linux-kernel - Re: [PATCH] uapi/netfilter: Change netfilter hook verdict code definition from macro to enum

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Date:   Thu, 7 Sep 2023 12:21:43 +1000
From:   Duncan Roe <duncan_roe@...usnet.com.au>
To:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uapi/netfilter: Change netfilter hook verdict code
 definition from macro to enum

On Wed, Sep 06, 2023 at 12:57:56AM +0800, David Wang wrote:
>
>
> At 2023-09-06 00:38:02, "Daniel Xu" <dxu@...uu.xyz> wrote:
> >Hi David,
> >
> >On Mon, Sep 04, 2023 at 09:02:02PM +0800, David Wang wrote:
>
> >>  #include <linux/in6.h>
> >>
> >>  /* Responses from hook functions. */
> >> -#define NF_DROP 0
> >> -#define NF_ACCEPT 1
> >> -#define NF_STOLEN 2
> >> -#define NF_QUEUE 3
> >> -#define NF_REPEAT 4
> >> -#define NF_STOP 5	/* Deprecated, for userspace nf_queue compatibility. */
> >> -#define NF_MAX_VERDICT NF_STOP
> >> +enum {
> >> +	NF_DROP        = 0,
> >> +	NF_ACCEPT      = 1,
> >> +	NF_STOLEN      = 2,
> >> +	NF_QUEUE       = 3,
> >> +	NF_REPEAT      = 4,
> >> +	NF_STOP        = 5,	/* Deprecated, for userspace nf_queue compatibility. */
> >> +	NF_MAX_VERDICT = NF_STOP,
> >> +};
> >
> >Switching from macro to enum works for almost all use cases, but not
> >all. If someone if #ifdefing the symbols (which is plausible) this
> >change would break them.
> >
> >I think I've seen some other networking code define both enums and
> >macros. But it was a little ugly. Not sure if that is acceptable here or
> >not.
> >
> >[...]
> >
> >Thanks,
> >Daniel
>
>
> Thanks for the review~
> I do not have a strong reasoning to deny the possibility of breaking unexpected usage of this macros,
>
> but I also agree that it is ugly to use both enum and macro at the same time.
>
> Kind of don't know how to proceed from here now...

I did see code like that somewhere and wondered what was going on. The #define
lines were interspersed with the enum members which indeed looked ugly to me.

I'd suggest a block of #defines after the enum close e.g.

> #define NF_DROP NF_DROP
>...

perhaps with a comment preceding to advise that the defines were there for
the benefit of anyone using #ifdef.

Cheers ... Duncan.