net: Replace the limit of TCP_LINGER2 with TCP_FIN_TIMEOUT_MAX

This patch changes the behavior of TCP_LINGER2 about its limit. The
sysctl_tcp_fin_timeout used to be the limit of TCP_LINGER2 but now it's
only the default value. A new macro named TCP_FIN_TIMEOUT_MAX is added
as the limit of TCP_LINGER2, which is 2 minutes.

Since TCP_LINGER2 used sysctl_tcp_fin_timeout as the default value
and the limit in the past, the system administrator cannot set the
default value for most of sockets and let some sockets have a greater
timeout. It might be a mistake that let the sysctl to be the limit of
the TCP_LINGER2. Maybe we can add a new sysctl to set the max of
TCP_LINGER2, but FIN-WAIT-2 timeout is usually no need to be too long
and 2 minutes are legal considering TCP specs.

Changes in v3:
- Remove the new socket option and change the TCP_LINGER2 behavior so
  that the timeout can be set to value between sysctl_tcp_fin_timeout
  and 2 minutes.

Changes in v2:
- Add int overflow check for the new socket option.

Changes in v1:
- Add a new socket option to set timeout greater than
  sysctl_tcp_fin_timeout.
Signed-off-by: Cambda Zhu <cambda@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -126,6 +126,7 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 				  * to combine FIN-WAIT-2 timeout with
 				  * TIME-WAIT timer.
 				  */
+#define TCP_FIN_TIMEOUT_MAX (120 * HZ) /* max TCP_LINGER2 value (two minutes) */
 
 #define TCP_DELACK_MAX	((unsigned)(HZ/5))	/* maximal time to delay before sending an ACK */
 #if HZ >= 100

net/ipv4/tcp.c

@@ -3035,8 +3035,8 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
 	case TCP_LINGER2:
 		if (val < 0)
 			tp->linger2 = -1;
-		else if (val > net->ipv4.sysctl_tcp_fin_timeout / HZ)
-			tp->linger2 = 0;
+		else if (val > TCP_FIN_TIMEOUT_MAX / HZ)
+			tp->linger2 = TCP_FIN_TIMEOUT_MAX;
 		else
 			tp->linger2 = val * HZ;
 		break;