Subject: [PATCH net-next v2 3/4] bpf-timestamp: keep track of the skb when wait_for_space occurs

From: Jason Xing <[email-masked]>
Date: Sat, 4 Apr 2026 23:04:51 +0800

From: Jason Xing <[email-masked]>

The patch is the 1/2 part of push-level granularity feature.

Tag the skb in tcp_sendmsg_locked() when wait_for_space occurs even
though it might not carry the last byte of the sendmsg.

Prior to the patch, BPF timestamping cannot cover this case:
The following steps reproduce this:
1) skb A is the current last skb before entering wait_for_space process
2) tcp_push() pushes A without any tag
3) A is transmitted from TCP to driver without putting any skb carrying
   timestamps in the error queue, like SCHED, DRV/HARDWARE.
4) sk_stream_wait_memory() sleeps for a while and then returns with an
   error code. Note that the socket lock is released.
5) skb A finally gets acked and removed from the rtx queue.
6) continue with the rest of tcp_sendmsg_locked(): it will jump to(goto)
   'do_error' label and then 'out' label.
7) at this moment, skb A turns out to be the last one in this send
   syscall, and miss the following tcp_bpf_tx_timestamp() opportunity
   before the final tcp_push()
8) BPF script fails to see any timestamps this time

Signed-off-by: Yushan Zhou <[email-masked]>
Signed-off-by: Jason Xing <[email-masked]>
---
 net/ipv4/tcp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index c603b90057f6..7d030a11d004 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1400,9 +1400,11 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 wait_for_space:
 		set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
 		tcp_remove_empty_skb(sk);
-		if (copied)
+		if (copied) {
+			tcp_bpf_tx_timestamp(sk);
 			tcp_push(sk, flags & ~MSG_MORE, mss_now,
 				 TCP_NAGLE_PUSH, size_goal);
+		}
 
 		err = sk_stream_wait_memory(sk, &timeo);
 		if (err != 0)
-- 
2.41.3