Re: [nf PATCH v2 1/2] net: nf_tables: Make nft_meta expression more robust

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Fri, Jul 19, 2019 at 02:39:20PM +0200, Phil Sutter wrote:
> nft_meta_get_eval()'s tendency to bail out setting NFT_BREAK verdict in
> situations where required data is missing breaks inverted checks
> like e.g.:
> 
> | meta iifname != eth0 accept
> 
> This rule will never match if there is no input interface (or it is not
> known) which is not intuitive and, what's worse, breaks consistency of
> iptables-nft with iptables-legacy.
> 
> Fix this by falling back to placing a value in dreg which never matches
> (avoiding accidental matches):
> 
> {I,O}IF:
> 	Use invalid ifindex value zero.
> 
> {BRI_,}{I,O}IFNAME, {I,O}IFKIND:
> 	Use an empty string which is neither a valid interface name nor
> 	kind.
> 
> {I,O}IFTYPE:
> 	Use ARPHRD_VOID (0xFFFF).

What could it be done with?

NFT_META_BRI_IIFPVID
NFT_META_BRI_IIFPVPROTO

Those will still not work for

        meta ibrpvid != 40

if interface is not available.

For VPROTO probably it's possible. I don't have a solution for
IIFPVID.