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613 lines
17 KiB
C
613 lines
17 KiB
C
/*doc*
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@section Relocations
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Bfd maintains relocations in much the same was as it maintains
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symbols; they are left alone until required, then read in en-mass and
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traslated into an internal form. There is a common routine
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@code{bfd_perform_relocation} which acts upon the canonical form to to
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the actual fixup.
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Note that relocations are maintained on a per section basis, whilst
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symbols are maintained on a per bfd basis.
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All a back end has to do to fit the bfd interface is to create as many
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@code{struct reloc_cache_entry} as there are relocations in a
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particuar section, and fill in the right bits:
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@menu
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* typedef arelent::
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* reloc handling functions::
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@end menu
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*/
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#include "sysdep.h"
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#include "bfd.h"
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#include "libbfd.h"
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/*doc
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*node typedef arelent, Relocations, reloc handling functions, Relocations
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@section typedef arelent
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*/
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/*proto* bfd_perform_relocation
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The relocation routine returns as a status an enumerated type:
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*+++
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$typedef enum bfd_reloc_status {
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No errors detected
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$ bfd_reloc_ok,
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The relocation was performed, but there was an overflow.
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$ bfd_reloc_overflow,
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The address to relocate was not within the section supplied
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$ bfd_reloc_outofrange,
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Used by special functions
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$ bfd_reloc_continue,
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Unused
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$ bfd_reloc_notsupported,
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Unsupported relocation size requested.
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$ bfd_reloc_other,
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The symbol to relocate against was undefined.
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$ bfd_reloc_undefined,
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The relocaction was performed, but may not be ok - presently generated
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only when linking i960 coff files with i960 b.out symbols.
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$ bfd_reloc_dangerous
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$ }
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$ bfd_reloc_status_enum_type;
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*---
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*/
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/*proto*
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*+++
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$typedef struct reloc_cache_entry
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${
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A pointer into the canonical table of pointers
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$ struct symbol_cache_entry **sym_ptr_ptr;
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offset in section
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$ rawdata_offset address;
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addend for relocation value
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$ bfd_vma addend;
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if sym is null this is the section
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$ struct sec *section;
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Pointer to how to perform the required relocation
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$ struct reloc_howto_struct *howto;
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$} arelent;
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*---
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*/
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/*doc*
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@table @code
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@item sym_ptr_ptr
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The symbol table pointer points to a pointer to the symbol ascociated with the
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relocation request. This would naturaly be the pointer into the table
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returned by the back end's get_symtab action. @xref{Symbols}. The
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symbol is referenced through a pointer to a pointer so that tools like
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the linker can fixup all the symbols of the same name by modifying
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only one pointer. The relocation routine looks in the symbol and uses
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the base of the section the symbol is attatched to and the value of
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the symbol as the initial relocation offset. If the symbol pointer is
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zero, then the section provided is looked up.
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@item address
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The address field gives the offset in bytes from the base of the
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section data which owns the relocation record to the first byte of
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relocatable information. The actual data relocated will be relative to
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this point - for example, a relocation type which modifies the bottom
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two bytes of a four byte word would not touch the first byte pointed
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to in a big endian world.
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@item addend
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The addend is a value provided by the back end to be added (!) to the
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relocation offset. It's interpretation is dependent upon the howto.
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For example, on the 68k the code:
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*+
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char foo[];
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main()
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{
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return foo[0x12345678];
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}
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*-
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Could be compiled into:
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*+
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linkw fp,#-4
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moveb @@#12345678,d0
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extbl d0
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unlk fp
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rts
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*-
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This could create a reloc pointing to foo, but leave the offset in the data
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(something like)
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*+
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RELOCATION RECORDS FOR [.text]:
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OFFSET TYPE VALUE
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00000006 32 _foo
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00000000 4e56 fffc ; linkw fp,#-4
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00000004 1039 1234 5678 ; moveb @@#12345678,d0
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0000000a 49c0 ; extbl d0
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0000000c 4e5e ; unlk fp
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0000000e 4e75 ; rts
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*-
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Using coff and an 88k, some instructions don't have enough space in them to
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represent the full address range, and pointers have to be loaded in
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two parts. So you'd get something like:
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*+
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or.u r13,r0,hi16(_foo+0x12345678)
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ld.b r2,r13,lo16(_foo+0x12345678)
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jmp r1
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*-
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This whould create two relocs, both pointing to _foo, and with 0x12340000
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in their addend field. The data would consist of:
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*+
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RELOCATION RECORDS FOR [.text]:
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OFFSET TYPE VALUE
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00000002 HVRT16 _foo+0x12340000
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00000006 LVRT16 _foo+0x12340000
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00000000 5da05678 ; or.u r13,r0,0x5678
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00000004 1c4d5678 ; ld.b r2,r13,0x5678
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00000008 f400c001 ; jmp r1
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*-
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The relocation routine digs out the value from the data, adds it to
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the addend to get the original offset and then adds the value of _foo.
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Note that all 32 bits have to be kept around somewhere, to cope with
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carry from bit 15 to bit 16.
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On further example is the sparc and the a.out format. The sparc has a
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similar problem to the 88k, in that some instructions don't have
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room for an entire offset, but on the sparc the parts are created odd
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sized lumps. The designers of the a.out format chose not to use the
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data within the section for storing part of the offset; all the offset
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is kept within the reloc. Any thing in the data should be ignored.
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*+
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save %sp,-112,%sp
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sethi %hi(_foo+0x12345678),%g2
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ldsb [%g2+%lo(_foo+0x12345678)],%i0
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ret
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restore
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*-
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Both relocs contains a pointer to foo, and the offsets would contain junk.
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*+
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RELOCATION RECORDS FOR [.text]:
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OFFSET TYPE VALUE
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00000004 HI22 _foo+0x12345678
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00000008 LO10 _foo+0x12345678
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00000000 9de3bf90 ; save %sp,-112,%sp
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00000004 05000000 ; sethi %hi(_foo+0),%g2
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00000008 f048a000 ; ldsb [%g2+%lo(_foo+0)],%i0
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0000000c 81c7e008 ; ret
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00000010 81e80000 ; restore
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*-
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@item section
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The section field is only used when the symbol pointer field is null.
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It supplies the section into which the data should be relocated. The
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field's main use comes from assemblers which do most of the symbol fixups
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themselves; an assembler may take an internal reference to a label,
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but since it knows where the label is, it can turn the relocation
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request from a symbol lookup into a section relative relocation - the
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relocation emitted has no symbol, just a section to relocate against.
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I'm not sure what it means when both a symbol pointer an a section
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pointer are present. Some formats use this sort of mechanism to
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describe PIC relocations, but bfd can't to that sort of thing yet.
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@item howto
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The howto field can be imagined as a relocation instruction. It is a
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pointer to a struct which contains information on what to do with all
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the other information in the reloc record and data section. A back end
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would normally have a relocation instruction set and turn relocations
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into pointers to the correct structure on input - but it would be
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possible to create each howto field on demand.
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@end table
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*/
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/*proto* reloc_howto_type
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The @code{reloc_howto_type} is a structure which contains all the
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information that bfd needs to know to tie up a back end's data.
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*+++
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$typedef CONST struct reloc_howto_struct
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${
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The type field has mainly a documetary use - the back end can to what
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it wants with it, though the normally the back end's external idea of
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what a reloc number would be would be stored in this field. For
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example, the a PC relative word relocation in a coff environment would
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have the type 023 - because that's what the outside world calls a
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R_PCRWORD reloc.
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$ unsigned int type;
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The value the final relocation is shifted right by. This drops
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unwanted data from the relocation.
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$ unsigned int rightshift;
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The size of the item to be relocated - 0, is one byte, 1 is 2 bytes, 3
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is four bytes.
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$ unsigned int size;
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Now obsolete
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$ unsigned int bitsize;
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Notes that the relocation is relative to the location in the data
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section of the addend. The relocation function will subtract from the
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relocation value the address of the location being relocated.
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$ boolean pc_relative;
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Now obsolete
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$ unsigned int bitpos;
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Now obsolete
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$ boolean absolute;
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Causes the relocation routine to return an error if overflow is
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detected when relocating.
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$ boolean complain_on_overflow;
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If this field is non null, then the supplied function is called rather
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than the normal function. This allows really strange relocation
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methods to be accomodated (eg, i960 callj instructions).
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$ bfd_reloc_status_enum_type (*special_function)();
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The textual name of the relocation type.
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$ char *name;
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When performing a partial link, some formats must modify the
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relocations rather than the data - this flag signals this.
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$ boolean partial_inplace;
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The src_mask is used to select what parts of the read in data are to
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be used in the relocation sum. Eg, if this was an 8 bit bit of data
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which we read and relocated, this would be 0x000000ff. When we have
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relocs which have an addend, such as sun4 extended relocs, the value
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in the offset part of a relocating field is garbage so we never use
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it. In this case the mask would be 0x00000000.
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$ bfd_word src_mask;
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The dst_mask is what parts of the instruction are replaced into the
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instruction. In most cases src_mask == dst_mask, except in the above
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special case, where dst_mask would be 0x000000ff, and src_mask would
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be 0x00000000.
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$ bfd_word dst_mask;
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When some formats create PC relative instructions, they leave the
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value of the pc of the place being relocated in the offset slot of the
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instruction, so that a PC relative relocation can be made just by
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adding in an ordinary offset (eg sun3 a.out). Some formats leave the
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displacement part of an instruction empty (eg m88k bcs), this flag
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signals the fact.
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$ boolean pcrel_offset;
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$} reloc_howto_type;
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*---
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*/
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/*proto* HOWTO
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The HOWTO define is horrible and will go away.
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*+
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#define HOWTO(C, R,S,B, P, BI, ABS, O, SF, NAME, INPLACE, MASKSRC, MASKDST, PC) \
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{(unsigned)C,R,S,B, P, BI, ABS,O,SF,NAME,INPLACE,MASKSRC,MASKDST,PC}
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*-
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*/
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/*proto* reloc_chain
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*+
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typedef unsigned char bfd_byte;
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typedef struct relent_chain {
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arelent relent;
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struct relent_chain *next;
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} arelent_chain;
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*-
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*/
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/*proto*
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If an output_bfd is supplied to this function the generated image
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will be relocatable, the relocations are copied to the output file
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after they have been changed to reflect the new state of the world.
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There are two ways of reflecting the results of partial linkage in an
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output file; by modifying the output data in place, and by modifying
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the relocation record. Some native formats (eg basic a.out and basic
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coff) have no way of specifying an addend in the relocation type, so
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the addend has to go in the output data. This is no big deal since in
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these formats the output data slot will always be big enough for the
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addend. Complex reloc types with addends were invented to solve just
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this problem.
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*; PROTO(bfd_reloc_status_enum_type,
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bfd_perform_relocation,
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(bfd * abfd,
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arelent *reloc_entry,
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PTR data,
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asection *input_section,
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bfd *output_bfd));
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*/
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bfd_reloc_status_enum_type
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DEFUN(bfd_perform_relocation,(abfd,
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reloc_entry,
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data,
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input_section,
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output_bfd),
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bfd *abfd AND
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arelent *reloc_entry AND
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PTR data AND
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asection *input_section AND
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bfd *output_bfd)
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{
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bfd_vma relocation;
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bfd_reloc_status_enum_type flag = bfd_reloc_ok;
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bfd_vma addr = reloc_entry->address ;
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bfd_vma output_base = 0;
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reloc_howto_type *howto = reloc_entry->howto;
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asection *reloc_target_output_section;
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asection *reloc_target_input_section;
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asymbol *symbol;
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if (reloc_entry->sym_ptr_ptr) {
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symbol = *( reloc_entry->sym_ptr_ptr);
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if ((symbol->flags & BSF_UNDEFINED) && output_bfd == (bfd *)NULL) {
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flag = bfd_reloc_undefined;
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}
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}
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else {
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symbol = (asymbol*)NULL;
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}
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if (howto->special_function){
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bfd_reloc_status_enum_type cont;
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cont = howto->special_function(abfd,
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reloc_entry,
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symbol,
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data,
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input_section);
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if (cont != bfd_reloc_continue) return cont;
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}
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/*
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Work out which section the relocation is targetted at and the
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initial relocation command value.
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*/
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if (symbol != (asymbol *)NULL){
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if (symbol->flags & BSF_FORT_COMM) {
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relocation = 0;
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}
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else {
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relocation = symbol->value;
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}
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if (symbol->section != (asection *)NULL)
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{
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reloc_target_input_section = symbol->section;
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}
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else {
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reloc_target_input_section = (asection *)NULL;
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}
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}
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else if (reloc_entry->section != (asection *)NULL)
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{
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relocation = 0;
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reloc_target_input_section = reloc_entry->section;
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}
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else {
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relocation = 0;
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reloc_target_input_section = (asection *)NULL;
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}
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if (reloc_target_input_section != (asection *)NULL) {
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reloc_target_output_section =
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reloc_target_input_section->output_section;
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if (output_bfd && howto->partial_inplace==false) {
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output_base = 0;
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}
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else {
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output_base = reloc_target_output_section->vma;
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}
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relocation += output_base + reloc_target_input_section->output_offset;
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}
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relocation += reloc_entry->addend ;
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if(reloc_entry->address > (bfd_vma)(input_section->size))
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{
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return bfd_reloc_outofrange;
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}
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if (howto->pc_relative == true)
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{
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/*
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Anything which started out as pc relative should end up that
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way too.
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There are two ways we can see a pcrel instruction. Sometimes
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the pcrel displacement has been partially calculated, it
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includes the distance from the start of the section to the
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instruction in it (eg sun3), and sometimes the field is
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totally blank - eg m88kbcs.
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*/
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relocation -=
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output_base + input_section->output_offset;
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if (howto->pcrel_offset == true) {
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relocation -= reloc_entry->address;
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}
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}
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if (output_bfd!= (bfd *)NULL) {
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if ( howto->partial_inplace == false) {
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/*
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This is a partial relocation, and we want to apply the relocation
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to the reloc entry rather than the raw data. Modify the reloc
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inplace to reflect what we now know.
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*/
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reloc_entry->addend = relocation ;
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reloc_entry->section = reloc_target_input_section;
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if (reloc_target_input_section != (asection *)NULL) {
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/* If we know the output section we can forget the symbol */
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reloc_entry->sym_ptr_ptr = (asymbol**)NULL;
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}
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reloc_entry->address +=
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input_section->output_offset;
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return flag;
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}
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else
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{
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/* This is a partial relocation, but inplace, so modify the
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reloc record a bit
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*/
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}
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}
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reloc_entry->addend = 0;
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/*
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Either we are relocating all the way, or we don't want to apply
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the relocation to the reloc entry (probably because there isn't
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any room in the output format to describe addends to relocs)
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*/
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relocation >>= howto->rightshift;
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/* Shift everything up to where it's going to be used */
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relocation <<= howto->bitpos;
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/* Wait for the day when all have the mask in them */
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/* What we do:
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i instruction to be left alone
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o offset within instruction
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r relocation offset to apply
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S src mask
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D dst mask
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N ~dst mask
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A part 1
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B part 2
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R result
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Do this:
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i i i i i o o o o o from bfd_get<size>
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and S S S S S to get the size offset we want
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+ r r r r r r r r r r to get the final value to place
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and D D D D D to chop to right size
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-----------------------
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A A A A A
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|
And this:
|
|
... i i i i i o o o o o from bfd_get<size>
|
|
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<size>
|
|
*/
|
|
|
|
#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;
|
|
}
|