Re: [PATCH net-next] tcp: Fix tcp_syn_flood_action() if CONFIG_IPV6=n

[Saeed Mahameed]

On 18 Nov 09:29, Geert Uytterhoeven wrote:
> Hi Jamie,
>
> On Fri, Nov 18, 2022 at 2:50 AM Jamie Bainbridge
> <jamie.bainbridge@xxxxxxxxx> wrote:
> > On Thu, 17 Nov 2022 at 08:15, Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> > > On Thu, 17 Nov 2022 08:39:43 +1100 Jamie Bainbridge wrote:
> > > > >         if (v6) {
> > > > > #ifdef v6
> > > > >                 expensive_call6();
> > > > > #endif
> > > > >         } else {
> > > > >                 expensive_call6();
> > > > >         }
> > > >
> > > > These should work, but I expect they cause a comparison which can't be
> > > > optimised out at compile time. This is probably why the first style
> > > > exists.
> > > >
> > > > In this SYN flood codepath optimisation doesn't matter because we're
> > > > doing ratelimited logging anyway. But if we're breaking with existing
> > > > style, then wouldn't the others also have to change to this style? I
> > > > haven't reviewed all the other usage to tell if they're in an oft-used
> > > > fastpath where such a thing might matter.
> > >
> > > I think the word style already implies subjectivity.
> >
> > You are right. Looking further, there are many other ways
> > IF_ENABLED(CONFIG_IPV6) is used, including similar to the ways you
> > have suggested.
> >
> > I don't mind Geert's original patch, but if you want a different
> > style, I like your suggestion with v4 first:
> >
> >         if (v4) {
> >                 expensive_call4();
> > #ifdef v6
> >         } else {
> >                 expensive_call6();
> > #endif
> >         }
>
> IMHO this is worse, as the #ifdef/#endif is spread across the two branches
> of an if-conditional.
>
> Hence this is usually written as:
>
>            if (cond1) {
>                    expensive_call1();
>            }
>    #ifdef cond2_enabled
>           else {
>                    expensive_call1();
>            }
>    #endif

I don't think any of this complication is needed, 

there's a macro inet6_rcv_saddr(sk), we can use it instead of directly
referencing &sk->sk_v6_rcv_saddr, it already handles the case where 
CONFIG_IPV6=n

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -6845,7 +6845,7 @@ static bool tcp_syn_flood_action(const struct sock *sk, const char *proto)
            xchg(&queue->synflood_warned, 1) == 0) {
                if (IS_ENABLED(CONFIG_IPV6) && sk->sk_family == AF_INET6) {
                        net_info_ratelimited("%s: Possible SYN flooding on port [%pI6c]:%u. %s.\n",
-                                       proto, &sk->sk_v6_rcv_saddr,
+                                       proto, inet6_rcv_saddr(sk),