Re: [PATCH net v2] tcp: Fix -Wc23-extensions in tcp_options_write()

[Eric Dumazet]

On Mon, Nov 6, 2023 at 4:36 PM Nathan Chancellor <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.
>
> While adding a semicolon after the label would resolve this, it is more
> in line with the kernel as a whole to refactor this block into a
> standalone function, which means the goto a label construct can just be
> replaced with a simple return. Do so to resolve the warning.
>
> 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>
> ---
> Please let me know if this function should have a different name. I
> think I got all the changes of the function shuffle correct but some
> testing would be appreciated.
>
> Changes in v2:
> - Break out problematic block into its own function so that goto can be
>   replaced with a simple return, instead of the simple semicolon
>   approach of v1 (Christoph)
> - Link to v1: https://lore.kernel.org/r/20231031-tcp-ao-fix-label-in-compound-statement-warning-v1-1-c9731d115f17@xxxxxxxxxx
> ---
>  net/ipv4/tcp_output.c | 69 ++++++++++++++++++++++++++++-----------------------
>  1 file changed, 38 insertions(+), 31 deletions(-)
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 0d8dd5b7e2e5..3f8dc74fbf40 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -601,6 +601,43 @@ static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
>  }
>  #endif
>
> +static void process_tcp_ao_options(struct tcp_sock *tp,
> +                                  const struct tcp_request_sock *tcprsk,
> +                                  struct tcp_out_options *opts,
> +                                  struct tcp_out_options *opts,

ptr has a different type than in the caller, this is a bit confusing

I would use

static __be32 * process_tcp_ao_options(struct tcp_sock *tp, const
struct tcp_request_sock *tcprsk,
                  struct tcp_out_options *opts,struct tcp_key *key, __be32 *ptr)

> +{
> +#ifdef CONFIG_TCP_AO
> +       u8 maclen = tcp_ao_maclen(key->ao_key);
> +
> +       if (tcprsk) {
> +               u8 aolen = maclen + sizeof(struct tcp_ao_hdr);
> +
> +               *(*ptr)++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
> +                                 (tcprsk->ao_keyid << 8) |
> +                                 (tcprsk->ao_rcv_next));


                 *ptr++ = ...

(and in all other *ptr uses in this helper)

> +       } else {
> +               struct tcp_ao_key *rnext_key;
> +               struct tcp_ao_info *ao_info;
> +
> +               ao_info = rcu_dereference_check(tp->ao_info,
> +                       lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
> +               rnext_key = READ_ONCE(ao_info->rnext_key);
> +               if (WARN_ON_ONCE(!rnext_key))
> +                       return;
> +               *(*ptr)++ = htonl((TCPOPT_AO << 24) |
> +                                 (tcp_ao_len(key->ao_key) << 16) |
> +                                 (key->ao_key->sndid << 8) |
> +                                 (rnext_key->rcvid));
> +       }
> +       opts->hash_location = (__u8 *)(*ptr);
> +       *ptr += maclen / sizeof(**ptr);
> +       if (unlikely(maclen % sizeof(**ptr))) {
> +               memset(*ptr, TCPOPT_NOP, sizeof(**ptr));
> +               (*ptr)++;
> +       }
> +#endif

    return ptr;
+}
> +
>  /* Write previously computed TCP options to the packet.
>   *
>   * Beware: Something in the Internet is very sensitive to the ordering of
> @@ -629,37 +666,7 @@ static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
>                 opts->hash_location = (__u8 *)ptr;
>                 ptr += 4;
>         } else if (tcp_key_is_ao(key)) {
> -#ifdef CONFIG_TCP_AO
> -               u8 maclen = tcp_ao_maclen(key->ao_key);
> -
> -               if (tcprsk) {
> -                       u8 aolen = maclen + sizeof(struct tcp_ao_hdr);
> -
> -                       *ptr++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
> -                                      (tcprsk->ao_keyid << 8) |
> -                                      (tcprsk->ao_rcv_next));
> -               } else {
> -                       struct tcp_ao_key *rnext_key;
> -                       struct tcp_ao_info *ao_info;
> -
> -                       ao_info = rcu_dereference_check(tp->ao_info,
> -                               lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
> -                       rnext_key = READ_ONCE(ao_info->rnext_key);
> -                       if (WARN_ON_ONCE(!rnext_key))
> -                               goto out_ao;
> -                       *ptr++ = htonl((TCPOPT_AO << 24) |
> -                                      (tcp_ao_len(key->ao_key) << 16) |
> -                                      (key->ao_key->sndid << 8) |
> -                                      (rnext_key->rcvid));
> -               }
> -               opts->hash_location = (__u8 *)ptr;
> -               ptr += maclen / sizeof(*ptr);
> -               if (unlikely(maclen % sizeof(*ptr))) {
> -                       memset(ptr, TCPOPT_NOP, sizeof(*ptr));
> -                       ptr++;
> -               }
> -out_ao:
> -#endif
> +               process_tcp_ao_options(tp, tcprsk, opts, key, &ptr);

ptr = process_tcp_ao_options(tp, tcprsk, opts, key, ptr);


>         }
>         if (unlikely(opts->mss)) {
>                 *ptr++ = htonl((TCPOPT_MSS << 24) |
>
> ---
> base-commit: c1ed833e0b3b7b9edc82b97b73b2a8a10ceab241
> change-id: 20231031-tcp-ao-fix-label-in-compound-statement-warning-ebd6c9978498
>
> Best regards,
> --
> Nathan Chancellor <nathan@xxxxxxxxxx>
>