195 lines
7.1 KiB
Plaintext
195 lines
7.1 KiB
Plaintext
$FreeBSD$
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This document describes some of the machine dependent parts in ld(1) and rtld(?)
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Most of the machine dependencies are a result of different ways in which
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relocation information is conveyed in an architecture's object files.
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Especially RISC architectures are likely candidates to have deviated from the
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traditional relocation record structure due a tendency to use odd sized
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"bitfields" to denote immediate operands within their fixed size instructions.
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Also, there may be slight differences in the model used for Position
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Independent Code generation by the compiler.
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Lastly, both ld and rtld must fiddle with actual machine instructions to setup
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a transfer vector to accommodate PIC code and dynamic linking.
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Machine dependent macros and data structures.
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typedef struct jmpslot { ... } jmpslot_t;
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The Procedure Linkage Table (PLT) is an array of these structures.
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The structure contains room for a control transfer instruction
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and a relocation index. md_make_jmpslot() and md_fix_jmpslot()
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are responsible for filling these in.
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JMPSLOT_NEEDS_RELOC()
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Macro indicating whether or not a jmpslot entry needs a run-time
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relocation when ld has already resolved the symbolic reference
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(eg. when `-Bsymbolic' was given). Usually the case if the control
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transfer instruction is PC relative or something.
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RELOC_STATICS_THROUGH_GOT_P(r)
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Predicate taking a `struct relocation_info *' as an argument. It
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decides whether variables with file scope are addressed relative to
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the start Global Offset Table (1) or an entry in GOT must be
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allocated (0). The compiler has a say in this.
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Some other random macros:
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BAD_MID(ex)
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Tells whether the machine ID in an input file header is acceptable.
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N_SET_FLAG(ex,f)
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Set flags F in a.out header. Must account for possible non-NetBSD
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headers; QMAGIC is still a documented ld output format.
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N_IS_DYNAMIC(ex)
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Return true if this appears to be a dynamically linked object.
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#define relocation_info reloc_info_<machine>
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Define (possibly machine dependent) relocation record format.
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Should convert to a typedef someday for greater opacity.
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md_got_reloc(r)
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Adjustment to be applied to the relocation value of references
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to "_GLOBAL_OFFSET_TABLE". It's here because of Sun's sparc as(1),
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(it's is a *bug*), and could have been `#ifdef SUN_COMPAT' if I
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had not let it slip into NetBSD's gas for compatibility.
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md_get_rt_segment_addend(r,a)
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Another SunOS bug workaround.
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The macros below have defaults defined in ld.h for the traditional relocation
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structure format; each takes a `struct relocation_info *' argument (see
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ld.h for more detailed comments):
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RELOC_ADDRESS(r) - the address at which to relocate
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RELOC_EXTERN_P(r) - relocation for external symbol
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RELOC_TYPE(r) - segment (text/data/bss), non-external relocs
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RELOC_SYMBOL(r) - symbol index, external relocs
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RELOC_MEMORY_SUB_P(r) - relocation involves something to subtract
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RELOC_MEMORY_ADD_P(r) - relocation involves something to add
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RELOC_ADD_EXTRA(r) - <disused> (moved into MD files)
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RELOC_PCREL_P(r) - relocation is PC relative
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RELOC_VALUE_RIGHTSHIFT(r) - <disused>
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RELOC_TARGET_SIZE(r) - size (in bytes) of relocated value
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RELOC_TARGET_BITPOS(r) - <disused> (moved into MD files)
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RELOC_TARGET_BITSIZE(r) - <disused> (moved into MD files)
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RELOC_JMPTAB_P(r) - relocation is for a PLT entry
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RELOC_BASEREL_P(r) - relocation is for a GOT entry
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RELOC_RELATIVE_P(r) - relocation is load address relative
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RELOC_COPY_P(r) - relocation involves an initialization
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RELOC_INIT_SEGMENT_RELOC(r) - initialize a relocation record pertaining to
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a segment; traditional archs can use bzero().
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RELOC_LAZY_P(r) - (run-time) resolution can be lazy.
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CHECK_GOT_RELOC(r) - consistency check on relocations involving
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the "_GLOBAL_OFFSET_TABLE" symbol
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Things which are currently defined as routines in <machine>/md.c:
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md_init_header(struct exec *hp, int magic, int flags)
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Initializes the output file header. Straightforward, unless
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multiple OS'es are supported.
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md_swap*_exec_hdr(struct exec *)
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Input/output a.out header in target byte-order.
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md_swap*_reloc(struct relocation_info *, n)
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Input/output N relocation records in target byte-order.
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md_get_addend(struct relocation_info *rp, char *addr)
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Return a relocation addend. Traditionally found in the object file
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at address ADDR. The relocation record determines the data width
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in bytes (using RELOC_TARGET_SIZE()).
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md_relocate(struct relocation_info *rp, long reloc, char *addr,
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int relocatable_output)
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Perform a relocation at the given address, usually by entering
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the specified value RELOC into the object file. Some architectures
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may store the relocation in RP when RELOCATABLE_OUTPUT is set.
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Again, the byte size of the relocation value is determined from
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RP through RELOC_TARGET_SIZE().
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md_make_reloc(struct relocation_info *rp, int type)
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Construct the machine dependent part of a relocation record used
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for run-time relocation. Sets RP's type field or one or more
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bitfields according to TYPE which is ld's internal relocation
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representation of the type of (run-time) relocation. TYPE is a
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combination of the following bits:
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RELTYPE_EXTERN - relocation is for unresolved symbol
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RELTYPE_JMPSLOT - relocation is for a PLT entry
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RELTYPE_BASEREL - <not used>
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RELTYPE_RELATIVE - relocation is load address relative
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RELTYPE_COPY - relocation is an initalization
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md_make_jmpreloc(struct relocation_info *rp, *r, int type)
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Set up a run-time relocation record pertaining to a jmpslot.
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This usually sets a bit or a relocation type dedicated to jmpslot
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relocations. R is the relocation record to be updated (ie. the
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run-time relocation record), while RP points at the relocation
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record from the object file on behalf of which we allocated a
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PLT entry. RP may not be needed.
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md_make_gotreloc(struct relocation_info *rp, *r, int type)
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Set up a run-time relocation record pertaining to a GOT entry.
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This usually sets a bit or a relocation type dedicated to GOT
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relocations. R is the relocation record to be updated (ie. the
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run-time relocation record), while RP points at the relocation
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record from the object file on behalf of which we allocated a
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GOT entry.
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md_make_cpyreloc(struct relocation_info *rp, *r)
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Mark the relocation record as pertaining to a location that needs
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run-time initializing from some shared object source.
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R and RP same as above.
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md_make_jmpslot(jmpslot_t *sp, long offset, long index)
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Construct a jmpslot, ie. fill in the machine opcodes that comprise
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a transfer to slot 0 of the PLT. OFFSET is the location of SP
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relative to the start of the PLT (ie. (int)sp - (int)&PLT[0] ).
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INDEX is the entry in the run-time relocation record table which
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determines what symbol this jmpslot is supposed to resolve.
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md_fix_jmpslot(jmpslot_t *sp, long offset, long addr)
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Fix up a jmpslot so that it will transfer directly to ADDR
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in stead of to PLT[0]. OFFSET has the same meaning as in
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md_make_jmpslot(). This function is called by binder() after
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it has resolved a symbol into a (run-time) address.
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md_set_breakpoint(long where, long *savep)
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Set a breakpoint. Used when run under a debugger. The breakpoint
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instruction is to be set at location WHERE. The original contents
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of *WHERE is to be saved in *SAVEP.
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