[PATCH] tcp: add an ability to dump and restore window parameters

From: Andrey Vagin
Date: Mon Jun 27 2016 - 18:34:18 EST


We found that sometimes a restored tcp socket doesn't work.

A reason of this bug is incorrect window parameters and in this case
tcp_acceptable_seq() returns tcp_wnd_end(tp) instead of tp->snd_nxt. The
other side drops packets with this seq, because seq is less than
tp->rcv_nxt ( tcp_sequence() ).

Data from a send queue is sent only if there is enough space in a
window, so when we restore unacked data, we need to expand a window to
fit this data.

This was in a first version of this patch:
"tcp: extend window to fit all restored unacked data in a send queue"

Then Alexey recommended me to restore window parameters instead of
adjusted them according with data in a sent queue. This sounds resonable.

rcv_wnd has to be restored, because it was reported to another side
and the offered window is never shrunk.
One of reasons why we need to restore snd_wnd was described above.

Cc: Pavel Emelyanov <xemul@xxxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Alexey Kuznetsov <kuznet@xxxxxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: Hideaki YOSHIFUJI <yoshfuji@xxxxxxxxxxxxxx>
Cc: Patrick McHardy <kaber@xxxxxxxxx>
Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
---
include/uapi/linux/tcp.h | 10 +++++++++
net/ipv4/tcp.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 67 insertions(+)

diff --git a/include/uapi/linux/tcp.h b/include/uapi/linux/tcp.h
index 53e8e3f..482898f 100644
--- a/include/uapi/linux/tcp.h
+++ b/include/uapi/linux/tcp.h
@@ -115,12 +115,22 @@ enum {
#define TCP_CC_INFO 26 /* Get Congestion Control (optional) info */
#define TCP_SAVE_SYN 27 /* Record SYN headers for new connections */
#define TCP_SAVED_SYN 28 /* Get SYN headers recorded for connection */
+#define TCP_REPAIR_WINDOW 29 /* Get/set window parameters */

struct tcp_repair_opt {
__u32 opt_code;
__u32 opt_val;
};

+struct tcp_repair_window {
+ __u32 snd_wl1;
+ __u32 snd_wnd;
+ __u32 max_window;
+
+ __u32 rcv_wnd;
+ __u32 rcv_wup;
+};
+
enum {
TCP_NO_QUEUE,
TCP_RECV_QUEUE,
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 5c7ed14..108ef2a 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2277,6 +2277,38 @@ static inline bool tcp_can_repair_sock(const struct sock *sk)
((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_ESTABLISHED));
}

+static int tcp_repair_set_window(struct tcp_sock *tp, char __user *optbuf, int len)
+{
+ struct tcp_repair_window opt;
+
+ if (!tp->repair)
+ return -EPERM;
+
+ if (len != sizeof(opt))
+ return -EINVAL;
+
+ if (copy_from_user(&opt, optbuf, sizeof(opt)))
+ return -EFAULT;
+
+ if (opt.max_window < opt.snd_wnd)
+ return -EINVAL;
+
+ if (after(opt.snd_wl1, tp->rcv_nxt + opt.rcv_wnd))
+ return -EINVAL;
+
+ if (after(opt.rcv_wup, tp->rcv_nxt))
+ return -EINVAL;
+
+ tp->snd_wl1 = opt.snd_wl1;
+ tp->snd_wnd = opt.snd_wnd;
+ tp->max_window = opt.max_window;
+
+ tp->rcv_wnd = opt.rcv_wnd;
+ tp->rcv_wup = opt.rcv_wup;
+
+ return 0;
+}
+
static int tcp_repair_options_est(struct tcp_sock *tp,
struct tcp_repair_opt __user *optbuf, unsigned int len)
{
@@ -2604,6 +2636,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
else
tp->tsoffset = val - tcp_time_stamp;
break;
+ case TCP_REPAIR_WINDOW:
+ err = tcp_repair_set_window(tp, optval, optlen);
+ break;
case TCP_NOTSENT_LOWAT:
tp->notsent_lowat = val;
sk->sk_write_space(sk);
@@ -2860,6 +2895,28 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
return -EINVAL;
break;

+ case TCP_REPAIR_WINDOW: {
+ struct tcp_repair_window opt;
+
+ if (get_user(len, optlen))
+ return -EFAULT;
+
+ if (len != sizeof(opt))
+ return -EINVAL;
+
+ if (!tp->repair)
+ return -EPERM;
+
+ opt.snd_wl1 = tp->snd_wl1;
+ opt.snd_wnd = tp->snd_wnd;
+ opt.max_window = tp->max_window;
+ opt.rcv_wnd = tp->rcv_wnd;
+ opt.rcv_wup = tp->rcv_wup;
+
+ if (copy_to_user(optval, &opt, len))
+ return -EFAULT;
+ return 0;
+ }
case TCP_QUEUE_SEQ:
if (tp->repair_queue == TCP_SEND_QUEUE)
val = tp->write_seq;
--
2.5.5