From: Marcelo Ricardo Leitner marcelo.leitner@gmail.com
commit 0b9aefea860063bb39e36bd7fe6c7087fed0ba87 upstream.
Markus Trippelsdorf reported that after commit dcb17d22e1c2 ("tcp: warn on bogus MSS and try to amend it") the kernel started logging the warning for a NIC driver that doesn't even support GRO.
It was diagnosed that it was possibly caused on connections that were using TCP Timestamps but some packets lacked the Timestamps option. As we reduce rcv_mss when timestamps are used, the lack of them would cause the packets to be bigger than expected, although this is a valid case.
As this warning is more as a hint, getting a clean-cut on the threshold is probably not worth the execution time spent on it. This patch thus alleviates the false-positives with 2 quick checks: by accounting for the entire TCP option space and also checking against the interface MTU if it's available.
These changes, specially the MTU one, might mask some real positives, though if they are really happening, it's possible that sooner or later it will be triggered anyway.
Reported-by: Markus Trippelsdorf markus@trippelsdorf.de Cc: Eric Dumazet eric.dumazet@gmail.com Signed-off-by: Marcelo Ricardo Leitner marcelo.leitner@gmail.com Signed-off-by: David S. Miller davem@davemloft.net Cc: Salvatore Bonaccorso carnil@debian.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- net/ipv4/tcp_input.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-)
--- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -129,7 +129,8 @@ int sysctl_tcp_invalid_ratelimit __read_ #define REXMIT_LOST 1 /* retransmit packets marked lost */ #define REXMIT_NEW 2 /* FRTO-style transmit of unsent/new packets */
-static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb) +static void tcp_gro_dev_warn(struct sock *sk, const struct sk_buff *skb, + unsigned int len) { static bool __once __read_mostly;
@@ -140,8 +141,9 @@ static void tcp_gro_dev_warn(struct sock
rcu_read_lock(); dev = dev_get_by_index_rcu(sock_net(sk), skb->skb_iif); - pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n", - dev ? dev->name : "Unknown driver"); + if (!dev || len >= dev->mtu) + pr_warn("%s: Driver has suspect GRO implementation, TCP performance may be compromised.\n", + dev ? dev->name : "Unknown driver"); rcu_read_unlock(); } } @@ -164,8 +166,10 @@ static void tcp_measure_rcv_mss(struct s if (len >= icsk->icsk_ack.rcv_mss) { icsk->icsk_ack.rcv_mss = min_t(unsigned int, len, tcp_sk(sk)->advmss); - if (unlikely(icsk->icsk_ack.rcv_mss != len)) - tcp_gro_dev_warn(sk, skb); + /* Account for possibly-removed options */ + if (unlikely(len > icsk->icsk_ack.rcv_mss + + MAX_TCP_OPTION_SPACE)) + tcp_gro_dev_warn(sk, skb, len); } else { /* Otherwise, we make more careful check taking into account, * that SACKs block is variable.