[RFC PATCH net] netfilter: flowtable: fix offloaded ct timeout never being extended

[Adrian Bențe]

> I guess we need to open-code expires, something like this (not even
> compile tested). Also see https://sashiko.dev/#/patchset/20260526060138.3924-1-adibente%40gmail.com
>
> diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
> --- a/net/netfilter/nf_flow_table_core.c
> +++ b/net/netfilter/nf_flow_table_core.c
> @@ -506,7 +506,12 @@ static u32 nf_flow_table_tcp_timeout(const struct nf_conn *ct)
>  static void nf_flow_table_extend_ct_timeout(struct nf_conn *ct)
>  {
>         static const u32 min_timeout = 5 * 60 * HZ;
> -       u32 expires = nf_ct_expires(ct);
> +       u32 ct_timeout = READ_ONCE(ct->timeout);
> +       s32 expires;
> +
> +       expires = ct_timeout - nfct_time_stamp;
> +       if (expires <= 0) /* already expired */
> +               return;
>
>         /* normal case: large enough timeout, nothing to do. */
>         if (likely(expires >= min_timeout))
> @@ -524,7 +529,7 @@ static void nf_flow_table_extend_ct_timeout(struct nf_conn *ct)
>         if (nf_ct_is_confirmed(ct) &&
>             test_bit(IPS_OFFLOAD_BIT, &ct->status)) {
>                 u8 l4proto = nf_ct_protonum(ct);
> -               u32 new_timeout = true;
> +               u32 new_timeout = 1;
>
>                 switch (l4proto) {
>                 case IPPROTO_UDP:
> @@ -549,7 +554,7 @@ static void nf_flow_table_extend_ct_timeout(struct nf_conn *ct)
>                  */
>                 if (new_timeout) {
>                         new_timeout += nfct_time_stamp;
> -                       cmpxchg(&ct->timeout, expires, new_timeout);
> +                       cmpxchg(&ct->timeout, ct_timeout, new_timeout);
>                 }
>         }
>

Thanks. Applied your diff to my OpenWrt 6.18 tree with one small
adjustment: changed min_timeout from u32 to s32 so the
"expires >= min_timeout" comparison has both operands signed.
Compiles clean.

Tested on MT7986 with the WED-offloaded flows that originally
reproduced the 300s drop. The flows now stay up well past 300s with
normal offloaded traffic, solution works fine.

I'll send v2 with this diff and Suggested-by: you, unless you'd
rather submit it yourself.