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

[Palmer Dabbelt]

On Wed, 01 Nov 2023 18:07:23 PDT (-0700), nathan@xxxxxxxxxx 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@xxxxxxxxxx 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@xxxxxxxxxx>
> > > ---
> > >  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