Re: [PATCH v3 1/2] net: nf_tables: Make nft_meta expression more robust

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On Tue, Jul 23, 2019 at 03:27:52PM +0200, Phil Sutter wrote:
> nft_meta_get_eval()'s tendency to bail out setting NFT_BREAK verdict in
> situations where required data is missing leads to unexpected behaviour
> with inverted checks like so:
> 
> | 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.e. zero for interface index and an
> empty string for interface name.
> ---
> Changes since v1:
> - Apply same fix to net/bridge/netfilter/nft_meta_bridge.c as well.
> 
> Changes since v2:
> - Limit this fix to address only expressions returning an interface
>   index or name. As Florian correctly criticized, these changes may be
>   problematic as they tend to change nftables' behaviour. Hence fix only
>   the cases needed to establish iptables-nft compatibility.

This leaves things in an inconsistent situation.

What's the concern here? Matching iifgroup/oifgroup from the wrong
packet path?



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