Subject: [PATCH bpf 2/3] bpf: sockmap, do not inc copied_seq when PEEK flag set

From: John Fastabend <[email-masked]>
Date: Wed, 20 Sep 2023 16:27:05 -0700

When data is peek'd off the receive queue we shouldn't considered it
copied from tcp_sock side. When we increment copied_seq this will confuse
tcp_data_ready() because copied_seq can be arbitrarily increased. From]
application side it results in poll() operations not waking up when
expected.

Notice tcp stack without BPF recvmsg programs also does not increment
copied_seq.

We broke this when we moved copied_seq into recvmsg to only update when
actual copy was happening. But, it wasn't working correctly either before
because the tcp_data_ready() tried to use the copied_seq value to see
if data was read by user yet. See fixes tags.

Fixes: e5c6de5fa0258 ("bpf, sockmap: Incorrectly handling copied_seq")
Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
Signed-off-by: John Fastabend <[email-masked]>
---
 net/ipv4/tcp_bpf.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index 81f0dff69e0b..327268203001 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -222,6 +222,7 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
 				  int *addr_len)
 {
 	struct tcp_sock *tcp = tcp_sk(sk);
+	int peek = flags & MSG_PEEK;
 	u32 seq = tcp->copied_seq;
 	struct sk_psock *psock;
 	int copied = 0;
@@ -311,7 +312,8 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk,
 		copied = -EAGAIN;
 	}
 out:
-	WRITE_ONCE(tcp->copied_seq, seq);
+	if (!peek)
+		WRITE_ONCE(tcp->copied_seq, seq);
 	tcp_rcv_space_adjust(sk);
 	if (copied > 0)
 		__tcp_cleanup_rbuf(sk, copied);
-- 
2.33.0