Re: [PATCH iptabes-nft] iptables-nft: allow removal of empty builtin chains

[Florian Westphal <fw@strlen.de>]

Phil Sutter <phil@nwl.cc> wrote:
> > Ok, so I will just send a simplified version of this patch that
> > will remove all empty basechains for -X too.
> 
> I believe there was a misunderstanding: How I read Pablo's comments, he
> was walking about '-X' with base-chain name explicitly given. If a user
> calls e.g. 'iptables-nft -X FORWARD', it is clear that the new behaviour
> is intended and dropping any non-standard policy is not a surprise. The
> code right now though behaves unexpectedly:
> 
> | # nft flush ruleset
> | # ./install/sbin/iptables-nft -P FORWARD DROP
> | # ./install/sbin/iptables-nft -X
> | # nft list ruleset
> | table ip filter {
> | }
> 
> So forward DROP policy is lost even though the user just wanted to make
> sure any user-defined chains are gone. But things are worse in practice:
> 
> | # iptables -A FORWARD -d 10.0.0.1 -j ACCEPT
> | # iptables -P FORWARD DROP
> | # iptables -X
> 
> With iptables-nft, the last command above fails (EBUSY). I expect users
> to be pedantic when it comes to unexpected firewall openings or bogus
> errors in iptables-wrapping scripts.
> 
> IMHO we're fine if chains with non-standard policy stay in place. Yet
> this might be racey because IIRC we don't have a "delete chain only if
> policy is accept" command flavour in kernel. This would be interesting,
> because we could drop a base chain also when it's flushed - just
> ignoring a rejected delete if it happens to be non-standard policy.
> 
> The safe option should be to delete base chains only if given
> explicitly, as suggested by Pablo already I suppose.

No idea, I won't change anything. V1 kept '-X' behaviour as-is:

https://patchwork.ozlabs.org/project/netfilter-devel/patch/20210814174643.130760-1-fw@strlen.de/

see the "don't delete built-in chain" comment, the reject-check was kept
in place for the case where iptables-nft is iterating over all the
chains; explict '-X $NAME' was required.

So I don't know what I should change now.  Feel free to update
as you see fit, including a revert.