocation -= output_base + input_section->output_offset; if (howto->pcrel_offset == true) { relocation -= reloc_entry->address; } } if (output_bfd!= (bfd *)NULL) { if ( howto->partial_inplace == false) { /* This is a partial relocation, and we want to apply the relocation to the reloc entry rather than the raw data. Modify the reloc inplace to reflect what we now know. */ reloc_entry->addend = relocation ; reloc_entry->section = reloc_target_input_section; if (reloc_target_input_section != (asection *)NULL) { /* If we know the output section we can forget the symbol */ reloc_entry->sym_ptr_ptr = (asymbol**)NULL; } reloc_entry->address += input_section->output_offset; return flag; } else { /* This is a partial relocation, but inplace, so modify the reloc record a bit */ } } reloc_entry->addend = 0; /* Either we are relocating all the way, or we don't want to apply the relocation to the reloc entry (probably because there isn't any room in the output format to describe addends to relocs) */ relocation >>= howto->rightshift; /* Shift everything up to where it's going to be used */ relocation <<= howto->bitpos; /* Wait for the day when all have the mask in them */ /* What we do: i instruction to be left alone o offset within instruction r relocation offset to apply S src mask D dst mask N ~dst mask A part 1 B part 2 R result Do this: i i i i i o o o o o from bfd_get and S S S S S to get the size offset we want + r r r r r r r r r r to get the final value to place and D D D D D to chop to right size ----------------------- A A A A A And this: ... i i i i i o o o o o from bfd_get and N N N N N get instruction ----------------------- ... B B B B B And then: B B B B B or A A A A A ----------------------- R R R R R R R R R R put into bfd_put */ #define DOIT(x) \ x = ( (x & ~howto->dst_mask) | (((x & howto->src_mask) + relocation) & howto->dst_mask)) switch (howto->size) { case 0: { char x = bfd_get_8(abfd, (char *)data + addr); DOIT(x); bfd_put_8(abfd,x, (unsigned char *) data + addr); } break; case 1: { short x = bfd_get_16(abfd, (bfd_byte *)data + addr); DOIT(x); bfd_put_16(abfd, x, (unsigned char *)data + addr); } break; case 2: { long x = bfd_get_32(abfd, (bfd_byte *) data + addr); DOIT(x); bfd_put_32(abfd,x, (bfd_byte *)data + addr); } break; case 3: /* Do nothing */ break; default: return bfd_reloc_other; } return flag; }