tcp: Fix -Wc23-extensions in tcp_options_write()

Clang warns (or errors with CONFIG_WERROR=y) when CONFIG_TCP_AO is set:

  net/ipv4/tcp_output.c:663:2: error: label at end of compound statement is a C23 extension [-Werror,-Wc23-extensions]
    663 |         }
        |         ^
  1 error generated.

On earlier releases (such as clang-11, the current minimum supported
version for building the kernel) that do not support C23, this was a
hard error unconditionally:

  net/ipv4/tcp_output.c:663:2: error: expected statement
          }
          ^
  1 error generated.

While adding a semicolon after the label would resolve this, it is more
in line with the kernel as a whole to refactor this block into a
standalone function, which means the goto a label construct can just be
replaced with a return statement. Do so to resolve the warning.

Closes: https://github.com/ClangBuiltLinux/linux/issues/1953
Fixes: 1e03d32b ("net/tcp: Add TCP-AO sign to outgoing packets")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Dmitry Safonov <dima@arista.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -601,34 +601,11 @@ static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
 }
 #endif
 
-/* Write previously computed TCP options to the packet.
- *
- * Beware: Something in the Internet is very sensitive to the ordering of
- * TCP options, we learned this through the hard way, so be careful here.
- * Luckily we can at least blame others for their non-compliance but from
- * inter-operability perspective it seems that we're somewhat stuck with
- * the ordering which we have been using if we want to keep working with
- * those broken things (not that it currently hurts anybody as there isn't
- * particular reason why the ordering would need to be changed).
- *
- * At least SACK_PERM as the first option is known to lead to a disaster
- * (but it may well be that other scenarios fail similarly).
- */
-static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
+static __be32 *process_tcp_ao_options(struct tcp_sock *tp,
 				      const struct tcp_request_sock *tcprsk,
 				      struct tcp_out_options *opts,
-			      struct tcp_key *key)
+				      struct tcp_key *key, __be32 *ptr)
 {
-	__be32 *ptr = (__be32 *)(th + 1);
-	u16 options = opts->options;	/* mungable copy */
-
-	if (tcp_key_is_md5(key)) {
-		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
-			       (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
-		/* overload cookie hash location */
-		opts->hash_location = (__u8 *)ptr;
-		ptr += 4;
-	} else if (tcp_key_is_ao(key)) {
 #ifdef CONFIG_TCP_AO
 	u8 maclen = tcp_ao_maclen(key->ao_key);
 
@@ -646,7 +623,7 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 			lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
 		rnext_key = READ_ONCE(ao_info->rnext_key);
 		if (WARN_ON_ONCE(!rnext_key))
-				goto out_ao;
+			return ptr;
 		*ptr++ = htonl((TCPOPT_AO << 24) |
 			       (tcp_ao_len(key->ao_key) << 16) |
 			       (key->ao_key->sndid << 8) |
@@ -658,8 +635,39 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 		memset(ptr, TCPOPT_NOP, sizeof(*ptr));
 		ptr++;
 	}
-out_ao:
 #endif
+	return ptr;
+}
+
+/* Write previously computed TCP options to the packet.
+ *
+ * Beware: Something in the Internet is very sensitive to the ordering of
+ * TCP options, we learned this through the hard way, so be careful here.
+ * Luckily we can at least blame others for their non-compliance but from
+ * inter-operability perspective it seems that we're somewhat stuck with
+ * the ordering which we have been using if we want to keep working with
+ * those broken things (not that it currently hurts anybody as there isn't
+ * particular reason why the ordering would need to be changed).
+ *
+ * At least SACK_PERM as the first option is known to lead to a disaster
+ * (but it may well be that other scenarios fail similarly).
+ */
+static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
+			      const struct tcp_request_sock *tcprsk,
+			      struct tcp_out_options *opts,
+			      struct tcp_key *key)
+{
+	__be32 *ptr = (__be32 *)(th + 1);
+	u16 options = opts->options;	/* mungable copy */
+
+	if (tcp_key_is_md5(key)) {
+		*ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
+			       (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
+		/* overload cookie hash location */
+		opts->hash_location = (__u8 *)ptr;
+		ptr += 4;
+	} else if (tcp_key_is_ao(key)) {
+		ptr = process_tcp_ao_options(tp, tcprsk, opts, key, ptr);
 	}
 	if (unlikely(opts->mss)) {
 		*ptr++ = htonl((TCPOPT_MSS << 24) |