cmd/go, cmd/link: enable buildmode=pie on darwin/amd64

Change some configurations to enable the feature. Also add the test.
This CL doesn't include internal linking support which is tentatively
disabled due to #18968. We could do that another day.

Fixes #21220

Change-Id: I601d2d78446d36332acc70be0d5b9461ac635208
Reviewed-on: https://go-review.googlesource.com/54790Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -6,6 +6,8 @@ package main_test
 
 import (
 	"bytes"
+	"debug/elf"
+	"debug/macho"
 	"fmt"
 	"go/format"
 	"internal/race"
@@ -4331,3 +4333,59 @@ func TestListTests(t *testing.T) {
 	t.Run("Example1", testWith("Example", "ExampleSimple"))
 	t.Run("Example2", testWith("Example", "ExampleWithEmptyOutput"))
 }
+
+func TestBuildmodePIE(t *testing.T) {
+	if runtime.Compiler == "gccgo" {
+		t.Skipf("skipping test because buildmode=pie is not supported on gccgo")
+	}
+
+	platform := fmt.Sprintf("%s/%s", runtime.GOOS, runtime.GOARCH)
+	switch platform {
+	case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/s390x",
+		"android/amd64", "android/arm", "android/arm64", "android/386":
+	case "darwin/amd64":
+	default:
+		t.Skipf("skipping test because buildmode=pie is not supported on %s", platform)
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+
+	tg.tempFile("main.go", `package main; func main() { print("hello") }`)
+	src := tg.path("main.go")
+	obj := tg.path("main")
+	tg.run("build", "-buildmode=pie", "-o", obj, src)
+
+	switch runtime.GOOS {
+	case "linux", "android":
+		f, err := elf.Open(obj)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if f.Type != elf.ET_DYN {
+			t.Errorf("PIE type must be ET_DYN, but %s", f.Type)
+		}
+	case "darwin":
+		f, err := macho.Open(obj)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if f.Flags&macho.FlagDyldLink == 0 {
+			t.Error("PIE must have DyldLink flag, but not")
+		}
+		if f.Flags&macho.FlagPIE == 0 {
+			t.Error("PIE must have PIE flag, but not")
+		}
+	default:
+		panic("unreachable")
+	}
+
+	out, err := exec.Command(obj).CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(out) != "hello" {
+		t.Errorf("got %q; want %q", out, "hello")
+	}
+}

src/cmd/go/internal/work/build.go

@@ -327,6 +327,8 @@ func BuildModeInit() {
 			case "linux/386", "linux/amd64", "linux/arm", "linux/arm64", "linux/ppc64le", "linux/s390x",
 				"android/amd64", "android/arm", "android/arm64", "android/386":
 				codegenArg = "-shared"
+			case "darwin/amd64":
+				codegenArg = "-shared"
 			default:
 				base.Fatalf("-buildmode=pie not supported on %s\n", platform)
 			}

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

@@ -44,6 +44,12 @@ func (mode *BuildMode) Set(s string) error {
 	case "pie":
 		switch objabi.GOOS {
 		case "android", "linux":
+		case "darwin":
+			switch objabi.GOARCH {
+			case "amd64":
+			default:
+				return badmode()
+			}
 		default:
 			return badmode()
 		}

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

@@ -1095,7 +1095,7 @@ func (l *Link) hostlink() {
 		argv = append(argv, "-Wl,-headerpad,1144")
 		if l.DynlinkingGo() {
 			argv = append(argv, "-Wl,-flat_namespace")
-		} else if !SysArch.InFamily(sys.ARM64) {
+		} else if !SysArch.InFamily(sys.ARM64) && Buildmode != BuildmodePIE {
 			argv = append(argv, "-Wl,-no_pie")
 		}
 	case objabi.Hopenbsd:
@@ -1114,10 +1114,13 @@ func (l *Link) hostlink() {
 			argv = append(argv, "-Wl,-pagezero_size,4000000")
 		}
 	case BuildmodePIE:
+		// ELF.
+		if Headtype != objabi.Hdarwin {
 			if UseRelro() {
 				argv = append(argv, "-Wl,-z,relro")
 			}
 			argv = append(argv, "-pie")
+		}
 	case BuildmodeCShared:
 		if Headtype == objabi.Hdarwin {
 			argv = append(argv, "-dynamiclib")

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

@@ -401,8 +401,8 @@ func machoshbits(ctxt *Link, mseg *MachoSeg, sect *Section, segname string) {
 
 	var msect *MachoSect
 	if sect.Rwx&1 == 0 && segname != "__DWARF" && (SysArch.Family == sys.ARM64 ||
-		(SysArch.Family == sys.AMD64 && (Buildmode == BuildmodeCShared || Buildmode == BuildmodeCArchive || Buildmode == BuildmodePlugin)) ||
-		(SysArch.Family == sys.ARM && (Buildmode == BuildmodeCShared || Buildmode == BuildmodeCArchive || Buildmode == BuildmodePlugin))) {
+		(SysArch.Family == sys.AMD64 && Buildmode != BuildmodeExe) ||
+		(SysArch.Family == sys.ARM && Buildmode != BuildmodeExe)) {
 		// Darwin external linker on arm64 and on amd64 and arm in c-shared/c-archive buildmode
 		// complains about absolute relocs in __TEXT, so if the section is not
 		// executable, put it in __DATA segment.