Initial commit: Go 1.23 release state

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

@@ -0,0 +1,103 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ld
+
+// This file provides helper functions for updating/rewriting the UUID
+// load command within a Go go binary generated on Darwin using
+// external linking. Why is it necessary to update the UUID load
+// command? See issue #64947 for more detail, but the short answer is
+// that newer versions of the Macos toolchain (the newer linker in
+// particular) appear to compute the UUID based not just on the
+// content of the object files being linked but also on things like
+// the timestamps/paths of the objects; this makes it
+// difficult/impossible to support reproducible builds. Since we try
+// hard to maintain build reproducibility for Go, the APIs here
+// compute a new UUID (based on the Go build ID) and write it to the
+// final executable generated by the external linker.
+
+import (
+	"cmd/internal/notsha256"
+	"debug/macho"
+	"io"
+	"os"
+	"unsafe"
+)
+
+// uuidFromGoBuildId hashes the Go build ID and returns a slice of 16
+// bytes suitable for use as the payload in a Macho LC_UUID load
+// command.
+func uuidFromGoBuildId(buildID string) []byte {
+	if buildID == "" {
+		return make([]byte, 16)
+	}
+	hashedBuildID := notsha256.Sum256([]byte(buildID))
+	rv := hashedBuildID[:16]
+
+	// RFC 4122 conformance (see RFC 4122 Sections 4.2.2, 4.1.3). We
+	// want the "version" of this UUID to appear as 'hashed' as opposed
+	// to random or time-based.  This is something of a fiction since
+	// we're not actually hashing using MD5 or SHA1, but it seems better
+	// 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] |= 0x30
+	rv[8] &= 0x3f
+	rv[8] |= 0xc0
+
+	return rv
+}
+
+// machoRewriteUuid copies over the contents of the Macho executable
+// exef into the output file outexe, and in the process updates the
+// LC_UUID command to a new value recomputed from the Go build id.
+func machoRewriteUuid(ctxt *Link, exef *os.File, exem *macho.File, outexe string) error {
+	outf, err := os.OpenFile(outexe, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0755)
+	if err != nil {
+		return err
+	}
+	defer outf.Close()
+
+	// Copy over the file.
+	if _, err := io.Copy(outf, exef); err != nil {
+		return err
+	}
+
+	// Locate the portion of the binary containing the load commands.
+	cmdOffset := unsafe.Sizeof(exem.FileHeader)
+	if is64bit := exem.Magic == macho.Magic64; is64bit {
+		// mach_header_64 has one extra uint32.
+		cmdOffset += unsafe.Sizeof(exem.Magic)
+	}
+	if _, err := outf.Seek(int64(cmdOffset), 0); err != nil {
+		return err
+	}
+
+	// Read the load commands, looking for the LC_UUID cmd. If/when we
+	// locate it, overwrite it with a new value produced by
+	// uuidFromGoBuildId.
+	reader := loadCmdReader{next: int64(cmdOffset),
+		f: outf, order: exem.ByteOrder}
+	for i := uint32(0); i < exem.Ncmd; i++ {
+		cmd, err := reader.Next()
+		if err != nil {
+			return err
+		}
+		if cmd.Cmd == LC_UUID {
+			var u uuidCmd
+			if err := reader.ReadAt(0, &u); err != nil {
+				return err
+			}
+			copy(u.Uuid[:], uuidFromGoBuildId(*flagBuildid))
+			if err := reader.WriteAt(0, &u); err != nil {
+				return err
+			}
+			break
+		}
+	}
+
+	// We're done
+	return nil
+}