cmd/link: generate Mach-O UUID when -B flag is specified

Currently, on Mach-O, the Go linker doesn't generate LC_UUID in
internal linking mode. This causes some macOS system tools unable
to track the binary, as well as in some cases the binary unable
to access local network on macOS 15.

This CL makes the linker start generate LC_UUID. Currently, the
UUID is generated if the -B flag is specified. And we'll make it
generate UUID by default in a later CL. The -B flag is currently
for generating GNU build ID on ELF, which is a similar concept to
Mach-O's UUID. Instead of introducing another flag, we just use
the same flag and the same setting. Specifically, "-B gobuildid"
will generate a UUID based on the Go build ID.

For golang#68678.

Cq-Include-Trybots: luci.golang.try:gotip-darwin-amd64_14,gotip-darwin-arm64_13
Change-Id: I90089a78ba144110bf06c1c6836daf2d737ff10a
Reviewed-on: https://go-review.googlesource.com/c/go/+/618595
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ingo Oeser <nightlyone@googlemail.com>
Reviewed-by: Than McIntosh <thanm@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -805,13 +805,19 @@ func elfwritefreebsdsig(out *OutBuf) int {
 	return int(sh.Size)
 }
 
-func addbuildinfo(val string) {
+func addbuildinfo(ctxt *Link) {
+	val := *flagHostBuildid
 	if val == "gobuildid" {
 		buildID := *flagBuildid
 		if buildID == "" {
 			Exitf("-B gobuildid requires a Go build ID supplied via -buildid")
 		}
 
+		if ctxt.IsDarwin() {
+			buildinfo = uuidFromGoBuildId(buildID)
+			return
+		}
+
 		hashedBuildID := hash.Sum32([]byte(buildID))
 		buildinfo = hashedBuildID[:20]
 
@@ -821,11 +827,13 @@ func addbuildinfo(val string) {
 	if !strings.HasPrefix(val, "0x") {
 		Exitf("-B argument must start with 0x: %s", val)
 	}
-
 	ov := val
 	val = val[2:]
 
-	const maxLen = 32
+	maxLen := 32
+	if ctxt.IsDarwin() {
+		maxLen = 16
+	}
 	if hex.DecodedLen(len(val)) > maxLen {
 		Exitf("-B option too long (max %d digits): %s", maxLen, ov)
 	}

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

@@ -297,6 +297,8 @@ func getMachoHdr() *MachoHdr {
 	return &machohdr
 }
 
+// Create a new Mach-O load command. ndata is the number of 32-bit words for
+// the data (not including the load command header).
 func newMachoLoad(arch *sys.Arch, type_ uint32, ndata uint32) *MachoLoad {
 	if arch.PtrSize == 8 && (ndata&1 != 0) {
 		ndata++
@@ -849,6 +851,20 @@ func asmbMacho(ctxt *Link) {
 			}
 		}
 
+		if ctxt.IsInternal() && len(buildinfo) > 0 {
+			ml := newMachoLoad(ctxt.Arch, LC_UUID, 4)
+			// Mach-O UUID is 16 bytes
+			if len(buildinfo) < 16 {
+				buildinfo = append(buildinfo, make([]byte, 16)...)
+			}
+			// By default, buildinfo is already in UUIDv3 format
+			// (see uuidFromGoBuildId).
+			ml.data[0] = ctxt.Arch.ByteOrder.Uint32(buildinfo)
+			ml.data[1] = ctxt.Arch.ByteOrder.Uint32(buildinfo[4:])
+			ml.data[2] = ctxt.Arch.ByteOrder.Uint32(buildinfo[8:])
+			ml.data[3] = ctxt.Arch.ByteOrder.Uint32(buildinfo[12:])
+		}
+
 		if ctxt.IsInternal() && ctxt.NeedCodeSign() {
 			ml := newMachoLoad(ctxt.Arch, LC_CODE_SIGNATURE, 2)
 			ml.data[0] = uint32(codesigOff)

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

@@ -42,7 +42,7 @@ func uuidFromGoBuildId(buildID string) []byte {
 	// to use this UUID flavor than any of the others. This is similar
 	// to how other linkers handle this (for example this code in lld:
 	// https://github.com/llvm/llvm-project/blob/2a3a79ce4c2149d7787d56f9841b66cacc9061d0/lld/MachO/Writer.cpp#L524).
-	rv[6] &= 0xcf
+	rv[6] &= 0x0f
 	rv[6] |= 0x30
 	rv[8] &= 0x3f
 	rv[8] |= 0xc0

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

@@ -95,6 +95,7 @@ var (
 	flagN             = flag.Bool("n", false, "no-op (deprecated)")
 	FlagS             = flag.Bool("s", false, "disable symbol table")
 	flag8             bool // use 64-bit addresses in symbol table
+	flagHostBuildid   = flag.String("B", "", "set ELF NT_GNU_BUILD_ID `note` or Mach-O UUID; use \"gobuildid\" to generate it from the Go build ID")
 	flagInterpreter   = flag.String("I", "", "use `linker` as ELF dynamic linker")
 	flagCheckLinkname = flag.Bool("checklinkname", true, "check linkname symbol references")
 	FlagDebugTramp    = flag.Int("debugtramp", 0, "debug trampolines")
@@ -196,7 +197,6 @@ func Main(arch *sys.Arch, theArch Arch) {
 	flag.Var(&ctxt.LinkMode, "linkmode", "set link `mode`")
 	flag.Var(&ctxt.BuildMode, "buildmode", "set build `mode`")
 	flag.BoolVar(&ctxt.compressDWARF, "compressdwarf", true, "compress DWARF if possible")
-	objabi.Flagfn1("B", "add an ELF NT_GNU_BUILD_ID `note` when using ELF; use \"gobuildid\" to generate it from the Go build ID", addbuildinfo)
 	objabi.Flagfn1("L", "add specified `directory` to library path", func(a string) { Lflag(ctxt, a) })
 	objabi.AddVersionFlag() // -V
 	objabi.Flagfn1("X", "add string value `definition` of the form importpath.name=value", func(s string) { addstrdata1(ctxt, s) })
@@ -294,6 +294,10 @@ func Main(arch *sys.Arch, theArch Arch) {
 		*flagBuildid = "go-openbsd"
 	}
 
+	if *flagHostBuildid != "" {
+		addbuildinfo(ctxt)
+	}
+
 	// enable benchmarking
 	var bench *benchmark.Metrics
 	if len(*benchmarkFlag) != 0 {

test/fixedbugs/issue14636.go

@@ -12,22 +12,29 @@ import (
 	"bytes"
 	"log"
 	"os/exec"
+	"runtime"
 	"strings"
 )
 
 func main() {
-	checkLinkOutput("", "-B argument must start with 0x")
+	// The cannot open file error indicates that the parsing of -B flag
+	// succeeded and it failed at a later step.
 	checkLinkOutput("0", "-B argument must start with 0x")
-	checkLinkOutput("0x", "usage")
+	checkLinkOutput("0x", "cannot open file nonexistent.o")
 	checkLinkOutput("0x0", "-B argument must have even number of digits")
-	checkLinkOutput("0x00", "usage")
+	checkLinkOutput("0x00", "cannot open file nonexistent.o")
 	checkLinkOutput("0xYZ", "-B argument contains invalid hex digit")
-	checkLinkOutput("0x"+strings.Repeat("00", 32), "usage")
-	checkLinkOutput("0x"+strings.Repeat("00", 33), "-B option too long (max 32 digits)")
+
+	maxLen := 32
+	if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
+		maxLen = 16
+	}
+	checkLinkOutput("0x"+strings.Repeat("00", maxLen), "cannot open file nonexistent.o")
+	checkLinkOutput("0x"+strings.Repeat("00", maxLen+1), "-B option too long")
 }
 
 func checkLinkOutput(buildid string, message string) {
-	cmd := exec.Command("go", "tool", "link", "-B", buildid)
+	cmd := exec.Command("go", "tool", "link", "-B", buildid, "nonexistent.o")
 	out, err := cmd.CombinedOutput()
 	if err == nil {
 		log.Fatalf("expected cmd/link to fail")
@@ -39,6 +46,6 @@ func checkLinkOutput(buildid string, message string) {
 	}
 
 	if !strings.Contains(firstLine, message) {
-		log.Fatalf("cmd/link output did not include expected message %q: %s", message, firstLine)
+		log.Fatalf("%s: cmd/link output did not include expected message %q: %s", buildid, message, firstLine)
 	}
 }