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Date:   Wed, 01 Nov 2023 17:41:10 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     nathan@...nel.org
CC:     edumazet@...gle.com, davem@...emloft.net, dsahern@...nel.org,
        kuba@...nel.org, pabeni@...hat.com, ndesaulniers@...gle.com,
        trix@...hat.com, 0x7f454c46@...il.com, fruggeri@...sta.com,
        noureddine@...sta.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        patches@...ts.linux.dev, nathan@...nel.org
Subject:     Re: [PATCH net] tcp: Fix -Wc23-extensions in tcp_options_write()

On Tue, 31 Oct 2023 13:23:35 PDT (-0700), nathan@...nel.org wrote:
> 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.
>
> Add a semicolon after the label to create an empty statement, which
> resolves the warning or error for all compilers.
>
> 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@...nel.org>
> ---
>  net/ipv4/tcp_output.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index f558c054cf6e..6064895daece 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -658,7 +658,7 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
>  			memset(ptr, TCPOPT_NOP, sizeof(*ptr));
>  			ptr++;
>  		}
> -out_ao:
> +out_ao:;
>  #endif
>  	}
>  	if (unlikely(opts->mss)) {
>
> ---
> base-commit: 55c900477f5b3897d9038446f72a281cae0efd86
> change-id: 20231031-tcp-ao-fix-label-in-compound-statement-warning-ebd6c9978498
>
> Best regards,

This gives me a

linux/net/ipv4/tcp_output.c:663:2: error: expected statement
        }

on GCC for me.  So I think something like

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index f558c054cf6e..ca09763acaa8 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -659,6 +659,11 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
 			ptr++;
 		}
 out_ao:
+	/*
+	 * Labels at the end of compound statements are a C23 feature, so
+	 * introduce a block to avoid a warning/error on strict toolchains.
+	 */
+	{}
 #endif
 	}
 	if (unlikely(opts->mss)) {

should do it (though it's still build testing...)

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