udplite: Print deprecation notice.

Recently syzkaller reported a 7-year-old null-ptr-deref [0] that occurs
when a UDP-Lite socket tries to allocate a buffer under memory pressure.

Someone should have stumbled on the bug much earlier if UDP-Lite had been
used in a real app.  Also, we do not always need a large UDP-Lite workload
to hit the bug since UDP and UDP-Lite share the same memory accounting
limit.

Removing UDP-Lite would simplify UDP code removing a bunch of conditionals
in fast path.

Let's add a deprecation notice when UDP-Lite socket is created and schedule
its removal to 2025.

Link: https://lore.kernel.org/netdev/20230523163305.66466-1-kuniyu@amazon.com/ [0]
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/udplite.c

@@ -22,6 +22,8 @@ static int udplite_sk_init(struct sock *sk)
 {
 	udp_init_sock(sk);
 	udp_sk(sk)->pcflag = UDPLITE_BIT;
+	pr_warn_once("UDP-Lite is deprecated and scheduled to be removed in 2025, "
+		     "please contact the netdev mailing list\n");
 	return 0;
 }
 

net/ipv6/udplite.c

@@ -8,6 +8,8 @@
  *  Changes:
  *  Fixes:
  */
+#define pr_fmt(fmt) "UDPLite6: " fmt
+
 #include <linux/export.h>
 #include <linux/proc_fs.h>
 #include "udp_impl.h"
@@ -16,6 +18,8 @@ static int udplitev6_sk_init(struct sock *sk)
 {
 	udpv6_init_sock(sk);
 	udp_sk(sk)->pcflag = UDPLITE_BIT;
+	pr_warn_once("UDP-Lite is deprecated and scheduled to be removed in 2025, "
+		     "please contact the netdev mailing list\n");
 	return 0;
 }