net-timestamp: sock_tx_timestamp() fix

sock_tx_timestamp() should not ignore initial *tx_flags value, as TCP
stack can store SKBTX_SHARED_FRAG in it.

Also first argument (struct sock *) can be const.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 4ed2d765 ("net-timestamp: TCP timestamping")
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2199,9 +2199,11 @@ static inline void sock_recv_ts_and_drops(struct msghdr *msg, struct sock *sk,
 /**
  * sock_tx_timestamp - checks whether the outgoing packet is to be time stamped
  * @sk:		socket sending this packet
- * @tx_flags:	filled with instructions for time stamping
+ * @tx_flags:	completed with instructions for time stamping
+ *
+ * Note : callers should take care of initial *tx_flags value (usually 0)
  */
-void sock_tx_timestamp(struct sock *sk, __u8 *tx_flags);
+void sock_tx_timestamp(const struct sock *sk, __u8 *tx_flags);
 
 /**
  * sk_eat_skb - Release a skb if it is no longer needed

net/socket.c

@@ -610,20 +610,26 @@ void sock_release(struct socket *sock)
 }
 EXPORT_SYMBOL(sock_release);
 
-void sock_tx_timestamp(struct sock *sk, __u8 *tx_flags)
+void sock_tx_timestamp(const struct sock *sk, __u8 *tx_flags)
 {
-	*tx_flags = 0;
+	u8 flags = *tx_flags;
+
 	if (sk->sk_tsflags & SOF_TIMESTAMPING_TX_HARDWARE)
-		*tx_flags |= SKBTX_HW_TSTAMP;
+		flags |= SKBTX_HW_TSTAMP;
+
 	if (sk->sk_tsflags & SOF_TIMESTAMPING_TX_SOFTWARE)
-		*tx_flags |= SKBTX_SW_TSTAMP;
+		flags |= SKBTX_SW_TSTAMP;
+
 	if (sk->sk_tsflags & SOF_TIMESTAMPING_TX_SCHED)
-		*tx_flags |= SKBTX_SCHED_TSTAMP;
+		flags |= SKBTX_SCHED_TSTAMP;
+
 	if (sk->sk_tsflags & SOF_TIMESTAMPING_TX_ACK)
-		*tx_flags |= SKBTX_ACK_TSTAMP;
+		flags |= SKBTX_ACK_TSTAMP;
 
 	if (sock_flag(sk, SOCK_WIFI_STATUS))
-		*tx_flags |= SKBTX_WIFI_STATUS;
+		flags |= SKBTX_WIFI_STATUS;
+
+	*tx_flags = flags;
 }
 EXPORT_SYMBOL(sock_tx_timestamp);