[dev.link] cmd/link: add SymbolBuilder helper

Add SymbolBuilder helper type -- this type provides a set of methods
intended to make it easy to manipulate the content of a symbol (type,
relocations, data, etc).

Change-Id: I579bf8d04650e66d33a9780a6c2347a576c94c6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/210178
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/internal/loader/symbolbuilder.go

@@ -0,0 +1,250 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package loader
+
+import (
+	"cmd/internal/objabi"
+	"cmd/internal/sys"
+	"cmd/link/internal/sym"
+)
+
+// SymbolBuilder is a helper designed to help with the construction
+// of new symbol contents.
+type SymbolBuilder struct {
+	*extSymPayload         // points to payload being updated
+	symIdx         Sym     // index of symbol being updated/constructed
+	l              *Loader // loader
+}
+
+// NewSymbolBuilder creates a symbol builder for use in constructing
+// an entirely new symbol.
+func (l *Loader) MakeSymbolBuilder(name string) *SymbolBuilder {
+	// for now assume that any new sym is intended to be static
+	symIdx := l.CreateExtSym(name)
+	if l.Syms[symIdx] != nil {
+		panic("can't build if sym.Symbol already present")
+	}
+	sb := &SymbolBuilder{l: l, symIdx: symIdx}
+	sb.extSymPayload = &l.payloads[symIdx-l.extStart]
+	return sb
+}
+
+// NewSymbolBuilder creates a symbol builder helper for an already-allocated
+// external symbol 'symIdx'.
+func (l *Loader) MakeSymbolUpdater(symIdx Sym) *SymbolBuilder {
+	if !l.IsExternal(symIdx) {
+		panic("can't build on non-external sym")
+	}
+	if l.Syms[symIdx] != nil {
+		panic("can't build if sym.Symbol already present")
+	}
+	sb := &SymbolBuilder{l: l, symIdx: symIdx}
+	sb.extSymPayload = &l.payloads[symIdx-l.extStart]
+	return sb
+}
+
+// Getters for properties of the symbol we're working on.
+
+func (sb *SymbolBuilder) Sym() Sym           { return sb.symIdx }
+func (sb *SymbolBuilder) Name() string       { return sb.name }
+func (sb *SymbolBuilder) Version() int       { return sb.ver }
+func (sb *SymbolBuilder) Type() sym.SymKind  { return sb.kind }
+func (sb *SymbolBuilder) Size() int64        { return sb.size }
+func (sb *SymbolBuilder) Data() []byte       { return sb.data }
+func (sb *SymbolBuilder) Value() int64       { return sb.l.SymValue(sb.symIdx) }
+func (sb *SymbolBuilder) Align() int32       { return sb.l.SymAlign(sb.symIdx) }
+func (sb *SymbolBuilder) Localentry() uint8  { return sb.l.SymLocalentry(sb.symIdx) }
+func (sb *SymbolBuilder) Extname() string    { return sb.l.SymExtname(sb.symIdx) }
+func (sb *SymbolBuilder) Dynimplib() string  { return sb.l.SymDynimplib(sb.symIdx) }
+func (sb *SymbolBuilder) Dynimpvers() string { return sb.l.SymDynimpvers(sb.symIdx) }
+
+// Setters for symbol properties.
+
+func (sb *SymbolBuilder) SetType(kind sym.SymKind)   { sb.kind = kind }
+func (sb *SymbolBuilder) SetSize(size int64)         { sb.size = size }
+func (sb *SymbolBuilder) SetData(data []byte)        { sb.data = data }
+func (sb *SymbolBuilder) SetValue(v int64)           { sb.l.SetSymValue(sb.symIdx, v) }
+func (sb *SymbolBuilder) SetAlign(align int32)       { sb.l.SetSymAlign(sb.symIdx, align) }
+func (sb *SymbolBuilder) SetLocalentry(value uint8)  { sb.l.SetSymLocalentry(sb.symIdx, value) }
+func (sb *SymbolBuilder) SetExtname(value string)    { sb.l.SetSymExtname(sb.symIdx, value) }
+func (sb *SymbolBuilder) SetDynimplib(value string)  { sb.l.SetSymDynimplib(sb.symIdx, value) }
+func (sb *SymbolBuilder) SetDynimpvers(value string) { sb.l.SetSymDynimpvers(sb.symIdx, value) }
+
+func (sb *SymbolBuilder) AddBytes(data []byte) {
+	sb.setReachable()
+	if sb.kind == 0 {
+		sb.kind = sym.SDATA
+	}
+	sb.data = append(sb.data, data...)
+	sb.size = int64(len(sb.data))
+}
+
+func (sb *SymbolBuilder) Relocs() []Reloc {
+	return sb.relocs
+}
+
+func (sb *SymbolBuilder) SetRelocs(rslice []Reloc) {
+	sb.relocs = rslice
+}
+
+func (sb *SymbolBuilder) AddReloc(r Reloc) {
+	sb.relocs = append(sb.relocs, r)
+}
+
+func (sb *SymbolBuilder) Reachable() bool {
+	return sb.l.AttrReachable(sb.symIdx)
+}
+
+func (sb *SymbolBuilder) setReachable() {
+	sb.l.SetAttrReachable(sb.symIdx, true)
+}
+
+func (sb *SymbolBuilder) Outer() Sym {
+	return sb.l.OuterSym(sb.symIdx)
+}
+
+func (sb *SymbolBuilder) Sub() Sym {
+	return sb.l.SubSym(sb.symIdx)
+}
+
+func (sb *SymbolBuilder) SortSub() {
+	sb.l.SortSub(sb.symIdx)
+}
+
+func (sb *SymbolBuilder) PrependSub(sub Sym) {
+	sb.l.PrependSub(sb.symIdx, sub)
+}
+
+func (sb *SymbolBuilder) AddUint8(v uint8) int64 {
+	off := sb.size
+	if sb.kind == 0 {
+		sb.kind = sym.SDATA
+	}
+	sb.setReachable()
+	sb.size++
+	sb.data = append(sb.data, v)
+	return off
+}
+
+func (sb *SymbolBuilder) AddUintXX(arch *sys.Arch, v uint64, wid int) int64 {
+	off := sb.size
+	sb.setReachable()
+	sb.setUintXX(arch, off, v, int64(wid))
+	return off
+}
+
+func (sb *SymbolBuilder) setUintXX(arch *sys.Arch, off int64, v uint64, wid int64) int64 {
+	if sb.kind == 0 {
+		sb.kind = sym.SDATA
+	}
+	if sb.size < off+wid {
+		sb.size = off + wid
+		sb.Grow(sb.size)
+	}
+
+	switch wid {
+	case 1:
+		sb.data[off] = uint8(v)
+	case 2:
+		arch.ByteOrder.PutUint16(sb.data[off:], uint16(v))
+	case 4:
+		arch.ByteOrder.PutUint32(sb.data[off:], uint32(v))
+	case 8:
+		arch.ByteOrder.PutUint64(sb.data[off:], v)
+	}
+
+	return off + wid
+}
+
+func (sb *SymbolBuilder) AddUint16(arch *sys.Arch, v uint16) int64 {
+	return sb.AddUintXX(arch, uint64(v), 2)
+}
+
+func (sb *SymbolBuilder) AddUint32(arch *sys.Arch, v uint32) int64 {
+	return sb.AddUintXX(arch, uint64(v), 4)
+}
+
+func (sb *SymbolBuilder) AddUint64(arch *sys.Arch, v uint64) int64 {
+	return sb.AddUintXX(arch, v, 8)
+}
+
+func (sb *SymbolBuilder) AddUint(arch *sys.Arch, v uint64) int64 {
+	return sb.AddUintXX(arch, v, arch.PtrSize)
+}
+
+func (sb *SymbolBuilder) SetUint8(arch *sys.Arch, r int64, v uint8) int64 {
+	sb.setReachable()
+	return sb.setUintXX(arch, r, uint64(v), 1)
+}
+
+func (sb *SymbolBuilder) SetUint16(arch *sys.Arch, r int64, v uint16) int64 {
+	sb.setReachable()
+	return sb.setUintXX(arch, r, uint64(v), 2)
+}
+
+func (sb *SymbolBuilder) SetUint32(arch *sys.Arch, r int64, v uint32) int64 {
+	sb.setReachable()
+	return sb.setUintXX(arch, r, uint64(v), 4)
+}
+
+func (sb *SymbolBuilder) SetUint(arch *sys.Arch, r int64, v uint64) int64 {
+	sb.setReachable()
+	return sb.setUintXX(arch, r, v, int64(arch.PtrSize))
+}
+
+func (sb *SymbolBuilder) Addstring(str string) int64 {
+	sb.setReachable()
+	if sb.kind == 0 {
+		sb.kind = sym.SNOPTRDATA
+	}
+	r := sb.size
+	if sb.name == ".shstrtab" {
+		// FIXME: find a better mechanism for this
+		sb.l.elfsetstring(nil, str, int(r))
+	}
+	sb.data = append(sb.data, str...)
+	sb.data = append(sb.data, 0)
+	sb.size = int64(len(sb.data))
+	return r
+}
+
+func (sb *SymbolBuilder) addRel() *Reloc {
+	sb.relocs = append(sb.relocs, Reloc{})
+	return &sb.relocs[len(sb.relocs)-1]
+}
+
+func (sb *SymbolBuilder) addAddrPlus(tgt Sym, add int64, typ objabi.RelocType, rsize int) int64 {
+	if sb.kind == 0 {
+		sb.kind = sym.SDATA
+	}
+	i := sb.size
+
+	sb.size += int64(rsize)
+	sb.Grow(sb.size)
+
+	r := sb.addRel()
+	r.Sym = tgt
+	r.Off = int32(i)
+	r.Size = uint8(rsize)
+	r.Type = typ
+	r.Add = add
+
+	return i + int64(r.Size)
+}
+
+func (sb *SymbolBuilder) AddAddrPlus(arch *sys.Arch, tgt Sym, add int64) int64 {
+	sb.setReachable()
+	return sb.addAddrPlus(tgt, add, objabi.R_ADDR, arch.PtrSize)
+}
+
+func (sb *SymbolBuilder) AddAddrPlus4(arch *sys.Arch, tgt Sym, add int64) int64 {
+	sb.setReachable()
+	return sb.addAddrPlus(tgt, add, objabi.R_ADDR, 4)
+}
+
+func (sb *SymbolBuilder) AddCURelativeAddrPlus(arch *sys.Arch, tgt Sym, add int64) int64 {
+	sb.setReachable()
+	return sb.addAddrPlus(tgt, add, objabi.R_ADDRCUOFF, arch.PtrSize)
+}