cmd/go: use ELF note instead of binary stamp on ELF systems

Other binary formats to follow.

For #11048.

Change-Id: Ia2d8b47c99c99d171c014b7cfd23c1c7ada5231c
Reviewed-on: https://go-review.googlesource.com/10707
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -2372,6 +2372,9 @@ func (gcToolchain) ld(b *builder, p *Package, out string, allactions []*action,
 	}
 	ldflags = setextld(ldflags, compiler)
 	ldflags = append(ldflags, "-buildmode="+ldBuildmode)
+	if p.buildID != "" {
+		ldflags = append(ldflags, "-buildid="+p.buildID)
+	}
 	ldflags = append(ldflags, buildLdflags...)
 	return b.run(".", p.ImportPath, nil, buildToolExec, tool("link"), "-o", out, importArgs, ldflags, mainpkg)
 }

src/cmd/go/note.go

@@ -5,10 +5,12 @@
 package main
 
 import (
+	"bytes"
 	"debug/elf"
 	"encoding/binary"
 	"fmt"
 	"io"
+	"os"
 )
 
 func readAligned4(r io.Reader, sz int32) ([]byte, error) {
@@ -64,3 +66,51 @@ func readELFNote(filename, name string, typ int32) ([]byte, error) {
 	}
 	return nil, nil
 }
+
+var elfGoNote = []byte("Go\x00\x00")
+
+// readELFGoBuildID the Go build ID string from an ELF binary.
+// The Go build ID is stored in a note described by an ELF PT_NOTE prog header.
+// The caller has already opened filename, to get f, and read the first 4 kB out, in data.
+func readELFGoBuildID(filename string, f *os.File, data []byte) (buildid string, err error) {
+	// Assume the note content is in the first 4 kB, already read.
+	// Rewrite the ELF header to set shnum to 0, so that we can pass
+	// the data to elf.NewFile and it will decode the Prog list but not
+	// try to read the section headers and the string table from disk.
+	// That's a waste of I/O when all we care about is the Prog list
+	// and the one ELF note.
+	switch elf.Class(data[elf.EI_CLASS]) {
+	case elf.ELFCLASS32:
+		data[48] = 0
+		data[49] = 0
+	case elf.ELFCLASS64:
+		data[60] = 0
+		data[61] = 0
+	}
+
+	const elfGoBuildIDTag = 4
+
+	ef, err := elf.NewFile(bytes.NewReader(data))
+	if err != nil {
+		return "", &os.PathError{Path: filename, Op: "parse", Err: err}
+	}
+	for _, p := range ef.Progs {
+		if p.Type != elf.PT_NOTE || p.Off >= uint64(len(data)) || p.Off+p.Filesz >= uint64(len(data)) || p.Filesz < 16 {
+			continue
+		}
+
+		note := data[p.Off : p.Off+p.Filesz]
+		nameSize := ef.ByteOrder.Uint32(note)
+		valSize := ef.ByteOrder.Uint32(note[4:])
+		tag := ef.ByteOrder.Uint32(note[8:])
+		name := note[12:16]
+		if nameSize != 4 || 16+valSize > uint32(len(note)) || tag != elfGoBuildIDTag || !bytes.Equal(name, elfGoNote) {
+			continue
+		}
+
+		return string(note[16 : 16+valSize]), nil
+	}
+
+	// No note. Treat as successful but build ID empty.
+	return "", nil
+}

src/cmd/go/note_test.go

@@ -0,0 +1,45 @@
+// Copyright 2015 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 main
+
+import (
+	"io/ioutil"
+	"os/exec"
+	"runtime"
+	"testing"
+)
+
+func TestNoteReading(t *testing.T) {
+	// TODO: Enable on non-ELF systems.
+	switch runtime.GOOS {
+	case "darwin", "windows", "plan9", "nacl":
+		t.Skipf("skipping on %q", runtime.GOOS)
+	}
+
+	// TODO: Replace with new test scaffolding by iant.
+	d, err := ioutil.TempDir("", "go-test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	out, err := exec.Command("go", "build", "-o", d+"/go.exe", "cmd/go").CombinedOutput()
+	if err != nil {
+		t.Fatalf("go build cmd/go: %v\n%s", err, out)
+	}
+
+	const buildID = "TestNoteReading-Build-ID"
+	out, err = exec.Command(d+"/go.exe", "build", "-ldflags", "-buildid="+buildID, "-o", d+"/hello.exe", "../../../test/helloworld.go").CombinedOutput()
+	if err != nil {
+		t.Fatalf("go build hello: %v\n%s", err, out)
+	}
+
+	id, err := readBuildIDFromBinary(d + "/hello.exe")
+	if err != nil {
+		t.Fatalf("reading build ID from hello binary: %v", err)
+	}
+
+	if id != buildID {
+		t.Fatalf("buildID in hello binary = %q, want %q", id, buildID)
+	}
+}

src/cmd/go/pkg.go

@@ -1091,7 +1091,7 @@ func readBuildID(p *Package) (id string, err error) {
 
 	// For commands, read build ID directly from binary.
 	if p.Name == "main" {
-		return readBuildIDFromBinary(p)
+		return readBuildIDFromBinary(p.Target)
 	}
 
 	// Otherwise, we expect to have an archive (.a) file,
@@ -1166,9 +1166,15 @@ var (
 	goBinary          = []byte("\x00\n\ngo binary\n")
 	endGoBinary       = []byte("\nend go binary\n")
 	newlineAndBuildid = []byte("\nbuild id ")
+
+	elfPrefix = []byte("ELF\x7F")
 )
 
 // readBuildIDFromBinary reads the build ID from a binary.
+//
+// The location of the build ID differs by object file type.
+// ELF uses a proper PT_NOTE section.
+//
 // Instead of trying to be good citizens and store the build ID in a
 // custom section of the binary, which would be different for each
 // of the four binary types we support (ELF, Mach-O, Plan 9, PE),
@@ -1182,17 +1188,30 @@ var (
 //	build id "XXX"
 //	end go binary
 //
-func readBuildIDFromBinary(p *Package) (id string, err error) {
-	if p.Target == "" {
-		return "", &os.PathError{Op: "parse", Path: p.Target, Err: errBuildIDUnknown}
+func readBuildIDFromBinary(filename string) (id string, err error) {
+	if filename == "" {
+		return "", &os.PathError{Op: "parse", Path: filename, Err: errBuildIDUnknown}
 	}
 
-	f, err := os.Open(p.Target)
+	f, err := os.Open(filename)
 	if err != nil {
 		return "", err
 	}
 	defer f.Close()
 
+	data := make([]byte, 4096)
+	_, err = io.ReadFull(f, data)
+	if err == io.ErrUnexpectedEOF {
+		err = nil
+	}
+	if err != nil {
+		return "", err
+	}
+
+	if bytes.HasPrefix(data, elfPrefix) {
+		return readELFGoBuildID(filename, f, data)
+	}
+
 	off, err := f.Seek(0, 2)
 	if err != nil {
 		return "", err
@@ -1204,7 +1223,7 @@ func readBuildIDFromBinary(p *Package) (id string, err error) {
 	if _, err := f.Seek(off-int64(n), 0); err != nil {
 		return "", err
 	}
-	data := make([]byte, n)
+	data = make([]byte, n)
 	if _, err := io.ReadFull(f, data); err != nil {
 		return "", err
 	}
@@ -1229,7 +1248,7 @@ func readBuildIDFromBinary(p *Package) (id string, err error) {
 	j := bytes.IndexByte(line, '\n') // must succeed - endGoBinary is at end and has newlines
 	id, err = strconv.Unquote(string(line[:j]))
 	if err != nil {
-		return "", &os.PathError{Op: "parse", Path: p.Target, Err: errBuildIDMalformed}
+		return "", &os.PathError{Op: "parse", Path: filename, Err: errBuildIDMalformed}
 	}
 	return id, nil
 }