netdev - [PATCH v1 net-next 4/6] net: don't make false software transmit timestamps

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Date:   Wed, 26 Apr 2017 16:50:33 +0200
From:   Miroslav Lichvar <mlichvar@...hat.com>
To:     netdev@...r.kernel.org
Cc:     Richard Cochran <richardcochran@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        "Keller, Jacob E" <jacob.e.keller@...el.com>,
        Denny Page <dennypage@...com>, Jiri Benc <jbenc@...hat.com>
Subject: [PATCH v1 net-next 4/6] net: don't make false software transmit timestamps

If software timestamping is enabled by the SO_TIMESTAMP(NS) option
when a message without timestamp is already waiting in the queue, the
__sock_recv_timestamp() function will read the current time to make a
timestamp in order to always have something for the application.

However, this applies also to outgoing packets looped back to the error
queue when hardware timestamping is enabled by the SO_TIMESTAMPING
option. A software transmit timestamp made after the actual transmission
is added to messages with hardware timestamps.

Modify the function to save the current time as a software timestamp
only if it's for a received packet (i.e. it's not in the error queue).

CC: Richard Cochran <richardcochran@...il.com>
CC: Willem de Bruijn <willemb@...gle.com>
Signed-off-by: Miroslav Lichvar <mlichvar@...hat.com>
---
 net/socket.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/socket.c b/net/socket.c
index 5ea5f29..68b9304 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -688,7 +688,8 @@ static void put_ts_pktinfo(struct msghdr *msg, struct sk_buff *skb)
 void __sock_recv_timestamp(struct msghdr *msg, struct sock *sk,
 	struct sk_buff *skb)
 {
-	int need_software_tstamp = sock_flag(sk, SOCK_RCVTSTAMP);
+	int need_software_tstamp = sock_flag(sk, SOCK_RCVTSTAMP) &&
+				   !skb_is_err_queue(skb);
 	struct scm_timestamping tss;
 	int empty = 1;
 	struct skb_shared_hwtstamps *shhwtstamps =
-- 
2.9.3