cmd/link: collect itablinks as a slice in moduledata

See #14874

This change tells the linker to collect all the itablink symbols and
collect them so that moduledata can have a slice of all compiler
generated itabs.

The logic is shamelessly adapted from what is done with typelink symbols.

Change-Id: Ie93b59acf0fcba908a876d506afbf796f222dbac
Reviewed-on: https://go-review.googlesource.com/20889Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/goobj/read.go

@@ -43,6 +43,7 @@ const (
 	SRODATA    SymKind = obj.SRODATA
 	SFUNCTAB   SymKind = obj.SFUNCTAB
 	STYPELINK  SymKind = obj.STYPELINK
+	SITABLINK  SymKind = obj.SITABLINK
 	SSYMTAB    SymKind = obj.SSYMTAB // TODO: move to unmapped section
 	SPCLNTAB   SymKind = obj.SPCLNTAB
 	SELFROSECT SymKind = obj.SELFROSECT
@@ -106,6 +107,7 @@ var symKindStrings = []string{
 	STLSBSS:           "STLSBSS",
 	STYPE:             "STYPE",
 	STYPELINK:         "STYPELINK",
+	SITABLINK:         "SITABLINK",
 	SWINDOWS:          "SWINDOWS",
 	SXREF:             "SXREF",
 }

src/cmd/internal/obj/link.go

@@ -397,6 +397,7 @@ const (
 	SFUNCTABRELRO
 
 	STYPELINK
+	SITABLINK
 	SSYMTAB
 	SPCLNTAB
 	SELFROSECT

src/cmd/internal/objfile/goobj.go

@@ -41,7 +41,7 @@ func (f *goobjFile) symbols() ([]Sym, error) {
 		switch s.Kind {
 		case goobj.STEXT, goobj.SELFRXSECT:
 			sym.Code = 'T'
-		case goobj.STYPE, goobj.SSTRING, goobj.SGOSTRING, goobj.SGOFUNC, goobj.SRODATA, goobj.SFUNCTAB, goobj.STYPELINK, goobj.SSYMTAB, goobj.SPCLNTAB, goobj.SELFROSECT:
+		case goobj.STYPE, goobj.SSTRING, goobj.SGOSTRING, goobj.SGOFUNC, goobj.SRODATA, goobj.SFUNCTAB, goobj.STYPELINK, goobj.SITABLINK, goobj.SSYMTAB, goobj.SPCLNTAB, goobj.SELFROSECT:
 			sym.Code = 'R'
 		case goobj.SMACHOPLT, goobj.SELFSECT, goobj.SMACHO, goobj.SMACHOGOT, goobj.SNOPTRDATA, goobj.SINITARR, goobj.SDATA, goobj.SWINDOWS:
 			sym.Code = 'D'

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

@@ -1605,6 +1605,24 @@ func dodata() {
 
 	sect.Length = uint64(datsize) - sect.Vaddr
 
+	/* itablink */
+	sect = addsection(segro, relro_prefix+".itablink", relro_perms)
+
+	sect.Align = maxalign(s, obj.SITABLINK)
+	datsize = Rnd(datsize, int64(sect.Align))
+	sect.Vaddr = uint64(datsize)
+	Linklookup(Ctxt, "runtime.itablink", 0).Sect = sect
+	Linklookup(Ctxt, "runtime.eitablink", 0).Sect = sect
+	for ; s != nil && s.Type == obj.SITABLINK; s = s.Next {
+		datsize = aligndatsize(datsize, s)
+		s.Sect = sect
+		s.Type = obj.SRODATA
+		s.Value = int64(uint64(datsize) - sect.Vaddr)
+		growdatsize(&datsize, s)
+	}
+
+	sect.Length = uint64(datsize) - sect.Vaddr
+
 	/* gosymtab */
 	sect = addsection(segro, relro_prefix+".gosymtab", relro_perms)
 
@@ -1835,7 +1853,8 @@ func address() {
 		// object on elf systems.
 		typelink = typelink.Next
 	}
-	symtab := typelink.Next
+	itablink := typelink.Next
+	symtab := itablink.Next
 	pclntab := symtab.Next
 
 	var sub *LSym
@@ -1862,6 +1881,8 @@ func address() {
 	xdefine("runtime.erodata", obj.SRODATA, int64(rodata.Vaddr+rodata.Length))
 	xdefine("runtime.typelink", obj.SRODATA, int64(typelink.Vaddr))
 	xdefine("runtime.etypelink", obj.SRODATA, int64(typelink.Vaddr+typelink.Length))
+	xdefine("runtime.itablink", obj.SRODATA, int64(itablink.Vaddr))
+	xdefine("runtime.eitablink", obj.SRODATA, int64(itablink.Vaddr+itablink.Length))
 
 	sym := Linklookup(Ctxt, "runtime.gcdata", 0)
 	sym.Attr |= AttrLocal

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

@@ -107,10 +107,11 @@ func deadcode(ctxt *Link) {
 	}
 
 	if Buildmode != BuildmodeShared {
-		// Keep a typelink if the symbol it points at is being kept.
-		// (When BuildmodeShared, always keep typelinks.)
+		// Keep a typelink or itablink if the symbol it points at is being kept.
+		// (When BuildmodeShared, always keep typelinks and itablinks.)
 		for _, s := range ctxt.Allsym {
-			if strings.HasPrefix(s.Name, "go.typelink.") {
+			if strings.HasPrefix(s.Name, "go.typelink.") ||
+				strings.HasPrefix(s.Name, "go.itablink.") {
 				s.Attr.Set(AttrReachable, len(s.R) == 1 && s.R[0].Sym.Attr.Reachable())
 			}
 		}

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

@@ -1740,6 +1740,7 @@ func doelf() {
 		relro_prefix = ".data.rel.ro"
 	}
 	Addstring(shstrtab, relro_prefix+".typelink")
+	Addstring(shstrtab, relro_prefix+".itablink")
 	Addstring(shstrtab, relro_prefix+".gosymtab")
 	Addstring(shstrtab, relro_prefix+".gopclntab")
 
@@ -1751,6 +1752,7 @@ func doelf() {
 			Addstring(shstrtab, ".rela.text")
 			Addstring(shstrtab, ".rela.rodata")
 			Addstring(shstrtab, ".rela"+relro_prefix+".typelink")
+			Addstring(shstrtab, ".rela"+relro_prefix+".itablink")
 			Addstring(shstrtab, ".rela"+relro_prefix+".gosymtab")
 			Addstring(shstrtab, ".rela"+relro_prefix+".gopclntab")
 			Addstring(shstrtab, ".rela.noptrdata")
@@ -1763,6 +1765,7 @@ func doelf() {
 			Addstring(shstrtab, ".rel.text")
 			Addstring(shstrtab, ".rel.rodata")
 			Addstring(shstrtab, ".rel"+relro_prefix+".typelink")
+			Addstring(shstrtab, ".rel"+relro_prefix+".itablink")
 			Addstring(shstrtab, ".rel"+relro_prefix+".gosymtab")
 			Addstring(shstrtab, ".rel"+relro_prefix+".gopclntab")
 			Addstring(shstrtab, ".rel.noptrdata")

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

@@ -1930,6 +1930,7 @@ func genasmsym(put func(*LSym, string, int, int64, int64, int, *LSym)) {
 			obj.SGCBITSRELRO,
 			obj.SRODATARELRO,
 			obj.STYPELINK,
+			obj.SITABLINK,
 			obj.SWINDOWS:
 			if !s.Attr.Reachable() {
 				continue

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

@@ -342,6 +342,8 @@ func symtab() {
 	xdefine("runtime.etext", obj.STEXT, 0)
 	xdefine("runtime.typelink", obj.SRODATA, 0)
 	xdefine("runtime.etypelink", obj.SRODATA, 0)
+	xdefine("runtime.itablink", obj.SRODATA, 0)
+	xdefine("runtime.eitablink", obj.SRODATA, 0)
 	xdefine("runtime.rodata", obj.SRODATA, 0)
 	xdefine("runtime.erodata", obj.SRODATA, 0)
 	xdefine("runtime.noptrdata", obj.SNOPTRDATA, 0)
@@ -428,6 +430,9 @@ func symtab() {
 	symtypelink := Linklookup(Ctxt, "runtime.typelink", 0)
 	symtypelink.Type = obj.STYPELINK
 
+	symitablink := Linklookup(Ctxt, "runtime.itablink", 0)
+	symitablink.Type = obj.SITABLINK
+
 	symt = Linklookup(Ctxt, "runtime.symtab", 0)
 	symt.Attr |= AttrLocal
 	symt.Type = obj.SSYMTAB
@@ -435,6 +440,7 @@ func symtab() {
 	symt.Attr |= AttrReachable
 
 	ntypelinks := 0
+	nitablinks := 0
 
 	// assign specific types so that they sort together.
 	// within a type they sort by size, so the .* symbols
@@ -463,6 +469,13 @@ func symtab() {
 			s.Outer = symtypelink
 		}
 
+		if strings.HasPrefix(s.Name, "go.itablink.") {
+			nitablinks++
+			s.Type = obj.SITABLINK
+			s.Attr |= AttrHidden
+			s.Outer = symitablink
+		}
+
 		if strings.HasPrefix(s.Name, "go.string.") {
 			s.Type = obj.SGOSTRING
 			s.Attr |= AttrHidden
@@ -543,6 +556,10 @@ func symtab() {
 	Addaddr(Ctxt, moduledata, Linklookup(Ctxt, "runtime.typelink", 0))
 	adduint(Ctxt, moduledata, uint64(ntypelinks))
 	adduint(Ctxt, moduledata, uint64(ntypelinks))
+	// The itablinks slice
+	Addaddr(Ctxt, moduledata, Linklookup(Ctxt, "runtime.itablink", 0))
+	adduint(Ctxt, moduledata, uint64(nitablinks))
+	adduint(Ctxt, moduledata, uint64(nitablinks))
 	if len(Ctxt.Shlibs) > 0 {
 		thismodulename := filepath.Base(outfile)
 		switch Buildmode {

src/runtime/symtab.go

@@ -129,6 +129,7 @@ type moduledata struct {
 	end, gcdata, gcbss    uintptr
 
 	typelinks []*_type
+	itablinks []*itab
 
 	modulename   string
 	modulehashes []modulehash