From: Neal Cardwell ncardwell@google.com
[ Upstream commit af38d07ed391b21f7405fa1f936ca9686787d6d2 ]
Fix tcp_ecn_withdraw_cwr() to clear the correct bit: TCP_ECN_QUEUE_CWR.
Rationale: basically, TCP_ECN_DEMAND_CWR is a bit that is purely about the behavior of data receivers, and deciding whether to reflect incoming IP ECN CE marks as outgoing TCP th->ece marks. The TCP_ECN_QUEUE_CWR bit is purely about the behavior of data senders, and deciding whether to send CWR. The tcp_ecn_withdraw_cwr() function is only called from tcp_undo_cwnd_reduction() by data senders during an undo, so it should zero the sender-side state, TCP_ECN_QUEUE_CWR. It does not make sense to stop the reflection of incoming CE bits on incoming data packets just because outgoing packets were spuriously retransmitted.
The bug has been reproduced with packetdrill to manifest in a scenario with RFC3168 ECN, with an incoming data packet with CE bit set and carrying a TCP timestamp value that causes cwnd undo. Before this fix, the IP CE bit was ignored and not reflected in the TCP ECE header bit, and sender sent a TCP CWR ('W') bit on the next outgoing data packet, even though the cwnd reduction had been undone. After this fix, the sender properly reflects the CE bit and does not set the W bit.
Note: the bug actually predates 2005 git history; this Fixes footer is chosen to be the oldest SHA1 I have tested (from Sep 2007) for which the patch applies cleanly (since before this commit the code was in a .h file).
Fixes: bdf1ee5d3bd3 ("[TCP]: Move code from tcp_ecn.h to tcp*.c and tcp.h & remove it") Signed-off-by: Neal Cardwell ncardwell@google.com Acked-by: Yuchung Cheng ycheng@google.com Acked-by: Soheil Hassas Yeganeh soheil@google.com Cc: Eric Dumazet edumazet@google.com Signed-off-by: Eric Dumazet edumazet@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/tcp_input.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -247,7 +247,7 @@ static void tcp_ecn_accept_cwr(struct tc
static void tcp_ecn_withdraw_cwr(struct tcp_sock *tp) { - tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR; + tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR; }
static void __tcp_ecn_check_ce(struct sock *sk, const struct sk_buff *skb)