net-timestamp: introduce SOF_TIMESTAMPING_OPT_RX_FILTER flag

introduce a new flag SOF_TIMESTAMPING_OPT_RX_FILTER in the receive
path. User can set it with SOF_TIMESTAMPING_SOFTWARE to filter
out rx software timestamp report, especially after a process turns on
netstamp_needed_key which can time stamp every incoming skb.

Previously, we found out if an application starts first which turns on
netstamp_needed_key, then another one only passing SOF_TIMESTAMPING_SOFTWARE
could also get rx timestamp. Now we handle this case by introducing this
new flag without breaking users.

Quoting Willem to explain why we need the flag:
"why a process would want to request software timestamp reporting, but
not receive software timestamp generation. The only use I see is when
the application does request
SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_TX_SOFTWARE."

Similarly, this new flag could also be used for hardware case where we
can set it with SOF_TIMESTAMPING_RAW_HARDWARE, then we won't receive
hardware receive timestamp.

Another thing about errqueue in this patch I have a few words to say:
In this case, we need to handle the egress path carefully, or else
reporting the tx timestamp will fail. Egress path and ingress path will
finally call sock_recv_timestamp(). We have to distinguish them.
Errqueue is a good indicator to reflect the flow direction.

Suggested-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240909015612.3856-2-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/timestamping.rst

@@ -267,6 +267,23 @@ SOF_TIMESTAMPING_OPT_TX_SWHW:
   two separate messages will be looped to the socket's error queue,
   each containing just one timestamp.
 
+SOF_TIMESTAMPING_OPT_RX_FILTER:
+  Filter out spurious receive timestamps: report a receive timestamp
+  only if the matching timestamp generation flag is enabled.
+
+  Receive timestamps are generated early in the ingress path, before a
+  packet's destination socket is known. If any socket enables receive
+  timestamps, packets for all socket will receive timestamped packets.
+  Including those that request timestamp reporting with
+  SOF_TIMESTAMPING_SOFTWARE and/or SOF_TIMESTAMPING_RAW_HARDWARE, but
+  do not request receive timestamp generation. This can happen when
+  requesting transmit timestamps only.
+
+  Receiving spurious timestamps is generally benign. A process can
+  ignore the unexpected non-zero value. But it makes behavior subtly
+  dependent on other sockets. This flag isolates the socket for more
+  deterministic behavior.
+
 New applications are encouraged to pass SOF_TIMESTAMPING_OPT_ID to
 disambiguate timestamps and SOF_TIMESTAMPING_OPT_TSONLY to operate
 regardless of the setting of sysctl net.core.tstamp_allow_data.

include/uapi/linux/net_tstamp.h

@@ -32,8 +32,9 @@ enum {
 	SOF_TIMESTAMPING_OPT_TX_SWHW = (1<<14),
 	SOF_TIMESTAMPING_BIND_PHC = (1 << 15),
 	SOF_TIMESTAMPING_OPT_ID_TCP = (1 << 16),
+	SOF_TIMESTAMPING_OPT_RX_FILTER = (1 << 17),
 
-	SOF_TIMESTAMPING_LAST = SOF_TIMESTAMPING_OPT_ID_TCP,
+	SOF_TIMESTAMPING_LAST = SOF_TIMESTAMPING_OPT_RX_FILTER,
 	SOF_TIMESTAMPING_MASK = (SOF_TIMESTAMPING_LAST - 1) |
 				 SOF_TIMESTAMPING_LAST
 };

net/ethtool/common.c

@@ -427,6 +427,7 @@ const char sof_timestamping_names[][ETH_GSTRING_LEN] = {
 	[const_ilog2(SOF_TIMESTAMPING_OPT_TX_SWHW)]  = "option-tx-swhw",
 	[const_ilog2(SOF_TIMESTAMPING_BIND_PHC)]     = "bind-phc",
 	[const_ilog2(SOF_TIMESTAMPING_OPT_ID_TCP)]   = "option-id-tcp",
+	[const_ilog2(SOF_TIMESTAMPING_OPT_RX_FILTER)] = "option-rx-filter",
 };
 static_assert(ARRAY_SIZE(sof_timestamping_names) == __SOF_TIMESTAMPING_CNT);
 

net/ipv4/tcp.c

@@ -2235,6 +2235,7 @@ void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
 			struct scm_timestamping_internal *tss)
 {
 	int new_tstamp = sock_flag(sk, SOCK_TSTAMP_NEW);
+	u32 tsflags = READ_ONCE(sk->sk_tsflags);
 	bool has_timestamping = false;
 
 	if (tss->ts[0].tv_sec || tss->ts[0].tv_nsec) {
@@ -2274,14 +2275,18 @@ void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
 			}
 		}
 
-		if (READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_SOFTWARE)
+		if (tsflags & SOF_TIMESTAMPING_SOFTWARE &&
+		    (tsflags & SOF_TIMESTAMPING_RX_SOFTWARE ||
+		     !(tsflags & SOF_TIMESTAMPING_OPT_RX_FILTER)))
 			has_timestamping = true;
 		else
 			tss->ts[0] = (struct timespec64) {0};
 	}
 
 	if (tss->ts[2].tv_sec || tss->ts[2].tv_nsec) {
-		if (READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_RAW_HARDWARE)
+		if (tsflags & SOF_TIMESTAMPING_RAW_HARDWARE &&
+		    (tsflags & SOF_TIMESTAMPING_RX_HARDWARE ||
+		     !(tsflags & SOF_TIMESTAMPING_OPT_RX_FILTER)))
 			has_timestamping = true;
 		else
 			tss->ts[2] = (struct timespec64) {0};

net/socket.c

@@ -946,11 +946,17 @@ void __sock_recv_timestamp(struct msghdr *msg, struct sock *sk,
 
 	memset(&tss, 0, sizeof(tss));
 	tsflags = READ_ONCE(sk->sk_tsflags);
-	if ((tsflags & SOF_TIMESTAMPING_SOFTWARE) &&
+	if ((tsflags & SOF_TIMESTAMPING_SOFTWARE &&
+	     (tsflags & SOF_TIMESTAMPING_RX_SOFTWARE ||
+	      skb_is_err_queue(skb) ||
+	      !(tsflags & SOF_TIMESTAMPING_OPT_RX_FILTER))) &&
 	    ktime_to_timespec64_cond(skb->tstamp, tss.ts + 0))
 		empty = 0;
 	if (shhwtstamps &&
-	    (tsflags & SOF_TIMESTAMPING_RAW_HARDWARE) &&
+	    (tsflags & SOF_TIMESTAMPING_RAW_HARDWARE &&
+	     (tsflags & SOF_TIMESTAMPING_RX_HARDWARE ||
+	      skb_is_err_queue(skb) ||
+	      !(tsflags & SOF_TIMESTAMPING_OPT_RX_FILTER))) &&
 	    !skb_is_swtx_tstamp(skb, false_tstamp)) {
 		if_index = 0;
 		if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP_NETDEV)