[PATCH net v3] tcp: Fix -Wc23-extensions in tcp_options_write()

[Nathan Chancellor]

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: 1e03d32bea8e ("net/tcp: Add TCP-AO sign to outgoing packets")
Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
Changes in v3:
- Don't use a pointer to a pointer for ptr parameter to avoid the extra
  indirection in process_tcp_ao_options(), just return the modified ptr
  value back to the caller (Eric)
- Link to v2: https://lore.kernel.org/r/20231106-tcp-ao-fix-label-in-compound-statement-warning-v2-1-91eff6e1648c@xxxxxxxxxx

Changes in v2:
- Break out problematic block into its own function so that goto can be
  replaced with a simple return, instead of the simple semicolon
  approach of v1 (Christoph)
- Link to v1: https://lore.kernel.org/r/20231031-tcp-ao-fix-label-in-compound-statement-warning-v1-1-c9731d115f17@xxxxxxxxxx
---
 net/ipv4/tcp_output.c | 70 ++++++++++++++++++++++++++++-----------------------
 1 file changed, 39 insertions(+), 31 deletions(-)

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 0d8dd5b7e2e5..eb13a55d660c 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -601,6 +601,44 @@ static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
 }
 #endif
 
+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, __be32 *ptr)
+{
+#ifdef CONFIG_TCP_AO
+	u8 maclen = tcp_ao_maclen(key->ao_key);
+
+	if (tcprsk) {
+		u8 aolen = maclen + sizeof(struct tcp_ao_hdr);
+
+		*ptr++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
+			       (tcprsk->ao_keyid << 8) |
+			       (tcprsk->ao_rcv_next));
+	} else {
+		struct tcp_ao_key *rnext_key;
+		struct tcp_ao_info *ao_info;
+
+		ao_info = rcu_dereference_check(tp->ao_info,
+			lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
+		rnext_key = READ_ONCE(ao_info->rnext_key);
+		if (WARN_ON_ONCE(!rnext_key))
+			return ptr;
+		*ptr++ = htonl((TCPOPT_AO << 24) |
+			       (tcp_ao_len(key->ao_key) << 16) |
+			       (key->ao_key->sndid << 8) |
+			       (rnext_key->rcvid));
+	}
+	opts->hash_location = (__u8 *)ptr;
+	ptr += maclen / sizeof(*ptr);
+	if (unlikely(maclen % sizeof(*ptr))) {
+		memset(ptr, TCPOPT_NOP, sizeof(*ptr));
+		ptr++;
+	}
+#endif
+	return ptr;
+}
+
 /* Write previously computed TCP options to the packet.
  *
  * Beware: Something in the Internet is very sensitive to the ordering of
@@ -629,37 +667,7 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 		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);
-
-		if (tcprsk) {
-			u8 aolen = maclen + sizeof(struct tcp_ao_hdr);
-
-			*ptr++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
-				       (tcprsk->ao_keyid << 8) |
-				       (tcprsk->ao_rcv_next));
-		} else {
-			struct tcp_ao_key *rnext_key;
-			struct tcp_ao_info *ao_info;
-
-			ao_info = rcu_dereference_check(tp->ao_info,
-				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;
-			*ptr++ = htonl((TCPOPT_AO << 24) |
-				       (tcp_ao_len(key->ao_key) << 16) |
-				       (key->ao_key->sndid << 8) |
-				       (rnext_key->rcvid));
-		}
-		opts->hash_location = (__u8 *)ptr;
-		ptr += maclen / sizeof(*ptr);
-		if (unlikely(maclen % sizeof(*ptr))) {
-			memset(ptr, TCPOPT_NOP, sizeof(*ptr));
-			ptr++;
-		}
-out_ao:
-#endif
+		ptr = process_tcp_ao_options(tp, tcprsk, opts, key, ptr);
 	}
 	if (unlikely(opts->mss)) {
 		*ptr++ = htonl((TCPOPT_MSS << 24) |

---
base-commit: c1ed833e0b3b7b9edc82b97b73b2a8a10ceab241
change-id: 20231031-tcp-ao-fix-label-in-compound-statement-warning-ebd6c9978498

Best regards,
-- 
Nathan Chancellor <nathan@xxxxxxxxxx>