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From: Luis Henriques <luis.henriques@canonical.com>
To: linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        kernel-team@lists.ubuntu.com
Cc: Eric Dumazet <edumazet@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Luis Henriques <luis.henriques@canonical.com>
Subject: [PATCH 3.16.y-ckt 008/180] tcp: tcp_make_synack() should clear skb->tstamp
Date: Thu,  7 May 2015 10:43:37 +0100
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3.16.7-ckt11 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>

commit b50edd7812852d989f2ef09dcfc729690f54a42d upstream.

I noticed tcpdump was giving funky timestamps for locally
generated SYNACK messages on loopback interface.

11:42:46.938990 IP 127.0.0.1.48245 > 127.0.0.2.23850: S
945476042:945476042(0) win 43690 <mss 65495,nop,nop,sackOK,nop,wscale 7>

20:28:58.502209 IP 127.0.0.2.23850 > 127.0.0.1.48245: S
3160535375:3160535375(0) ack 945476043 win 43690 <mss
65495,nop,nop,sackOK,nop,wscale 7>

This is because we need to clear skb->tstamp before
entering lower stack, otherwise net_timestamp_check()
does not set skb->tstamp.

Fixes: 7faee5c0d514 ("tcp: remove TCP_SKB_CB(skb)->when")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
 net/ipv4/tcp_output.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 1ea0a07ab0b5..144af9539ceb 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2855,6 +2855,8 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 	}
 #endif
 
+	/* Do not fool tcpdump (if any), clean our debris */
+	skb->tstamp.tv64 = 0;
 	return skb;
 }
 EXPORT_SYMBOL(tcp_make_synack);
--
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