cmd/link: fix contents of debug_pubnames/debug_pubtypes

The contents of debug_pubnames and debug_pubtypes have been wrong since
Go 1.12.
CL golang.org/cl/137235 moved global variables DIE to their respective
compilation unit, unfortunately writepub can't emit correct sections
for anything but the first compilation unit.

This commit moves the code generating debug_pubnames and debug_pubtypes
inside writeinfo and fixes it.
Gets rid of a number of unnecessary relocations as well as a hack that
writeinfo used to communicate to writepub the size of each compilation
unit.

Fixes #30573

Change-Id: Ibdaa80c02746ae81661c2cfe1d218092c5ae9236
Reviewed-on: https://go-review.googlesource.com/c/go/+/165337
Run-TryBot: Alessandro Arzilli <alessandro.arzilli@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/link/internal/ld/dwarf.go

@@ -264,15 +264,6 @@ func newrefattr(die *dwarf.DWDie, attr uint16, ref *sym.Symbol) *dwarf.DWAttr {
 	return newattr(die, attr, dwarf.DW_CLS_REFERENCE, 0, ref)
 }
 
-func putdies(linkctxt *Link, ctxt dwarf.Context, syms []*sym.Symbol, die *dwarf.DWDie) []*sym.Symbol {
-	for ; die != nil; die = die.Link {
-		syms = putdie(linkctxt, ctxt, syms, die)
-	}
-	syms[len(syms)-1].AddUint8(0)
-
-	return syms
-}
-
 func dtolsym(s dwarf.Sym) *sym.Symbol {
 	if s == nil {
 		return nil
@@ -294,7 +285,10 @@ func putdie(linkctxt *Link, ctxt dwarf.Context, syms []*sym.Symbol, die *dwarf.D
 	dwarf.Uleb128put(ctxt, s, int64(die.Abbrev))
 	dwarf.PutAttrs(ctxt, s, die.Abbrev, die.Attr)
 	if dwarf.HasChildren(die) {
-		return putdies(linkctxt, ctxt, syms, die.Child)
+		for die := die.Child; die != nil; die = die.Link {
+			syms = putdie(linkctxt, ctxt, syms, die)
+		}
+		syms[len(syms)-1].AddUint8(0)
 	}
 	return syms
 }
@@ -1517,7 +1511,7 @@ const (
 	COMPUNITHEADERSIZE = 4 + 2 + 4 + 1
 )
 
-func writeinfo(ctxt *Link, syms []*sym.Symbol, units []*compilationUnit, abbrevsym *sym.Symbol) []*sym.Symbol {
+func writeinfo(ctxt *Link, syms []*sym.Symbol, units []*compilationUnit, abbrevsym *sym.Symbol, pubNames, pubTypes *pubWriter) []*sym.Symbol {
 	infosec := ctxt.Syms.Lookup(".debug_info", 0)
 	infosec.Type = sym.SDWARFINFO
 	infosec.Attr |= sym.AttrReachable
@@ -1533,6 +1527,9 @@ func writeinfo(ctxt *Link, syms []*sym.Symbol, units []*compilationUnit, abbrevs
 			continue
 		}
 
+		pubNames.beginCompUnit(compunit)
+		pubTypes.beginCompUnit(compunit)
+
 		// Write .debug_info Compilation Unit Header (sec 7.5.1)
 		// Fields marked with (*) must be changed for 64-bit dwarf
 		// This must match COMPUNITHEADERSIZE above.
@@ -1553,11 +1550,32 @@ func writeinfo(ctxt *Link, syms []*sym.Symbol, units []*compilationUnit, abbrevs
 		if u.consts != nil {
 			cu = append(cu, u.consts)
 		}
-		cu = putdies(ctxt, dwarfctxt, cu, compunit.Child)
 		var cusize int64
 		for _, child := range cu {
 			cusize += child.Size
 		}
+
+		for die := compunit.Child; die != nil; die = die.Link {
+			l := len(cu)
+			lastSymSz := cu[l-1].Size
+			cu = putdie(ctxt, dwarfctxt, cu, die)
+			if ispubname(die) {
+				pubNames.add(die, cusize)
+			}
+			if ispubtype(die) {
+				pubTypes.add(die, cusize)
+			}
+			if lastSymSz != cu[l-1].Size {
+				// putdie will sometimes append directly to the last symbol of the list
+				cusize = cusize - lastSymSz + cu[l-1].Size
+			}
+			for _, child := range cu[l:] {
+				cusize += child.Size
+			}
+		}
+		cu[len(cu)-1].AddUint8(0) // closes compilation unit DIE
+		cusize++
+
 		// Save size for AIX symbol table.
 		if ctxt.HeadType == objabi.Haix {
 			saveDwsectCUSize(".debug_info", getPkgFromCUSym(s), uint64(cusize))
@@ -1569,9 +1587,8 @@ func writeinfo(ctxt *Link, syms []*sym.Symbol, units []*compilationUnit, abbrevs
 			cusize -= 4 // exclude the length field.
 			s.SetUint32(ctxt.Arch, 0, uint32(cusize))
 		}
-		// Leave a breadcrumb for writepub. This does not
-		// appear in the DWARF output.
-		newattr(compunit, dwarf.DW_AT_byte_size, dwarf.DW_CLS_CONSTANT, cusize, 0)
+		pubNames.endCompUnit(compunit, uint32(cusize)+4)
+		pubTypes.endCompUnit(compunit, uint32(cusize)+4)
 		syms = append(syms, cu...)
 	}
 	return syms
@@ -1595,52 +1612,57 @@ func ispubtype(die *dwarf.DWDie) bool {
 	return die.Abbrev >= dwarf.DW_ABRV_NULLTYPE
 }
 
-func writepub(ctxt *Link, sname string, ispub func(*dwarf.DWDie) bool, syms []*sym.Symbol) []*sym.Symbol {
+type pubWriter struct {
+	ctxt  *Link
+	s     *sym.Symbol
+	sname string
+
+	sectionstart int64
+	culengthOff  int64
+}
+
+func newPubWriter(ctxt *Link, sname string) *pubWriter {
 	s := ctxt.Syms.Lookup(sname, 0)
 	s.Type = sym.SDWARFSECT
-	syms = append(syms, s)
+	return &pubWriter{ctxt: ctxt, s: s, sname: sname}
+}
 
-	for _, u := range ctxt.compUnits {
-		if len(u.lib.Textp) == 0 && u.dwinfo.Child == nil {
-			continue
-		}
-		compunit := u.dwinfo
-		sectionstart := s.Size
-		culength := uint32(getattr(compunit, dwarf.DW_AT_byte_size).Value) + 4
+func (pw *pubWriter) beginCompUnit(compunit *dwarf.DWDie) {
+	pw.sectionstart = pw.s.Size
 
 	// Write .debug_pubnames/types	Header (sec 6.1.1)
-		createUnitLength(ctxt, s, 0)                    // unit_length (*), will be filled in later.
-		s.AddUint16(ctxt.Arch, 2)                       // dwarf version (appendix F)
-		addDwarfAddrRef(ctxt, s, dtolsym(compunit.Sym)) // debug_info_offset (of the Comp unit Header)
-		addDwarfAddrField(ctxt, s, uint64(culength))    // debug_info_length
+	createUnitLength(pw.ctxt, pw.s, 0)                    // unit_length (*), will be filled in later.
+	pw.s.AddUint16(pw.ctxt.Arch, 2)                       // dwarf version (appendix F)
+	addDwarfAddrRef(pw.ctxt, pw.s, dtolsym(compunit.Sym)) // debug_info_offset (of the Comp unit Header)
+	pw.culengthOff = pw.s.Size
+	addDwarfAddrField(pw.ctxt, pw.s, uint64(0)) // debug_info_length, will be filled in later.
 
-		for die := compunit.Child; die != nil; die = die.Link {
-			if !ispub(die) {
-				continue
-			}
+}
+
+func (pw *pubWriter) add(die *dwarf.DWDie, offset int64) {
 	dwa := getattr(die, dwarf.DW_AT_name)
 	name := dwa.Data.(string)
 	if die.Sym == nil {
 		fmt.Println("Missing sym for ", name)
 	}
-			addDwarfAddrRef(ctxt, s, dtolsym(die.Sym))
-			Addstring(s, name)
-		}
+	addDwarfAddrField(pw.ctxt, pw.s, uint64(offset))
+	Addstring(pw.s, name)
+}
 
-		addDwarfAddrField(ctxt, s, 0) // Null offset
+func (pw *pubWriter) endCompUnit(compunit *dwarf.DWDie, culength uint32) {
+	addDwarfAddrField(pw.ctxt, pw.s, 0) // Null offset
 
 	// On AIX, save the current size of this compilation unit.
-		if ctxt.HeadType == objabi.Haix {
-			saveDwsectCUSize(sname, getPkgFromCUSym(dtolsym(compunit.Sym)), uint64(s.Size-sectionstart))
+	if pw.ctxt.HeadType == objabi.Haix {
+		saveDwsectCUSize(pw.sname, getPkgFromCUSym(dtolsym(compunit.Sym)), uint64(pw.s.Size-pw.sectionstart))
 	}
-		if isDwarf64(ctxt) {
-			s.SetUint(ctxt.Arch, sectionstart+4, uint64(s.Size-sectionstart)-12) // exclude the length field.
+	if isDwarf64(pw.ctxt) {
+		pw.s.SetUint(pw.ctxt.Arch, pw.sectionstart+4, uint64(pw.s.Size-pw.sectionstart)-12) // exclude the length field.
+		pw.s.SetUint(pw.ctxt.Arch, pw.culengthOff, uint64(culength))
 	} else {
-			s.SetUint32(ctxt.Arch, sectionstart, uint32(s.Size-sectionstart)-4) // exclude the length field.
-		}
+		pw.s.SetUint32(pw.ctxt.Arch, pw.sectionstart, uint32(pw.s.Size-pw.sectionstart)-4) // exclude the length field.
+		pw.s.SetUint32(pw.ctxt.Arch, pw.culengthOff, culength)
 	}
-
-	return syms
 }
 
 func writegdbscript(ctxt *Link, syms []*sym.Symbol) []*sym.Symbol {
@@ -1878,13 +1900,14 @@ func dwarfGenerateDebugSyms(ctxt *Link) {
 	reversetree(&dwtypes.Child)
 	movetomodule(ctxt, &dwtypes)
 
+	pubNames := newPubWriter(ctxt, ".debug_pubnames")
+	pubTypes := newPubWriter(ctxt, ".debug_pubtypes")
+
 	// Need to reorder symbols so sym.SDWARFINFO is after all sym.SDWARFSECT
-	// (but we need to generate dies before writepub)
-	infosyms := writeinfo(ctxt, nil, ctxt.compUnits, abbrev)
+	infosyms := writeinfo(ctxt, nil, ctxt.compUnits, abbrev, pubNames, pubTypes)
 
 	syms = writeframes(ctxt, syms)
-	syms = writepub(ctxt, ".debug_pubnames", ispubname, syms)
-	syms = writepub(ctxt, ".debug_pubtypes", ispubtype, syms)
+	syms = append(syms, pubNames.s, pubTypes.s)
 	syms = writegdbscript(ctxt, syms)
 	// Now we're done writing SDWARFSECT symbols, so we can write
 	// other SDWARF* symbols.