Patchwork [net-next] tcp: add ability to set a timestamp offset

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Submitter Andrey Vagin
Date Jan. 22, 2013, 8:52 p.m.
Message ID <1358887953-28665-1-git-send-email-avagin@openvz.org>
Download mbox | patch
Permalink /patch/214657/
State Changes Requested
Delegated to: David Miller
Headers show

Comments

Andrey Vagin - Jan. 22, 2013, 8:52 p.m.
If a TCP socket will get live-migrated from one box to another the
timestamps (which are typically ON) will get screwed up -- the new
kernel will generate TS values that has nothing to do with what they
were on dump. The solution is to yet again fix the kernel and put a
"timestamp offset" on a socket.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: James Morris <jmorris@namei.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
---
 include/linux/tcp.h      | 2 ++
 include/uapi/linux/tcp.h | 1 +
 net/ipv4/tcp.c           | 6 ++++++
 net/ipv4/tcp_output.c    | 7 ++++---
 4 files changed, 13 insertions(+), 3 deletions(-)
Rick Jones - Jan. 22, 2013, 9:16 p.m.
On 01/22/2013 12:52 PM, Andrey Vagin wrote:
> If a TCP socket will get live-migrated from one box to another the
> timestamps (which are typically ON) will get screwed up -- the new
> kernel will generate TS values that has nothing to do with what they
> were on dump. The solution is to yet again fix the kernel and put a
> "timestamp offset" on a socket.

Is there a chance a connection can be moved more than once within the 
"lifetime" of a given timestamp value?

rick jones

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Neal Cardwell - Jan. 22, 2013, 9:18 p.m.
On Tue, Jan 22, 2013 at 3:52 PM, Andrey Vagin <avagin@openvz.org> wrote:
> If a TCP socket will get live-migrated from one box to another the
> timestamps (which are typically ON) will get screwed up -- the new
> kernel will generate TS values that has nothing to do with what they
> were on dump. The solution is to yet again fix the kernel and put a
> "timestamp offset" on a socket.

One serious issue with this patch is that outgoing timestamp values
will no longer correspond to tcp_time_stamp, so echoed timestamp
values will also no longer have a meaningful relationship to
tcp_time_stamp. That violates assumptions made in several places in
the code, which assumes that we can compare echoed timestamp values to
tcp_time_stamp; for example, there are several places where we do
things like subtracting:
   tcp_time_stamp - tp->rx_opt.rcv_tsecr
to find the estimated RTT for a segment.

neal
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David Miller - Jan. 22, 2013, 9:24 p.m.
From: Neal Cardwell <ncardwell@google.com>
Date: Tue, 22 Jan 2013 16:18:04 -0500

> On Tue, Jan 22, 2013 at 3:52 PM, Andrey Vagin <avagin@openvz.org> wrote:
>> If a TCP socket will get live-migrated from one box to another the
>> timestamps (which are typically ON) will get screwed up -- the new
>> kernel will generate TS values that has nothing to do with what they
>> were on dump. The solution is to yet again fix the kernel and put a
>> "timestamp offset" on a socket.
> 
> One serious issue with this patch is that outgoing timestamp values
> will no longer correspond to tcp_time_stamp, so echoed timestamp
> values will also no longer have a meaningful relationship to
> tcp_time_stamp. That violates assumptions made in several places in
> the code, which assumes that we can compare echoed timestamp values to
> tcp_time_stamp; for example, there are several places where we do
> things like subtracting:
>    tcp_time_stamp - tp->rx_opt.rcv_tsecr
> to find the estimated RTT for a segment.

Right, this change seems pretty bogus as-is.
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Patch

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 4e1d228..746dad5 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -162,6 +162,8 @@  struct tcp_sock {
 	u32	rcv_tstamp;	/* timestamp of last received ACK (for keepalives) */
 	u32	lsndtime;	/* timestamp of last sent data packet (for restart window) */
 
+	u32	snd_tsval_offset; /* offset for snd_tsval */
+
 	struct list_head tsq_node; /* anchor in tsq_tasklet.head list */
 	unsigned long	tsq_flags;
 
diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index e962faa..6b1ead0 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -111,6 +111,7 @@  enum {
 #define TCP_QUEUE_SEQ		21
 #define TCP_REPAIR_OPTIONS	22
 #define TCP_FASTOPEN		23	/* Enable FastOpen on listeners */
+#define TCP_TIMESTAMP		24
 
 struct tcp_repair_opt {
 	__u32	opt_code;
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 1ca2536..72dee28 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2704,6 +2704,9 @@  static int do_tcp_setsockopt(struct sock *sk, int level,
 		else
 			err = -EINVAL;
 		break;
+	case TCP_TIMESTAMP:
+		tp->snd_tsval_offset = val - tcp_time_stamp;
+		break;
 	default:
 		err = -ENOPROTOOPT;
 		break;
@@ -2952,6 +2955,9 @@  static int do_tcp_getsockopt(struct sock *sk, int level,
 	case TCP_USER_TIMEOUT:
 		val = jiffies_to_msecs(icsk->icsk_user_timeout);
 		break;
+	case TCP_TIMESTAMP:
+		val = tcp_time_stamp + tp->snd_tsval_offset;
+		break;
 	default:
 		return -ENOPROTOOPT;
 	}
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 5d45159..9b6d485 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -622,7 +622,7 @@  static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 
 	if (likely(sysctl_tcp_timestamps && *md5 == NULL)) {
 		opts->options |= OPTION_TS;
-		opts->tsval = TCP_SKB_CB(skb)->when;
+		opts->tsval = TCP_SKB_CB(skb)->when + tp->snd_tsval_offset;
 		opts->tsecr = tp->rx_opt.ts_recent;
 		remaining -= TCPOLEN_TSTAMP_ALIGNED;
 	}
@@ -705,6 +705,7 @@  static unsigned int tcp_synack_options(struct sock *sk,
 				   struct tcp_extend_values *xvp,
 				   struct tcp_fastopen_cookie *foc)
 {
+	struct tcp_sock *tp = tcp_sk(sk);
 	struct inet_request_sock *ireq = inet_rsk(req);
 	unsigned int remaining = MAX_TCP_OPTION_SPACE;
 	u8 cookie_plus = (xvp != NULL && !xvp->cookie_out_never) ?
@@ -739,7 +740,7 @@  static unsigned int tcp_synack_options(struct sock *sk,
 	}
 	if (likely(ireq->tstamp_ok)) {
 		opts->options |= OPTION_TS;
-		opts->tsval = TCP_SKB_CB(skb)->when;
+		opts->tsval = TCP_SKB_CB(skb)->when + tp->snd_tsval_offset;
 		opts->tsecr = req->ts_recent;
 		remaining -= TCPOLEN_TSTAMP_ALIGNED;
 	}
@@ -806,7 +807,7 @@  static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
 
 	if (likely(tp->rx_opt.tstamp_ok)) {
 		opts->options |= OPTION_TS;
-		opts->tsval = tcb ? tcb->when : 0;
+		opts->tsval = tcb ? tcb->when + tp->snd_tsval_offset : 0;
 		opts->tsecr = tp->rx_opt.ts_recent;
 		size += TCPOLEN_TSTAMP_ALIGNED;
 	}