Re: [PATCH net-next 1/7] net: ip: add drop reason to ip_route_input_noref()

[Eric Dumazet]

On Fri, Oct 4, 2024 at 6:36 PM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> no longer applies, please respin
>
> On Tue,  1 Oct 2024 13:59:59 +0800 Menglong Dong wrote:
> > +     enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
> >       const struct iphdr *iph = ip_hdr(skb);
> > -     int err, drop_reason;
> > +     int err;
> >       struct rtable *rt;
>
> reverse xmas tree
>
> >
> > -     drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
> > -
> >       if (ip_can_use_hint(skb, iph, hint)) {
> >               err = ip_route_use_hint(skb, iph->daddr, iph->saddr, iph->tos,
> >                                       dev, hint);
> > @@ -363,7 +362,7 @@ static int ip_rcv_finish_core(struct net *net, struct sock *sk,
> >        */
> >       if (!skb_valid_dst(skb)) {
> >               err = ip_route_input_noref(skb, iph->daddr, iph->saddr,
> > -                                        iph->tos, dev);
> > +                                        iph->tos, dev, &drop_reason);
>
> I find the extra output argument quite ugly.
> I can't apply this now to try to suggest something better, perhaps you
> can come up with a better solution..

Also, passing a local variable by address forces the compiler to emit
more canary checks in more
networking core functions.


See :


config STACKPROTECTOR_STRONG
bool "Strong Stack Protector"
depends on STACKPROTECTOR
depends on $(cc-option,-fstack-protector-strong)
default y
help
  Functions will have the stack-protector canary logic added in any
  of the following conditions:

  - local variable's address used as part of the right hand side of an
    assignment or function argument
  - local variable is an array (or union containing an array),
    regardless of array type or length
  - uses register local variables

  This feature requires gcc version 4.9 or above, or a distribution
  gcc with the feature backported ("-fstack-protector-strong").

  On an x86 "defconfig" build, this feature adds canary checks to
  about 20% of all kernel functions, which increases the kernel code
  size by about 2%.