Walk the control buffer by hand instead of allocating a message list per frame

ts.go

@@ -80,21 +80,31 @@ func enableRxTimestamps(fd int) error {
 // means the mac did not produce a stamp.
 const scmTimestampingLen = 3 * int(unsafe.Sizeof(unix.Timespec{}))
 
+// A control message's payload starts at its header rounded up to a pointer, and
+// the next starts at its length rounded up the same way.
+const (
+	cmsgAlignMask = unix.SizeofPtr - 1
+	cmsgDataOff   = (unix.SizeofCmsghdr + cmsgAlignMask) &^ cmsgAlignMask
+)
+
+// Walked by hand rather than through ParseSocketControlMessage, which allocates
+// per call on a path that runs for every frame received.
 func hwTimestamp(oob []byte) (int64, bool) {
-	msgs, err := unix.ParseSocketControlMessage(oob)
-	if err != nil {
-		return 0, false
-	}
-	for _, m := range msgs {
-		if m.Header.Level != unix.SOL_SOCKET || m.Header.Type != unix.SCM_TIMESTAMPING {
-			continue
-		}
-		if len(m.Data) < scmTimestampingLen {
+	for off := 0; off+unix.SizeofCmsghdr <= len(oob); {
+		h := (*unix.Cmsghdr)(unsafe.Pointer(&oob[off]))
+		n := int(h.Len)
+		if n < unix.SizeofCmsghdr || off+n > len(oob) {
 			return 0, false
 		}
-		ts := (*[3]unix.Timespec)(unsafe.Pointer(&m.Data[0]))
-		ns := ts[2].Nano()
-		return ns, ns != 0
+		if h.Level == unix.SOL_SOCKET && h.Type == unix.SCM_TIMESTAMPING {
+			if cmsgDataOff+scmTimestampingLen > n {
+				return 0, false
+			}
+			ts := (*[3]unix.Timespec)(unsafe.Pointer(&oob[off+cmsgDataOff]))
+			ns := ts[2].Nano()
+			return ns, ns != 0
+		}
+		off += (n + cmsgAlignMask) &^ cmsgAlignMask
 	}
 	return 0, false
 }