cmd/go: fix reading PT_NOTE segment with multiple notes

The old code was assuming that a PT_NOTE segment never had more than one
note, but there is no such requirement.

Fixes #13364.

Change-Id: I3f6b3716130bf7af6abe81b8e10571a8c7cd943c
Reviewed-on: https://go-review.googlesource.com/17331
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/note.go

@@ -121,15 +121,26 @@ func readELFGoBuildID(filename string, f *os.File, data []byte) (buildid string,
 				return "", err
 			}
 		}
+
+		filesz := p.Filesz
+		for filesz >= 16 {
 			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
+			if nameSize == 4 && 16+valSize <= uint32(len(note)) && tag == elfGoBuildIDTag && bytes.Equal(name, elfGoNote) {
+				return string(note[16 : 16+valSize]), nil
 			}
 
-		return string(note[16 : 16+valSize]), nil
+			nameSize = (nameSize + 3) &^ 3
+			valSize = (valSize + 3) &^ 3
+			notesz := uint64(12 + nameSize + valSize)
+			if filesz <= notesz {
+				break
+			}
+			filesz -= notesz
+			note = note[notesz:]
+		}
 	}
 
 	// No note. Treat as successful but build ID empty.

src/cmd/go/note_test.go

@@ -28,9 +28,6 @@ func TestNoteReading2K(t *testing.T) {
 }
 
 func testNoteReading(t *testing.T) {
-	if runtime.GOOS == "dragonfly" {
-		t.Skipf("TestNoteReading is broken on dragonfly - golang.org/issue/13364", runtime.GOOS)
-	}
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.tempFile("hello.go", `package main; func main() { print("hello, world\n") }`)