netdev - [PATCH net] tcp: warn on bogus MSS and try to amend it

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Date:   Wed, 30 Nov 2016 11:14:32 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     netdev@...r.kernel.org
Cc:     Jon Maxwell <jmaxwell37@...il.com>,
        Alex Sidorenko <alexandre.sidorenko@....com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>, tlfalcon@...ux.vnet.ibm.com,
        Brian King <brking@...ux.vnet.ibm.com>,
        Eric Dumazet <eric.dumazet@...il.com>, davem@...emloft.net
Subject: [PATCH net] tcp: warn on bogus MSS and try to amend it

There have been some reports lately about TCP connection stalls caused
by NIC drivers that aren't setting gso_size on aggregated packets on rx
path. This causes TCP to assume that the MSS is actually the size of the
aggregated packet, which is invalid.

Although the proper fix is to be done at each driver, it's often hard
and cumbersome for one to debug, come to such root cause and report/fix
it.

This patch amends this situation in two ways. First, it adds a warning
on when this situation occurs, so it gives a hint to those trying to
debug this. It also limit the maximum probed MSS to the adverised MSS,
as it should never be any higher than that.

The result is that the connection may not have the best performance ever
but it shouldn't stall, and the admin will have a hint on what to look
for.

Tested with virtio by forcing gso_size to 0.

Cc: Jonathan Maxwell <jmaxwell37@...il.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
---
 net/ipv4/tcp_input.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a27b9c0e27c08b4e4aeaff3d0bfdf3ae561ba4d8..ecc86105eb479de9b80db71af6a16a5af612a61c 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -144,7 +144,10 @@ static void tcp_measure_rcv_mss(struct sock *sk, const struct sk_buff *skb)
 	 */
 	len = skb_shinfo(skb)->gso_size ? : skb->len;
 	if (len >= icsk->icsk_ack.rcv_mss) {
-		icsk->icsk_ack.rcv_mss = len;
+		icsk->icsk_ack.rcv_mss = min_t(unsigned int, len,
+					       tcp_sk(sk)->advmss);
+		if (icsk->icsk_ack.rcv_mss != len)
+			pr_warn_once("Seems your NIC driver is doing bad RX acceleration. TCP performance may be compromised.\n");
 	} else {
 		/* Otherwise, we make more careful check taking into account,
 		 * that SACKs block is variable.
-- 
2.9.3