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Date:   Thu, 02 Nov 2023 13:23:54 -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 18:42:10 PDT (-0700), Palmer Dabbelt wrote:
> 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...

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 <palmer@...osinc.com>

Sorry for the confusion!

>> 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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