[PATCH RFC net-next 3/9] tcp: record a start time in the tx path for SK_BPF_CB_TIMESTAMPING_V2

[Jason Xing <kerneljasonxing@gmail.com>]

Add a new field sk_start_time to 'struct sock' to support different
types of sockets (TCP/UDP/ICMP...) to record the pre-set start time
value and then propagates it into skbs.

Start time might vary across different protocols, so for TCP the
patch picks the entry of tcp_sendmsg_locked as the start time because
1) it already holds the socket lock,
2) it nearly reflects when to trigger a sendmsg syscall from applications.

The value for TCP is a real (wall-clock) time from ktime_get_real() so
it stays comparable with the timestamps produced elsewhere in the stack
and can be consumed directly by a BPF prog using newly added
bpf_ktime_get_real_ns()).

A later patch copies this value onto every skb entailed by the TCP
write queue.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
 include/net/sock.h | 2 ++
 net/ipv4/tcp.c     | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/include/net/sock.h b/include/net/sock.h
index 51185222aac2..782ad5fc8ff5 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -314,6 +314,7 @@ struct sk_filter;
   *	@mptcp_retransmit_timer: mptcp retransmit timer
   *	@sk_stamp: time stamp of last packet received
   *	@sk_stamp_seq: lock for accessing sk_stamp on 32 bit architectures only
+  *	@sk_start_time: start time recorded for SK_BPF_CB_TIMESTAMPING_V2
   *	@sk_tsflags: SO_TIMESTAMPING flags
   *	@sk_bpf_cb_flags: used in bpf_setsockopt()
   *	@sk_use_task_frag: allow sk_page_frag() to use current->task_frag.
@@ -552,6 +553,7 @@ struct sock {
 #if BITS_PER_LONG==32
 	seqlock_t		sk_stamp_seq;
 #endif
+	ktime_t			sk_start_time;
 	int			sk_disconnects;
 
 	union {
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b4237d0e994d..28730a3e4473 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1129,6 +1129,10 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 	flags = msg->msg_flags;
 
+	if (static_branch_unlikely(&bpfts_v2_needed_key) &&
+	    SK_BPF_CB_FLAG_TEST(sk, SK_BPF_CB_TIMESTAMPING_V2))
+		sk->sk_start_time = ktime_get_real();
+
 	sockc = (struct sockcm_cookie){ .tsflags = READ_ONCE(sk->sk_tsflags) };
 	if (msg->msg_controllen) {
 		sockc_err = sock_cmsg_send(sk, msg, &sockc);
-- 
2.43.7