Subject: [PATCH net-next 1/2] tcp: make SOF_TIMESTAMPING_RX_SOFTWARE feature per socket

From: Jason Xing <[email-masked]>
Date: Sun, 25 Aug 2024 23:24:39 +0800

From: Jason Xing <[email-masked]>

Normally, if we want to record and print the rx timestamp after
tcp_recvmsg_locked(), we must enable both SOF_TIMESTAMPING_SOFTWARE
and SOF_TIMESTAMPING_RX_SOFTWARE flags, from which we also can notice
through running rxtimestamp binary in selftests (see testcase 7).

However, there is one particular case that fails the selftests with
"./rxtimestamp: Expected swtstamp to not be set." error printing in
testcase 6.

How does it happen? When we keep running a thread starting a socket
and set SOF_TIMESTAMPING_RX_HARDWARE option first, then running
./rxtimestamp, it will fail. The reason is the former thread
switching on netstamp_needed_key that makes the feature global,
every skb going through netif_receive_skb_list_internal() function
will get a current timestamp in net_timestamp_check(). So the skb
will have timestamp regardless of whether its socket option has
SOF_TIMESTAMPING_RX_SOFTWARE or not.

After this patch, we can pass the selftest and control each socket
as we want when using rx timestamp feature.

Signed-off-by: Jason Xing <[email-masked]>
---
 net/ipv4/tcp.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 8514257f4ecd..49e73d66c57d 100644
--- a/net/ipv4/tcp.c
+++ b/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 +2275,19 @@ void tcp_recv_timestamp(struct msghdr *msg, const struct sock *sk,
 			}
 		}
 
-		if (READ_ONCE(sk->sk_tsflags) & SOF_TIMESTAMPING_SOFTWARE)
+		/* skb may contain timestamp because another socket
+		 * turned on netstamp_needed_key which allows generate
+		 * the timestamp. So we need to check the current socket.
+		 */
+		if (tsflags & SOF_TIMESTAMPING_SOFTWARE &&
+		    tsflags & SOF_TIMESTAMPING_RX_SOFTWARE)
 			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)
 			has_timestamping = true;
 		else
 			tss->ts[2] = (struct timespec64) {0};
-- 
2.37.3