Re: [PATCH nf] netfilter: flowtable: do not clear the offload bit twice

[João Silva]

Hi,

The fix was LLM generated but I wouldn't say it's just "for correctness".
For some context, this issue was hit while developing hardware-offloaded 
flows for the RTL930x target in openwrt. There was a point in 
development where this was causing flows to pile up and eventually 
exhaust the TCAM of the PIE engine on the switch.

Thanks for considering.

João Silva


On 03/11/26 13:59, Pablo Neira Ayuso wrote:
> On Tue, Nov 03, 2026 at 09:52:56AM +0000, João Silva via B4 Relay wrote:
> > From: João Silva <jgc3silva@xxxxxxxxx>
> >
> > An expired hardware flow remains in the flowtable until its asynchronous
> > hardware deletion completes. Teardown clears IPS_OFFLOAD_BIT, so a packet
> > can create a replacement flow referencing the same conntrack entry while
> > the old flow is still waiting for deletion.
> >
> > The next GC pass calls flow_offload_teardown() on the expired old flow
> > again, clearing the replacement's offload bit. More packets can then
> > create additional flows for the same conntrack entry. A pending hardware
> > work item can also delay deletion and expose the same repeated teardown.
> >
> > Commit d2d31ea8cd80 ("netfilter: conntrack: fix erronous removal of
> > offload bit") closed this window for software flows by unlinking them in
> > the same GC pass and made the conntrack state fixup one-shot, but
> > hardware flows stay linked until NF_FLOW_HW_DEAD is set.
> >
> > Make clearing IPS_OFFLOAD_BIT part of the existing one-time teardown
> > transition, alongside the conntrack state fixup. Subsequent teardown calls
> > must not alter the offload state now owned by a replacement flow.
> Let's better target nf-next for this, this looks like LLM triggered
> for correctness.
>
> Thanks.
>
> > Fixes: e5eaac2beb54 ("netfilter: flowtable: fix TCP flow teardown")
> > Signed-off-by: João Silva <jgc3silva@xxxxxxxxx>
> > ---
> >   net/netfilter/nf_flow_table_core.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
> > index 0d72dc8ae47a..709b55b74cc7 100644
> > --- a/net/netfilter/nf_flow_table_core.c
> > +++ b/net/netfilter/nf_flow_table_core.c
> > @@ -398,9 +398,11 @@ static void flow_offload_del(struct nf_flowtable *flow_table,
> >   
> >   void flow_offload_teardown(struct flow_offload *flow)
> >   {
> > +	if (test_and_set_bit(NF_FLOW_TEARDOWN, &flow->flags))
> > +		return;
> > +
> >   	clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
> > -	if (!test_and_set_bit(NF_FLOW_TEARDOWN, &flow->flags))
> > -		flow_offload_fixup_ct(flow);
> > +	flow_offload_fixup_ct(flow);
> >   }
> >   EXPORT_SYMBOL_GPL(flow_offload_teardown);
> >   
> >
> > ---
> > base-commit: 734bef548a96e8774d6eef5dfa2026c6edc4e80b
> > change-id: 20261103-flowtable-offload-bit-f15bfd1d38ed
> >
> > Best regards,
> > --
> > João Silva <jgc3silva@xxxxxxxxx>