[PATCH net-next 1/4] tcp: sync MSS after enabling MTU probing on passive connections

[Yongjian Wang via B4 Relay]

From: Yongjian Wang <echoowall@xxxxxxxxx>

tcp_v4_syn_recv_sock() and tcp_v6_syn_recv_sock() call tcp_sync_mss()
before MTU probing is enabled, and tcp_init_transfer() enables it later
without syncing the MSS again.

With net.ipv4.tcp_mtu_probing=2, a passive connection therefore starts
at the full MSS instead of tcp_base_mss as documented, while an active
connection starts at tcp_base_mss. This has two visible effects, shown
here with tcp_base_mss=1024:

 - The MSS drops when the first probe is acknowledged, from 1460 to 1242,
   although that probe has just shown the path can carry more.

 - On a black hole path the MSS never tries tcp_base_mss. It goes from
   1460 straight to 512, since MTU probing is already enabled when black
   hole detection triggers.

Sync the MSS right after tcp_mtup_init(). tcp_sync_mss() only applies
search_low when MTU probing is enabled, so the other tcp_mtu_probing
values keep their behaviour.

The Fixes commit kept tcp_mtup_init() in a single place on purpose, so
sync the MSS next to it instead of moving it back into the
syn_recv_sock() handlers.

Fixes: 4d83e1773031 ("tcp: delete redundant calls of tcp_mtup_init()")
Signed-off-by: Yongjian Wang <echoowall@xxxxxxxxx>
---
 net/ipv4/tcp_input.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e1f2d9765..c13f005b2 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6760,6 +6760,7 @@ void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	tcp_mtup_init(sk);
+	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 	icsk->icsk_af_ops->rebuild_header(sk);
 	tcp_init_metrics(sk);
 

-- 
Git-157)