Re: [PATCH v4] netfilter: Exclude LEGACY TABLES on PREEMPT_RT.

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

On Fri, Apr 04, 2025 at 05:28:15PM +0200, Sebastian Andrzej Siewior wrote:
> From: Pablo Neira Ayuso <pablo@netfilter.org>
> 
> The seqcount xt_recseq is used to synchronize the replacement of
> xt_table::private in xt_replace_table() against all readers such as
> ipt_do_table()
> 
> To ensure that there is only one writer, the writing side disables
> bottom halves. The sequence counter can be acquired recursively. Only the
> first invocation modifies the sequence counter (signaling that a writer
> is in progress) while the following (recursive) writer does not modify
> the counter.
> The lack of a proper locking mechanism for the sequence counter can lead
> to live lock on PREEMPT_RT if the high prior reader preempts the
> writer. Additionally if the per-CPU lock on PREEMPT_RT is removed from
> local_bh_disable() then there is no synchronisation for the per-CPU
> sequence counter.
> 
> The affected code is "just" the legacy netfilter code which is replaced
> by "netfilter tables". That code can be disabled without sacrificing
> functionality because everything is provided by the newer
> implementation. This will only requires the usage of the "-nft" tools
> instead of the "-legacy" ones.
> The long term plan is to remove the legacy code so lets accelerate the
> progress.
> 
> Relax dependencies on iptables legacy, replace select with depends on,
> this should cause no harm to existing kernel configs and users can still
> toggle IP{6}_NF_IPTABLES_LEGACY in any case.
> Make EBTABLES_LEGACY, IPTABLES_LEGACY and ARPTABLES depend on
> NETFILTER_LEGACY. Hide xt_recseq and its users, xt_register_table() and
> xt_percpu_counter_alloc() behind NETFILTER_LEGACY. Let NETFILTER_LEGACY
> depend on !PREEMPT_RT.
> 
> Replace CONFIG_IP6_NF_MANGLE->CONFIG_IP6_NF_IPTABLES for TCPOPTSTRIP and
> add CONFIG_NFT_COMPAT_ARP to the MARK target for the IPv6 and ARP target
> to keep it enabled without the LEGACY code for NFT.

Applied to nf-next.

Thanks for keeping me as author, I don't deserve it.