net: packetmmap: fix only tx timestamp on request

The packetmmap tx ring should only return timestamps if requested via
setsockopt PACKET_TIMESTAMP, as documented. This allows compatibility
with non-timestamp aware user-space code which checks
tp_status == TP_STATUS_AVAILABLE; not expecting additional timestamp
flags to be set in tp_status.

Fixes: b9c32fb2 ("packet: if hw/sw ts enabled in rx/tx ring, report which ts we got")
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Signed-off-by: Richard Sanger <rsanger@wand.net.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -422,7 +422,8 @@ static __u32 tpacket_get_timestamp(struct sk_buff *skb, struct timespec64 *ts,
 	    ktime_to_timespec64_cond(shhwtstamps->hwtstamp, ts))
 		return TP_STATUS_TS_RAW_HARDWARE;
 
-	if (ktime_to_timespec64_cond(skb->tstamp, ts))
+	if ((flags & SOF_TIMESTAMPING_SOFTWARE) &&
+	    ktime_to_timespec64_cond(skb->tstamp, ts))
 		return TP_STATUS_TS_SOFTWARE;
 
 	return 0;
@@ -2340,7 +2341,12 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 
 	skb_copy_bits(skb, 0, h.raw + macoff, snaplen);
 
-	if (!(ts_status = tpacket_get_timestamp(skb, &ts, po->tp_tstamp)))
+	/* Always timestamp; prefer an existing software timestamp taken
+	 * closer to the time of capture.
+	 */
+	ts_status = tpacket_get_timestamp(skb, &ts,
+					  po->tp_tstamp | SOF_TIMESTAMPING_SOFTWARE);
+	if (!ts_status)
 		ktime_get_real_ts64(&ts);
 
 	status |= ts_status;