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Date:   Wed, 01 Nov 2023 18:42:10 -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
Subject:     Re: [PATCH net] tcp: Fix -Wc23-extensions in tcp_options_write()

On Wed, 01 Nov 2023 18:07:23 PDT (-0700), nathan@...nel.org wrote:
> On Wed, Nov 01, 2023 at 05:41:10PM -0700, Palmer Dabbelt wrote:
>> 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.
>
> 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...

> 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.
>
>> 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

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