1 files changed, 40 insertions, 0 deletions
diff --git a/src/cmd/go/note.go b/src/cmd/go/note.go
index 97e18651e4..f8d6588b73 100644
--- a/src/cmd/go/note.go
+++ b/src/cmd/go/note.go
@@ -7,6 +7,7 @@ package main
 import (
 	"bytes"
 	"debug/elf"
+	"debug/macho"
 	"encoding/binary"
 	"fmt"
 	"io"
@@ -114,3 +115,42 @@ func readELFGoBuildID(filename string, f *os.File, data []byte) (buildid string,
 	// No note. Treat as successful but build ID empty.
 	return "", nil
 }
+
+// The Go build ID is stored at the beginning of the Mach-O __text segment.
+// The caller has already opened filename, to get f, and read a few kB out, in data.
+// Sadly, that's not guaranteed to hold the note, because there is an arbitrary amount
+// of other junk placed in the file ahead of the main text.
+func readMachoGoBuildID(filename string, f *os.File, data []byte) (buildid string, err error) {
+	// If the data we want has already been read, don't worry about Mach-O parsing.
+	// This is both an optimization and a hedge against the Mach-O parsing failing
+	// in the future due to, for example, the name of the __text section changing.
+	if b, err := readRawGoBuildID(filename, data); b != "" && err == nil {
+		return b, err
+	}
+
+	mf, err := macho.NewFile(f)
+	if err != nil {
+		return "", &os.PathError{Path: filename, Op: "parse", Err: err}
+	}
+
+	sect := mf.Section("__text")
+	if sect == nil {
+		// Every binary has a __text section. Something is wrong.
+		return "", &os.PathError{Path: filename, Op: "parse", Err: fmt.Errorf("cannot find __text section")}
+	}
+
+	// It should be in the first few bytes, but read a lot just in case,
+	// especially given our past problems on OS X with the build ID moving.
+	// There shouldn't be much difference between reading 4kB and 32kB:
+	// the hard part is getting to the data, not transferring it.
+	n := sect.Size
+	if n > uint64(BuildIDReadSize) {
+		n = uint64(BuildIDReadSize)
+	}
+	buf := make([]byte, n)
+	if _, err := f.ReadAt(buf, int64(sect.Offset)); err != nil {
+		return "", err
+	}
+
+	return readRawGoBuildID(filename, buf)
+}