net: ping6: support setting basic SOL_IPV6 options via cmsg

Support setting IPV6_HOPLIMIT, IPV6_TCLASS, IPV6_DONTFRAG
during sendmsg via SOL_IPV6 cmsgs.

tclass and dontfrag are init'ed from struct ipv6_pinfo in
ipcm6_init_sk(), while hlimit is inited to -1, so we need
to handle it being populated via cmsg explicitly.

Leave extension headers and flowlabel unimplemented.
Those are slightly more laborious to test and users
seem to primarily care about IPV6_TCLASS.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1487,6 +1487,7 @@ static int __ip6_append_data(struct sock *sk,
 
 	if (cork->length + length > mtu - headersize && ipc6->dontfrag &&
 	    (sk->sk_protocol == IPPROTO_UDP ||
+	     sk->sk_protocol == IPPROTO_ICMPV6 ||
 	     sk->sk_protocol == IPPROTO_RAW)) {
 		ipv6_local_rxpmtu(sk, fl6, mtu - headersize +
 				sizeof(struct ipv6hdr));

net/ipv6/ping.c

@@ -101,11 +101,21 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	ipc6.sockc.tsflags = sk->sk_tsflags;
 	ipc6.sockc.mark = sk->sk_mark;
 
-	err = sock_cmsg_send(sk, msg, &ipc6.sockc);
-	if (err)
-		return err;
+	if (msg->msg_controllen) {
+		struct ipv6_txoptions opt = {};
+
+		opt.tot_len = sizeof(opt);
+		ipc6.opt = &opt;
 
-	/* TODO: use ip6_datagram_send_ctl to get options from cmsg */
+		err = ip6_datagram_send_ctl(sock_net(sk), sk, msg, &fl6, &ipc6);
+		if (err < 0)
+			return err;
+
+		/* Changes to txoptions and flow info are not implemented, yet.
+		 * Drop the options, fl6 is wiped below.
+		 */
+		ipc6.opt = NULL;
+	}
 
 	memset(&fl6, 0, sizeof(fl6));
 
@@ -140,7 +150,8 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	pfh.wcheck = 0;
 	pfh.family = AF_INET6;
 
-	ipc6.hlimit = ip6_sk_dst_hoplimit(np, &fl6, dst);
+	if (ipc6.hlimit < 0)
+		ipc6.hlimit = ip6_sk_dst_hoplimit(np, &fl6, dst);
 
 	lock_sock(sk);
 	err = ip6_append_data(sk, ping_getfrag, &pfh, len,