On Wed, 01 Nov 2023 18:42:10 PDT (-0700), Palmer Dabbelt wrote:

On Wed, 01 Nov 2023 18:07:23 PDT (-0700), nathan@kernel.org wrote:

quoted

On Wed, Nov 01, 2023 at 05:41:10PM -0700, Palmer Dabbelt wrote:

quoted

On Tue, 31 Oct 2023 13:23:35 PDT (-0700), nathan@kernel.org wrote:

quoted

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@kernel.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.

What GCC version?

12.1, though I can't get a smaller reproducer so I'm going to roll back
to your change and double-check.  Might take a bit...

Looks like there was just some bug in my test scripts and the original 
patch wasn't actually picked up for all the configs.  It's working now, 
so

Reviewed-by: Palmer Dabbelt <redacted>

Sorry for the confusion!

quoted

I cannot reproduce that error with my patch applied. I tested mainline
at commit deefd5024f07 ("Merge tag 'vfio-v6.7-rc1' of
https://github.com/awilliam/linux-vfio") using GCC 6 from kernel.org and
I can reproduce a similar failure with ARCH=x86_64 allyesconfig:

  net/ipv4/tcp_output.c: In function 'tcp_options_write':
  net/ipv4/tcp_output.c:661:1: error: label at end of compound statement
   out_ao:
   ^~~~~~

With this change applied, the error disappears for GCC 6 and GCC 13
continues to build without error. I can try the other supported versions
later, I just did an older and newer one for a quick test.

quoted

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...)

I am not opposed to this once we understand what versions are affected
by this so that we have some timeline of removing this workaround.

Cheers,
Nathan