cmd/link: apply -B UUID to external linking on Mach-O

Currently, on Mach-O, the -B UUID setting is only applied in
internal linking mode, whereas in external linking mode the UUID
is always rewritten to a hash of Go build ID. This CL makes it
apply to external linking as well. This makes the behavior
consistent on both linkmodes, and also consistent with the -B
flag's behavior for GNU build ID on ELF.

Add tests.

Updates #68678.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64_14,gotip-darwin-arm64_13
Change-Id: I276a5930e231141440cdba16e8812df28ac4237b
Reviewed-on: https://go-review.googlesource.com/c/go/+/618599
Reviewed-by: Than McIntosh <thanm@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

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

@@ -1468,6 +1468,9 @@ func (ctxt *Link) hostlink() {
 				argv = append(argv, "-Wl,-x")
 			}
 		}
+		if *flagHostBuildid == "none" {
+			argv = append(argv, "-Wl,-no_uuid")
+		}
 	case objabi.Hopenbsd:
 		argv = append(argv, "-pthread")
 		if ctxt.BuildMode != BuildModePIE {
@@ -2059,7 +2062,7 @@ func (ctxt *Link) hostlink() {
 			uuidUpdated = true
 		}
 	}
-	if ctxt.IsDarwin() && !uuidUpdated && *flagBuildid != "" {
+	if ctxt.IsDarwin() && !uuidUpdated && len(buildinfo) > 0 {
 		updateMachoOutFile("rewriting uuid",
 			func(ctxt *Link, exef *os.File, exem *macho.File, outexe string) error {
 				return machoRewriteUuid(ctxt, exef, exem, outexe)

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

@@ -195,8 +195,9 @@ func machoCombineDwarf(ctxt *Link, exef *os.File, exem *macho.File, dsym, outexe
 		case imacho.LC_UUID:
 			var u uuidCmd
 			err = reader.ReadAt(0, &u)
-			if err == nil {
-				copy(u.Uuid[:], uuidFromGoBuildId(*flagBuildid))
+			if err == nil && len(buildinfo) > 0 {
+				clear(u.Uuid[:])
+				copy(u.Uuid[:], buildinfo)
 				err = reader.WriteAt(0, &u)
 			}
 		case macho.LoadCmdDylib, macho.LoadCmdThread, macho.LoadCmdUnixThread,

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

@@ -86,7 +86,8 @@ func machoRewriteUuid(ctxt *Link, exef *os.File, exem *macho.File, outexe string
 			if err := reader.ReadAt(0, &u); err != nil {
 				return err
 			}
-			copy(u.Uuid[:], uuidFromGoBuildId(*flagBuildid))
+			clear(u.Uuid[:])
+			copy(u.Uuid[:], buildinfo)
 			if err := reader.WriteAt(0, &u); err != nil {
 				return err
 			}

src/cmd/link/link_test.go

@@ -19,6 +19,7 @@ import (
 	"strings"
 	"testing"
 
+	imacho "cmd/internal/macho"
 	"cmd/internal/sys"
 )
 
@@ -386,7 +387,6 @@ func TestMachOBuildVersion(t *testing.T) {
 		t.Fatal(err)
 	}
 	found := false
-	const LC_BUILD_VERSION = 0x32
 	checkMin := func(ver uint32) {
 		major, minor, patch := (ver>>16)&0xff, (ver>>8)&0xff, (ver>>0)&0xff
 		if major < 11 {
@@ -396,7 +396,7 @@ func TestMachOBuildVersion(t *testing.T) {
 	for _, cmd := range exem.Loads {
 		raw := cmd.Raw()
 		type_ := exem.ByteOrder.Uint32(raw)
-		if type_ != LC_BUILD_VERSION {
+		if type_ != imacho.LC_BUILD_VERSION {
 			continue
 		}
 		osVer := exem.ByteOrder.Uint32(raw[12:])
@@ -411,6 +411,78 @@ func TestMachOBuildVersion(t *testing.T) {
 	}
 }
 
+func TestMachOUUID(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	if runtime.GOOS != "darwin" {
+		t.Skip("this is only for darwin")
+	}
+
+	t.Parallel()
+
+	tmpdir := t.TempDir()
+
+	src := filepath.Join(tmpdir, "main.go")
+	err := os.WriteFile(src, []byte(trivialSrc), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	extractUUID := func(exe string) string {
+		exem, err := macho.Open(exe)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer exem.Close()
+		for _, cmd := range exem.Loads {
+			raw := cmd.Raw()
+			type_ := exem.ByteOrder.Uint32(raw)
+			if type_ != imacho.LC_UUID {
+				continue
+			}
+			return string(raw[8:24])
+		}
+		return ""
+	}
+
+	tests := []struct{ name, ldflags, expect string }{
+		{"default", "", "gobuildid"},
+		{"gobuildid", "-B=gobuildid", "gobuildid"},
+		{"specific", "-B=0x0123456789ABCDEF0123456789ABCDEF", "\x01\x23\x45\x67\x89\xAB\xCD\xEF\x01\x23\x45\x67\x89\xAB\xCD\xEF"},
+		{"none", "-B=none", ""},
+	}
+	if testenv.HasCGO() {
+		for _, test := range tests {
+			t1 := test
+			t1.name += "_external"
+			t1.ldflags += " -linkmode=external"
+			tests = append(tests, t1)
+		}
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			exe := filepath.Join(tmpdir, test.name)
+			cmd := testenv.Command(t, testenv.GoToolPath(t), "build", "-ldflags="+test.ldflags, "-o", exe, src)
+			if out, err := cmd.CombinedOutput(); err != nil {
+				t.Fatalf("%v: %v:\n%s", cmd.Args, err, out)
+			}
+			uuid := extractUUID(exe)
+			if test.expect == "gobuildid" {
+				// Go buildid is not known in source code. Check UUID is present,
+				// and satisifies UUIDv3.
+				if uuid == "" {
+					t.Fatal("expect nonempty UUID, got empty")
+				}
+				// The version number is the high 4 bits of byte 6.
+				if uuid[6]>>4 != 3 {
+					t.Errorf("expect v3 UUID, got %X (version %d)", uuid, uuid[6]>>4)
+				}
+			} else if uuid != test.expect {
+				t.Errorf("UUID mismatch: got %X, want %X", uuid, test.expect)
+			}
+		})
+	}
+}
+
 const Issue34788src = `
 
 package blah