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

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Message-ID: <cc6e3tukgqhi5y4uhepntrpf272o652pytuynj4nijsf5bkgjq@rgnbhckr3p4w>
Date:   Tue, 5 Sep 2023 10:38:02 -0600
From:   Daniel Xu <dxu@...uu.xyz>
To:     David Wang <00107082@....com>
Cc:     Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH] uapi/netfilter: Change netfilter hook verdict code
 definition from macro to enum

Hi David,

On Mon, Sep 04, 2023 at 09:02:02PM +0800, David Wang wrote:
> As BPF_PROG_TYPE_NETFILTER was added in 6.4, a netfilter
> bpf program can attach to netfilter hooks, process package
> and return verdict back to netfilter. But those verdict
> codes are defined as macro, which could not be compiled
> into BTF with btf.c. libbpf, and maybe other bpf tools,
> would extract information from BTF and generate a
> common header "vmlinux.h". With macro definition, netfilter
> bpf program would have to redefine those macro again,
> besides including "vmlinux.h".
> 
> This code change netfilter hook verdict code definition to
> enum, this way,  make it into BTF.
> 
> Signed-off-by: David Wang <00107082@....com>
> ---
>  include/uapi/linux/netfilter.h | 16 +++++++++-------
>  1 file changed, 9 insertions(+), 7 deletions(-)
> 
> diff --git a/include/uapi/linux/netfilter.h b/include/uapi/linux/netfilter.h
> index 5a79ccb76701..d2f5dfab20dc 100644
> --- a/include/uapi/linux/netfilter.h
> +++ b/include/uapi/linux/netfilter.h
> @@ -8,13 +8,15 @@
>  #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