asm.go

// Inferno utils/6l/asm.c
// https://bitbucket.org/inferno-os/inferno-os/src/master/utils/6l/asm.c
//
//	Copyright © 1994-1999 Lucent Technologies Inc.  All rights reserved.
//	Portions Copyright © 1995-1997 C H Forsyth (forsyth@terzarima.net)
//	Portions Copyright © 1997-1999 Vita Nuova Limited
//	Portions Copyright © 2000-2007 Vita Nuova Holdings Limited (www.vitanuova.com)
//	Portions Copyright © 2004,2006 Bruce Ellis
//	Portions Copyright © 2005-2007 C H Forsyth (forsyth@terzarima.net)
//	Revisions Copyright © 2000-2007 Lucent Technologies Inc. and others
//	Portions Copyright © 2009 The Go Authors. All rights reserved.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.

package amd64

import (
	"cmd/internal/objabi"
	"cmd/internal/sys"
	"cmd/link/internal/ld"
	"cmd/link/internal/loader"
	"cmd/link/internal/sym"
	"debug/elf"
	"log"
)

func PADDR(x uint32) uint32 {
	return x &^ 0x80000000
}

func gentext(ctxt *ld.Link, ldr *loader.Loader) {
	initfunc, addmoduledata := ld.PrepareAddmoduledata(ctxt)
	if initfunc == nil {
		return
	}

	o := func(op ...uint8) {
		for _, op1 := range op {
			initfunc.AddUint8(op1)
		}
	}

	// 0000000000000000 <local.dso_init>:
	//    0:	48 8d 3d 00 00 00 00 	lea    0x0(%rip),%rdi        # 7 <local.dso_init+0x7>
	// 			3: R_X86_64_PC32	runtime.firstmoduledata-0x4
	o(0x48, 0x8d, 0x3d)
	initfunc.AddPCRelPlus(ctxt.Arch, ctxt.Moduledata, 0)
	//    7:	e8 00 00 00 00       	callq  c <local.dso_init+0xc>
	// 			8: R_X86_64_PLT32	runtime.addmoduledata-0x4
	o(0xe8)
	initfunc.AddSymRef(ctxt.Arch, addmoduledata, 0, objabi.R_CALL, 4)
	//    c:	c3                   	retq
	o(0xc3)
}

func adddynrel(target *ld.Target, ldr *loader.Loader, syms *ld.ArchSyms, s loader.Sym, r loader.Reloc, rIdx int) bool {
	targ := r.Sym()
	var targType sym.SymKind
	if targ != 0 {
		targType = ldr.SymType(targ)
	}

	switch rt := r.Type(); rt {
	default:
		if rt >= objabi.ElfRelocOffset {
			ldr.Errorf(s, "unexpected relocation type %d (%s)", r.Type(), sym.RelocName(target.Arch, r.Type()))
			return false
		}

		// Handle relocations found in ELF object files.
	case objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_PC32):
		if targType == sym.SDYNIMPORT {
			ldr.Errorf(s, "unexpected R_X86_64_PC32 relocation for dynamic symbol %s", ldr.SymName(targ))
		}
		if targType == 0 || targType == sym.SXREF {
			ldr.Errorf(s, "unknown symbol %s in pcrel", ldr.SymName(targ))
		}
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_PCREL)
		su.SetRelocAdd(rIdx, r.Add()+4)
		return true

	case objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_PC64):
		if targType == sym.SDYNIMPORT {
			ldr.Errorf(s, "unexpected R_X86_64_PC64 relocation for dynamic symbol %s", ldr.SymName(targ))
		}
		if targType == 0 || targType == sym.SXREF {
			ldr.Errorf(s, "unknown symbol %s in pcrel", ldr.SymName(targ))
		}
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_PCREL)
		su.SetRelocAdd(rIdx, r.Add()+8)
		return true

	case objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_PLT32):
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_PCREL)
		su.SetRelocAdd(rIdx, r.Add()+4)
		if targType == sym.SDYNIMPORT {
			addpltsym(target, ldr, syms, targ)
			su.SetRelocSym(rIdx, syms.PLT)
			su.SetRelocAdd(rIdx, r.Add()+int64(ldr.SymPlt(targ)))
		}

		return true

	case objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_GOTPCREL),
		objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_GOTPCRELX),
		objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_REX_GOTPCRELX):
		su := ldr.MakeSymbolUpdater(s)
		if targType != sym.SDYNIMPORT {
			// have symbol
			sData := ldr.Data(s)
			if r.Off() >= 2 && sData[r.Off()-2] == 0x8b {
				su.MakeWritable()
				// turn MOVQ of GOT entry into LEAQ of symbol itself
				writeableData := su.Data()
				writeableData[r.Off()-2] = 0x8d
				su.SetRelocType(rIdx, objabi.R_PCREL)
				su.SetRelocAdd(rIdx, r.Add()+4)
				return true
			}
		}

		// fall back to using GOT and hope for the best (CMOV*)
		// TODO: just needs relocation, no need to put in .dynsym
		ld.AddGotSym(target, ldr, syms, targ, uint32(elf.R_X86_64_GLOB_DAT))

		su.SetRelocType(rIdx, objabi.R_PCREL)
		su.SetRelocSym(rIdx, syms.GOT)
		su.SetRelocAdd(rIdx, r.Add()+4+int64(ldr.SymGot(targ)))
		return true

	case objabi.ElfRelocOffset + objabi.RelocType(elf.R_X86_64_64):
		if targType == sym.SDYNIMPORT {
			ldr.Errorf(s, "unexpected R_X86_64_64 relocation for dynamic symbol %s", ldr.SymName(targ))
		}
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_ADDR)
		if target.IsPIE() && target.IsInternal() {
			// For internal linking PIE, this R_ADDR relocation cannot
			// be resolved statically. We need to generate a dynamic
			// relocation. Let the code below handle it.
			break
		}
		return true

	// Handle relocations found in Mach-O object files.
	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_UNSIGNED*2 + 0,
		objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_SIGNED*2 + 0,
		objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_BRANCH*2 + 0:
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_ADDR)

		if targType == sym.SDYNIMPORT {
			ldr.Errorf(s, "unexpected reloc for dynamic symbol %s", ldr.SymName(targ))
		}
		if target.IsPIE() && target.IsInternal() {
			// For internal linking PIE, this R_ADDR relocation cannot
			// be resolved statically. We need to generate a dynamic
			// relocation. Let the code below handle it.
			if rt == objabi.MachoRelocOffset+ld.MACHO_X86_64_RELOC_UNSIGNED*2 {
				break
			} else {
				// MACHO_X86_64_RELOC_SIGNED or MACHO_X86_64_RELOC_BRANCH
				// Can this happen? The object is expected to be PIC.
				ldr.Errorf(s, "unsupported relocation for PIE: %v", rt)
			}
		}
		return true

	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_BRANCH*2 + 1:
		if targType == sym.SDYNIMPORT {
			addpltsym(target, ldr, syms, targ)
			su := ldr.MakeSymbolUpdater(s)
			su.SetRelocSym(rIdx, syms.PLT)
			su.SetRelocType(rIdx, objabi.R_PCREL)
			su.SetRelocAdd(rIdx, int64(ldr.SymPlt(targ)))
			return true
		}
		fallthrough

	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_UNSIGNED*2 + 1,
		objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_SIGNED*2 + 1,
		objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_SIGNED_1*2 + 1,
		objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_SIGNED_2*2 + 1,
		objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_SIGNED_4*2 + 1:
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_PCREL)

		if targType == sym.SDYNIMPORT {
			ldr.Errorf(s, "unexpected pc-relative reloc for dynamic symbol %s", ldr.SymName(targ))
		}
		return true

	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_GOT_LOAD*2 + 1:
		if targType != sym.SDYNIMPORT {
			// have symbol
			// turn MOVQ of GOT entry into LEAQ of symbol itself
			sdata := ldr.Data(s)
			if r.Off() < 2 || sdata[r.Off()-2] != 0x8b {
				ldr.Errorf(s, "unexpected GOT_LOAD reloc for non-dynamic symbol %s", ldr.SymName(targ))
				return false
			}

			su := ldr.MakeSymbolUpdater(s)
			su.MakeWritable()
			sdata = su.Data()
			sdata[r.Off()-2] = 0x8d
			su.SetRelocType(rIdx, objabi.R_PCREL)
			return true
		}
		fallthrough

	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_GOT*2 + 1:
		if targType != sym.SDYNIMPORT {
			ldr.Errorf(s, "unexpected GOT reloc for non-dynamic symbol %s", ldr.SymName(targ))
		}
		ld.AddGotSym(target, ldr, syms, targ, 0)
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocType(rIdx, objabi.R_PCREL)
		su.SetRelocSym(rIdx, syms.GOT)
		su.SetRelocAdd(rIdx, r.Add()+int64(ldr.SymGot(targ)))
		return true
	}

	// Reread the reloc to incorporate any changes in type above.
	relocs := ldr.Relocs(s)
	r = relocs.At(rIdx)

	switch r.Type() {
	case objabi.R_CALL:
		if targType != sym.SDYNIMPORT {
			// nothing to do, the relocation will be laid out in reloc
			return true
		}
		if target.IsExternal() {
			// External linker will do this relocation.
			return true
		}
		// Internal linking, for both ELF and Mach-O.
		// Build a PLT entry and change the relocation target to that entry.
		addpltsym(target, ldr, syms, targ)
		su := ldr.MakeSymbolUpdater(s)
		su.SetRelocSym(rIdx, syms.PLT)
		su.SetRelocAdd(rIdx, int64(ldr.SymPlt(targ)))
		return true

	case objabi.R_PCREL:
		if targType == sym.SDYNIMPORT && ldr.SymType(s) == sym.STEXT && target.IsDarwin() {
			// Loading the address of a dynamic symbol. Rewrite to use GOT.
			// turn LEAQ symbol address to MOVQ of GOT entry
			if r.Add() != 0 {
				ldr.Errorf(s, "unexpected nonzero addend for dynamic symbol %s", ldr.SymName(targ))
				return false
			}
			su := ldr.MakeSymbolUpdater(s)
			if r.Off() >= 2 && su.Data()[r.Off()-2] == 0x8d {
				su.MakeWritable()
				su.Data()[r.Off()-2] = 0x8b
				if target.IsInternal() {
					ld.AddGotSym(target, ldr, syms, targ, 0)
					su.SetRelocSym(rIdx, syms.GOT)
					su.SetRelocAdd(rIdx, int64(ldr.SymGot(targ)))
				} else {
					su.SetRelocType(rIdx, objabi.R_GOTPCREL)
				}
				return true
			}
			ldr.Errorf(s, "unexpected R_PCREL reloc for dynamic symbol %s: not preceded by LEAQ instruction", ldr.SymName(targ))
		}

	case objabi.R_ADDR:
		if ldr.SymType(s) == sym.STEXT && target.IsElf() {
			su := ldr.MakeSymbolUpdater(s)
			if target.IsSolaris() {
				addpltsym(target, ldr, syms, targ)
				su.SetRelocSym(rIdx, syms.PLT)
				su.SetRelocAdd(rIdx, r.Add()+int64(ldr.SymPlt(targ)))
				return true
			}
			// The code is asking for the address of an external
			// function. We provide it with the address of the
			// correspondent GOT symbol.
			ld.AddGotSym(target, ldr, syms, targ, uint32(elf.R_X86_64_GLOB_DAT))

			su.SetRelocSym(rIdx, syms.GOT)
			su.SetRelocAdd(rIdx, r.Add()+int64(ldr.SymGot(targ)))
			return true
		}

		// Process dynamic relocations for the data sections.
		if target.IsPIE() && target.IsInternal() {
			// When internally linking, generate dynamic relocations
			// for all typical R_ADDR relocations. The exception
			// are those R_ADDR that are created as part of generating
			// the dynamic relocations and must be resolved statically.
			//
			// There are three phases relevant to understanding this:
			//
			//	dodata()  // we are here
			//	address() // symbol address assignment
			//	reloc()   // resolution of static R_ADDR relocs
			//
			// At this point symbol addresses have not been
			// assigned yet (as the final size of the .rela section
			// will affect the addresses), and so we cannot write
			// the Elf64_Rela.r_offset now. Instead we delay it
			// until after the 'address' phase of the linker is
			// complete. We do this via Addaddrplus, which creates
			// a new R_ADDR relocation which will be resolved in
			// the 'reloc' phase.
			//
			// These synthetic static R_ADDR relocs must be skipped
			// now, or else we will be caught in an infinite loop
			// of generating synthetic relocs for our synthetic
			// relocs.
			//
			// Furthermore, the rela sections contain dynamic
			// relocations with R_ADDR relocations on
			// Elf64_Rela.r_offset. This field should contain the
			// symbol offset as determined by reloc(), not the
			// final dynamically linked address as a dynamic
			// relocation would provide.
			switch ldr.SymName(s) {
			case ".dynsym", ".rela", ".rela.plt", ".got.plt", ".dynamic":
				return false
			}
		} else {
			// Either internally linking a static executable,
			// in which case we can resolve these relocations
			// statically in the 'reloc' phase, or externally
			// linking, in which case the relocation will be
			// prepared in the 'reloc' phase and passed to the
			// external linker in the 'asmb' phase.
			if ldr.SymType(s) != sym.SDATA && ldr.SymType(s) != sym.SRODATA {
				break
			}
		}

		if target.IsElf() {
			// Generate R_X86_64_RELATIVE relocations for best
			// efficiency in the dynamic linker.
			//
			// As noted above, symbol addresses have not been
			// assigned yet, so we can't generate the final reloc
			// entry yet. We ultimately want:
			//
			// r_offset = s + r.Off
			// r_info = R_X86_64_RELATIVE
			// r_addend = targ + r.Add
			//
			// The dynamic linker will set *offset = base address +
			// addend.
			//
			// AddAddrPlus is used for r_offset and r_addend to
			// generate new R_ADDR relocations that will update
			// these fields in the 'reloc' phase.
			rela := ldr.MakeSymbolUpdater(syms.Rela)
			rela.AddAddrPlus(target.Arch, s, int64(r.Off()))
			if r.Siz() == 8 {
				rela.AddUint64(target.Arch, elf.R_INFO(0, uint32(elf.R_X86_64_RELATIVE)))
			} else {
				ldr.Errorf(s, "unexpected relocation for dynamic symbol %s", ldr.SymName(targ))
			}
			rela.AddAddrPlus(target.Arch, targ, int64(r.Add()))
			// Not mark r done here. So we still apply it statically,
			// so in the file content we'll also have the right offset
			// to the relocation target. So it can be examined statically
			// (e.g. go version).
			return true
		}

		if target.IsDarwin() {
			// Mach-O relocations are a royal pain to lay out.
			// They use a compact stateful bytecode representation.
			// Here we record what are needed and encode them later.
			ld.MachoAddRebase(s, int64(r.Off()))
			// Not mark r done here. So we still apply it statically,
			// so in the file content we'll also have the right offset
			// to the relocation target. So it can be examined statically
			// (e.g. go version).
			return true
		}
	}

	return false
}

func elfreloc1(ctxt *ld.Link, out *ld.OutBuf, ldr *loader.Loader, s loader.Sym, r loader.ExtReloc, ri int, sectoff int64) bool {
	out.Write64(uint64(sectoff))

	elfsym := ld.ElfSymForReloc(ctxt, r.Xsym)
	siz := r.Size
	switch r.Type {
	default:
		return false
	case objabi.R_ADDR, objabi.R_DWARFSECREF:
		if siz == 4 {
			out.Write64(uint64(elf.R_X86_64_32) | uint64(elfsym)<<32)
		} else if siz == 8 {
			out.Write64(uint64(elf.R_X86_64_64) | uint64(elfsym)<<32)
		} else {
			return false
		}
	case objabi.R_TLS_LE:
		if siz == 4 {
			out.Write64(uint64(elf.R_X86_64_TPOFF32) | uint64(elfsym)<<32)
		} else {
			return false
		}
	case objabi.R_TLS_IE:
		if siz == 4 {
			out.Write64(uint64(elf.R_X86_64_GOTTPOFF) | uint64(elfsym)<<32)
		} else {
			return false
		}
	case objabi.R_CALL:
		if siz == 4 {
			if ldr.SymType(r.Xsym) == sym.SDYNIMPORT {
				out.Write64(uint64(elf.R_X86_64_PLT32) | uint64(elfsym)<<32)
			} else {
				out.Write64(uint64(elf.R_X86_64_PC32) | uint64(elfsym)<<32)
			}
		} else {
			return false
		}
	case objabi.R_PCREL:
		if siz == 4 {
			if ldr.SymType(r.Xsym) == sym.SDYNIMPORT && ldr.SymElfType(r.Xsym) == elf.STT_FUNC {
				out.Write64(uint64(elf.R_X86_64_PLT32) | uint64(elfsym)<<32)
			} else {
				out.Write64(uint64(elf.R_X86_64_PC32) | uint64(elfsym)<<32)
			}
		} else {
			return false
		}
	case objabi.R_GOTPCREL:
		if siz == 4 {
			out.Write64(uint64(elf.R_X86_64_GOTPCREL) | uint64(elfsym)<<32)
		} else {
			return false
		}
	}

	out.Write64(uint64(r.Xadd))
	return true
}

func machoreloc1(arch *sys.Arch, out *ld.OutBuf, ldr *loader.Loader, s loader.Sym, r loader.ExtReloc, sectoff int64) bool {
	var v uint32

	rs := r.Xsym
	rt := r.Type

	if !ldr.SymType(s).IsDWARF() {
		if ldr.SymDynid(rs) < 0 {
			ldr.Errorf(s, "reloc %d (%s) to non-macho symbol %s type=%d (%s)", rt, sym.RelocName(arch, rt), ldr.SymName(rs), ldr.SymType(rs), ldr.SymType(rs))
			return false
		}

		v = uint32(ldr.SymDynid(rs))
		v |= 1 << 27 // external relocation
	} else {
		v = uint32(ldr.SymSect(rs).Extnum)
		if v == 0 {
			ldr.Errorf(s, "reloc %d (%s) to symbol %s in non-macho section %s type=%d (%s)", rt, sym.RelocName(arch, rt), ldr.SymName(rs), ldr.SymSect(rs).Name, ldr.SymType(rs), ldr.SymType(rs))
			return false
		}
	}

	switch rt {
	default:
		return false

	case objabi.R_ADDR:
		v |= ld.MACHO_X86_64_RELOC_UNSIGNED << 28

	case objabi.R_CALL:
		v |= 1 << 24 // pc-relative bit
		v |= ld.MACHO_X86_64_RELOC_BRANCH << 28

		// NOTE: Only works with 'external' relocation. Forced above.
	case objabi.R_PCREL:
		v |= 1 << 24 // pc-relative bit
		v |= ld.MACHO_X86_64_RELOC_SIGNED << 28
	case objabi.R_GOTPCREL:
		v |= 1 << 24 // pc-relative bit
		v |= ld.MACHO_X86_64_RELOC_GOT_LOAD << 28
	}

	switch r.Size {
	default:
		return false

	case 1:
		v |= 0 << 25

	case 2:
		v |= 1 << 25

	case 4:
		v |= 2 << 25

	case 8:
		v |= 3 << 25
	}

	out.Write32(uint32(sectoff))
	out.Write32(v)
	return true
}

func pereloc1(arch *sys.Arch, out *ld.OutBuf, ldr *loader.Loader, s loader.Sym, r loader.ExtReloc, sectoff int64) bool {
	var v uint32

	rs := r.Xsym
	rt := r.Type

	if ldr.SymDynid(rs) < 0 {
		ldr.Errorf(s, "reloc %d (%s) to non-coff symbol %s type=%d (%s)", rt, sym.RelocName(arch, rt), ldr.SymName(rs), ldr.SymType(rs), ldr.SymType(rs))
		return false
	}

	out.Write32(uint32(sectoff))
	out.Write32(uint32(ldr.SymDynid(rs)))

	switch rt {
	default:
		return false

	case objabi.R_DWARFSECREF:
		v = ld.IMAGE_REL_AMD64_SECREL

	case objabi.R_ADDR:
		if r.Size == 8 {
			v = ld.IMAGE_REL_AMD64_ADDR64
		} else {
			v = ld.IMAGE_REL_AMD64_ADDR32
		}

	case objabi.R_PEIMAGEOFF:
		v = ld.IMAGE_REL_AMD64_ADDR32NB

	case objabi.R_CALL,
		objabi.R_PCREL:
		v = ld.IMAGE_REL_AMD64_REL32
	}

	out.Write16(uint16(v))

	return true
}

func archreloc(*ld.Target, *loader.Loader, *ld.ArchSyms, loader.Reloc, loader.Sym, int64) (int64, int, bool) {
	return -1, 0, false
}

func archrelocvariant(*ld.Target, *loader.Loader, loader.Reloc, sym.RelocVariant, loader.Sym, int64, []byte) int64 {
	log.Fatalf("unexpected relocation variant")
	return -1
}

func elfsetupplt(ctxt *ld.Link, ldr *loader.Loader, plt, got *loader.SymbolBuilder, dynamic loader.Sym) {
	if plt.Size() == 0 {
		// pushq got+8(IP)
		plt.AddUint8(0xff)

		plt.AddUint8(0x35)
		plt.AddPCRelPlus(ctxt.Arch, got.Sym(), 8)

		// jmpq got+16(IP)
		plt.AddUint8(0xff)

		plt.AddUint8(0x25)
		plt.AddPCRelPlus(ctxt.Arch, got.Sym(), 16)

		// nopl 0(AX)
		plt.AddUint32(ctxt.Arch, 0x00401f0f)

		// assume got->size == 0 too
		got.AddAddrPlus(ctxt.Arch, dynamic, 0)

		got.AddUint64(ctxt.Arch, 0)
		got.AddUint64(ctxt.Arch, 0)
	}
}

func addpltsym(target *ld.Target, ldr *loader.Loader, syms *ld.ArchSyms, s loader.Sym) {
	if ldr.SymPlt(s) >= 0 {
		return
	}

	ld.Adddynsym(ldr, target, syms, s)

	if target.IsElf() {
		plt := ldr.MakeSymbolUpdater(syms.PLT)
		got := ldr.MakeSymbolUpdater(syms.GOTPLT)
		rela := ldr.MakeSymbolUpdater(syms.RelaPLT)
		if plt.Size() == 0 {
			panic("plt is not set up")
		}

		// jmpq *got+size(IP)
		plt.AddUint8(0xff)

		plt.AddUint8(0x25)
		plt.AddPCRelPlus(target.Arch, got.Sym(), got.Size())

		// add to got: pointer to current pos in plt
		got.AddAddrPlus(target.Arch, plt.Sym(), plt.Size())

		// pushq $x
		plt.AddUint8(0x68)

		plt.AddUint32(target.Arch, uint32((got.Size()-24-8)/8))

		// jmpq .plt
		plt.AddUint8(0xe9)

		plt.AddUint32(target.Arch, uint32(-(plt.Size() + 4)))

		// rela
		rela.AddAddrPlus(target.Arch, got.Sym(), got.Size()-8)

		sDynid := ldr.SymDynid(s)
		rela.AddUint64(target.Arch, elf.R_INFO(uint32(sDynid), uint32(elf.R_X86_64_JMP_SLOT)))
		rela.AddUint64(target.Arch, 0)

		ldr.SetPlt(s, int32(plt.Size()-16))
	} else if target.IsDarwin() {
		ld.AddGotSym(target, ldr, syms, s, 0)

		sDynid := ldr.SymDynid(s)
		lep := ldr.MakeSymbolUpdater(syms.LinkEditPLT)
		lep.AddUint32(target.Arch, uint32(sDynid))

		plt := ldr.MakeSymbolUpdater(syms.PLT)
		ldr.SetPlt(s, int32(plt.Size()))

		// jmpq *got+size(IP)
		plt.AddUint8(0xff)
		plt.AddUint8(0x25)
		plt.AddPCRelPlus(target.Arch, syms.GOT, int64(ldr.SymGot(s)))
	} else {
		ldr.Errorf(s, "addpltsym: unsupported binary format")
	}
}

func tlsIEtoLE(P []byte, off, size int) {
	// Transform the PC-relative instruction into a constant load.
	// That is,
	//
	//	MOVQ X(IP), REG  ->  MOVQ $Y, REG
	//
	// To determine the instruction and register, we study the op codes.
	// Consult an AMD64 instruction encoding guide to decipher this.
	if off < 3 {
		log.Fatal("R_X86_64_GOTTPOFF reloc not preceded by MOVQ or ADDQ instruction")
	}
	op := P[off-3 : off]
	reg := op[2] >> 3

	if op[1] == 0x8b || reg == 4 {
		// MOVQ
		if op[0] == 0x4c {
			op[0] = 0x49
		} else if size == 4 && op[0] == 0x44 {
			op[0] = 0x41
		}
		if op[1] == 0x8b {
			op[1] = 0xc7
		} else {
			op[1] = 0x81 // special case for SP
		}
		op[2] = 0xc0 | reg
	} else {
		// An alternate op is ADDQ. This is handled by GNU gold,
		// but right now is not generated by the Go compiler:
		//	ADDQ X(IP), REG  ->  ADDQ $Y, REG
		// Consider adding support for it here.
		log.Fatalf("expected TLS IE op to be MOVQ, got %v", op)
	}
}