cmd/link: put read-only data in __DATA_CONST segment

On darwin, we put read-only data in __TEXT segment on AMD64 in
exe (non-PIE) buildmode, and in __DATA on everywhere else. This
is not ideal: things in __DATA segment are not read-only, and
being mapped R/W may use more run-time resources.

In fact, newer darwin systems support a __DATA_CONST segment,
which the dynamic linker will map it read-only after applying
relocations. Use that.

Fixes #38830.

Change-Id: Ic281e6c6ca8ef5fec4bb7c5b71c50dd5393e78ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/253919
Reviewed-by: Than McIntosh <thanm@google.com>

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

@@ -930,7 +930,7 @@ func writeBlock(ctxt *Link, out *OutBuf, ldr *loader.Loader, syms []loader.Sym,
 			break
 		}
 		if val < addr {
-			ldr.Errorf(s, "phase error: addr=%#x but sym=%#x type=%d", addr, val, ldr.SymType(s))
+			ldr.Errorf(s, "phase error: addr=%#x but sym=%#x type=%v sect=%v", addr, val, ldr.SymType(s), ldr.SymSect(s).Name)
 			errorexit()
 		}
 		if addr < val {
@@ -1308,9 +1308,9 @@ func (state *dodataState) makeRelroForSharedLib(target *Link) {
 				// relro Type before it reaches here.
 				isRelro = true
 			case sym.SFUNCTAB:
-				if target.IsAIX() && ldr.SymName(s) == "runtime.etypes" {
+				if ldr.SymName(s) == "runtime.etypes" {
 					// runtime.etypes must be at the end of
-					// the relro datas.
+					// the relro data.
 					isRelro = true
 				}
 			}
@@ -1706,7 +1706,7 @@ func (state *dodataState) allocateDataSections(ctxt *Link) {
 	ldr.SetSymSect(ldr.LookupOrCreateSym("runtime.ebss", 0), sect)
 	bssGcEnd := state.datsize - int64(sect.Vaddr)
 
-	// Emit gcdata for bcc symbols now that symbol values have been assigned.
+	// Emit gcdata for bss symbols now that symbol values have been assigned.
 	gcsToEmit := []struct {
 		symName string
 		symKind sym.SymKind
@@ -1826,13 +1826,16 @@ func (state *dodataState) allocateDataSections(ctxt *Link) {
 	const fallbackPerm = 04
 	relroSecPerm := fallbackPerm
 	genrelrosecname := func(suffix string) string {
+		if suffix == "" {
+			return ".rodata"
+		}
 		return suffix
 	}
 	seg := segro
 
 	if ctxt.UseRelro() {
 		segrelro := &Segrelrodata
-		if ctxt.LinkMode == LinkExternal && ctxt.HeadType != objabi.Haix {
+		if ctxt.LinkMode == LinkExternal && !ctxt.IsAIX() && !ctxt.IsDarwin() {
 			// Using a separate segment with an external
 			// linker results in some programs moving
 			// their data sections unexpectedly, which
@@ -1845,9 +1848,12 @@ func (state *dodataState) allocateDataSections(ctxt *Link) {
 			state.datsize = 0
 		}
 
-		genrelrosecname = func(suffix string) string {
-			return ".data.rel.ro" + suffix
+		if !ctxt.IsDarwin() { // We don't need the special names on darwin.
+			genrelrosecname = func(suffix string) string {
+				return ".data.rel.ro" + suffix
+			}
 		}
+
 		relroReadOnly := []sym.SymKind{}
 		for _, symnro := range sym.ReadOnly {
 			symn := sym.RelROMap[symnro]

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

@@ -499,16 +499,7 @@ func machoadddynlib(lib string, linkmode LinkMode) {
 func machoshbits(ctxt *Link, mseg *MachoSeg, sect *sym.Section, segname string) {
 	buf := "__" + strings.Replace(sect.Name[1:], ".", "_", -1)
 
-	var msect *MachoSect
-	if sect.Rwx&1 == 0 && segname != "__DWARF" && (ctxt.Arch.Family == sys.ARM64 ||
-		(ctxt.Arch.Family == sys.AMD64 && ctxt.BuildMode != BuildModeExe)) {
-		// Darwin external linker on arm64, and on amd64 in c-shared/c-archive buildmode
-		// complains about absolute relocs in __TEXT, so if the section is not
-		// executable, put it in __DATA segment.
-		msect = newMachoSect(mseg, buf, "__DATA")
-	} else {
-		msect = newMachoSect(mseg, buf, segname)
-	}
+	msect := newMachoSect(mseg, buf, segname)
 
 	if sect.Rellen > 0 {
 		msect.reloc = uint32(sect.Reloff)
@@ -633,13 +624,28 @@ func asmbMacho(ctxt *Link) {
 		machoshbits(ctxt, ms, sect, "__TEXT")
 	}
 
+	/* rodata */
+	if ctxt.LinkMode != LinkExternal && Segrelrodata.Length > 0 {
+		ms = newMachoSeg("__DATA_CONST", 20)
+		ms.vaddr = Segrelrodata.Vaddr
+		ms.vsize = Segrelrodata.Length
+		ms.fileoffset = Segrelrodata.Fileoff
+		ms.filesize = Segrelrodata.Filelen
+		ms.prot1 = 3
+		ms.prot2 = 3
+		ms.flag = 0x10 // SG_READ_ONLY
+	}
+
+	for _, sect := range Segrelrodata.Sections {
+		machoshbits(ctxt, ms, sect, "__DATA_CONST")
+	}
+
 	/* data */
 	if ctxt.LinkMode != LinkExternal {
-		w := int64(Segdata.Length)
 		ms = newMachoSeg("__DATA", 20)
-		ms.vaddr = uint64(va) + uint64(v)
-		ms.vsize = uint64(w)
-		ms.fileoffset = uint64(v)
+		ms.vaddr = Segdata.Vaddr
+		ms.vsize = Segdata.Length
+		ms.fileoffset = Segdata.Fileoff
 		ms.filesize = Segdata.Filelen
 		ms.prot1 = 3
 		ms.prot2 = 3
@@ -695,7 +701,7 @@ func asmbMacho(ctxt *Link) {
 
 		if ctxt.LinkMode != LinkExternal {
 			ms := newMachoSeg("__LINKEDIT", 0)
-			ms.vaddr = uint64(va) + uint64(v) + uint64(Rnd(int64(Segdata.Length), int64(*FlagRound)))
+			ms.vaddr = uint64(Rnd(int64(Segdata.Vaddr+Segdata.Length), int64(*FlagRound)))
 			ms.vsize = uint64(s1) + uint64(s2) + uint64(s3) + uint64(s4)
 			ms.fileoffset = uint64(linkoff)
 			ms.filesize = ms.vsize
@@ -1008,7 +1014,7 @@ func doMachoLink(ctxt *Link) int64 {
 	size := int(ldr.SymSize(s1) + ldr.SymSize(s2) + ldr.SymSize(s3) + ldr.SymSize(s4))
 
 	if size > 0 {
-		linkoff = Rnd(int64(uint64(HEADR)+Segtext.Length), int64(*FlagRound)) + Rnd(int64(Segdata.Filelen), int64(*FlagRound)) + Rnd(int64(Segdwarf.Filelen), int64(*FlagRound))
+		linkoff = Rnd(int64(uint64(HEADR)+Segtext.Length), int64(*FlagRound)) + Rnd(int64(Segrelrodata.Filelen), int64(*FlagRound)) + Rnd(int64(Segdata.Filelen), int64(*FlagRound)) + Rnd(int64(Segdwarf.Filelen), int64(*FlagRound))
 		ctxt.Out.SeekSet(linkoff)
 
 		ctxt.Out.Write(ldr.Data(s1))
@@ -1086,6 +1092,9 @@ func machoEmitReloc(ctxt *Link) {
 	for _, sect := range Segtext.Sections[1:] {
 		relocSect(ctxt, sect, ctxt.datap)
 	}
+	for _, sect := range Segrelrodata.Sections {
+		relocSect(ctxt, sect, ctxt.datap)
+	}
 	for _, sect := range Segdata.Sections {
 		relocSect(ctxt, sect, ctxt.datap)
 	}

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

@@ -74,8 +74,12 @@ func (t *Target) IsDynlinkingGo() bool {
 func (t *Target) UseRelro() bool {
 	switch t.BuildMode {
 	case BuildModeCArchive, BuildModeCShared, BuildModeShared, BuildModePIE, BuildModePlugin:
-		return t.IsELF || t.HeadType == objabi.Haix
+		return t.IsELF || t.HeadType == objabi.Haix || t.HeadType == objabi.Hdarwin
 	default:
+		if t.HeadType == objabi.Hdarwin && t.IsARM64() {
+			// On darwin/ARM64, everything is PIE.
+			return true
+		}
 		return t.linkShared || (t.HeadType == objabi.Haix && t.LinkMode == LinkExternal)
 	}
 }