tcp: annotate data-races around tp->dsack_dups

[ Upstream commit a984705 ]

tcp_get_timestamping_opt_stats() intentionally runs lockless, we must
add READ_ONCE() and WRITE_ONCE() annotations to keep KCSAN happy.

Fixes: 7e10b65 ("tcp: add dsack blocks received stats")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260416200319.3608680-10-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: edumazet

net/ipv4/tcp.c

@@ -4306,7 +4306,7 @@ struct sk_buff *tcp_get_timestamping_opt_stats(const struct sock *sk,
 			  TCP_NLA_PAD);
 	nla_put_u64_64bit(stats, TCP_NLA_BYTES_RETRANS,
 			  READ_ONCE(tp->bytes_retrans), TCP_NLA_PAD);
-	nla_put_u32(stats, TCP_NLA_DSACK_DUPS, tp->dsack_dups);
+	nla_put_u32(stats, TCP_NLA_DSACK_DUPS, READ_ONCE(tp->dsack_dups));
 	nla_put_u32(stats, TCP_NLA_REORD_SEEN, tp->reord_seen);
 	nla_put_u32(stats, TCP_NLA_SRTT, tp->srtt_us >> 3);
 	nla_put_u16(stats, TCP_NLA_TIMEOUT_REHASH, tp->timeout_rehash);

net/ipv4/tcp_input.c

@@ -1059,7 +1059,7 @@ static u32 tcp_dsack_seen(struct tcp_sock *tp, u32 start_seq,
 	else if (tp->tlp_high_seq && tp->tlp_high_seq == end_seq)
 		state->flag |= FLAG_DSACK_TLP;
 
-	tp->dsack_dups += dup_segs;
+	WRITE_ONCE(tp->dsack_dups, tp->dsack_dups + dup_segs);
 	/* Skip the DSACK if dup segs weren't retransmitted by sender */
 	if (tp->dsack_dups > tp->total_retrans)
 		return 0;