tcp: Move tcp_ns_to_ts() to tcp.h

We will support arbitrary SYN Cookie with BPF.

When BPF prog validates ACK and kfunc allocates a reqsk, we need
to call tcp_ns_to_ts() to calculate an offset of TSval for later
use:

  time
  t0 : Send SYN+ACK
       -> tsval = Initial TSval (Random Number)

  t1 : Recv ACK of 3WHS
       -> tsoff = TSecr - tcp_ns_to_ts(usec_ts_ok, tcp_clock_ns())
                = Initial TSval - t1

  t2 : Send ACK
       -> tsval = t2 + tsoff
                = Initial TSval + (t2 - t1)
                = Initial TSval + Time Delta (x)

  (x) Note that the time delta does not include the initial RTT
      from t0 to t1.

Let's move tcp_ns_to_ts() to tcp.h.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240115205514.68364-2-kuniyu@amazon.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/net/tcp.h b/include/net/tcp.h
index dd78a1181031..114000e71a46 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -577,6 +577,15 @@ static inline u32 tcp_cookie_time(void)
 	return val;
 }
 
+/* Convert one nsec 64bit timestamp to ts (ms or usec resolution) */
+static inline u64 tcp_ns_to_ts(bool usec_ts, u64 val)
+{
+	if (usec_ts)
+		return div_u64(val, NSEC_PER_USEC);
+
+	return div_u64(val, NSEC_PER_MSEC);
+}
+
 u32 __cookie_v4_init_sequence(const struct iphdr *iph, const struct tcphdr *th,
 			      u16 *mssp);
 __u32 cookie_v4_init_sequence(const struct sk_buff *skb, __u16 *mss);