Re: [PATCH v3 11/14] flow, fwd: Consult rules table when forwarding a new flow from socket

[Stefano Brivio <sbrivio@redhat.com>]

Silly nits only and a couple of remarks that will probably be clarified
at a later point:

On Thu,  8 Jan 2026 13:29:45 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> We now have a formal array of forwarding rules.  However, we don't actually
> consult it when we forward a new flow.  Instead we rely on (a) implicit
> information (we wouldn't be here if there wasn't a listening socket for the
> rule) and (b) the legacy delta[] data structure.
> 
> Start addressing this, by searching for a matching forwarding rule when
> attempting to forward a new flow.  For now this is incomplete:
>  * We only do this for socket-initiated flows
>  * We make a potentially costly linear lookup
>  * We don't actually use the matching rule for anything yet
> 
> We'll address each of those in later patches.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  flow.c | 43 ++++++++++++++++++++++++++++++++++---------
>  fwd.c  | 33 +++++++++++++++++++++++++++++++++
>  fwd.h  |  2 ++
>  3 files changed, 69 insertions(+), 9 deletions(-)
> 
> diff --git a/flow.c b/flow.c
> index 4f534865..045e9712 100644
> --- a/flow.c
> +++ b/flow.c
> @@ -489,7 +489,7 @@ struct flowside *flow_initiate_sa(union flow *flow, uint8_t pif,
>  struct flowside *flow_target(const struct ctx *c, union flow *flow,
>  			     uint8_t proto)
>  {
> -	char estr[INANY_ADDRSTRLEN], fstr[INANY_ADDRSTRLEN];
> +	char estr[INANY_ADDRSTRLEN], ostr[INANY_ADDRSTRLEN];
>  	struct flow_common *f = &flow->f;
>  	const struct flowside *ini = &f->side[INISIDE];
>  	struct flowside *tgt = &f->side[TGTSIDE];
> @@ -500,6 +500,30 @@ struct flowside *flow_target(const struct ctx *c, union flow *flow,
>  	ASSERT(f->pif[INISIDE] != PIF_NONE && f->pif[TGTSIDE] == PIF_NONE);
>  	ASSERT(flow->f.state == FLOW_STATE_INI);
>  
> +	if (pif_is_socket(f->pif[INISIDE])) {
> +		const struct fwd_ports *fwd;
> +
> +		if (f->pif[INISIDE] == PIF_HOST && proto == IPPROTO_TCP)
> +			fwd = &c->tcp.fwd_in;
> +		else if (f->pif[INISIDE] == PIF_HOST && proto == IPPROTO_UDP)
> +			fwd = &c->udp.fwd_in;
> +		else if (f->pif[INISIDE] == PIF_SPLICE && proto == IPPROTO_TCP)
> +			fwd = &c->tcp.fwd_out;
> +		else if (f->pif[INISIDE] == PIF_SPLICE && proto == IPPROTO_UDP)
> +			fwd = &c->udp.fwd_out;
> +		else
> +			goto nofwd;
> +
> +		if (!fwd_rule_search(fwd, ini)) {
> +			/* This shouldn't happen, because if there's no rule for
> +			 * it we should have no listening socket that would let
> +			 * us get here
> +			 */
> +			flow_dbg(flow, "Unexpected missing forward rule");
> +			goto nofwd;
> +		}
> +	}
> +
>  	switch (f->pif[INISIDE]) {
>  	case PIF_TAP:
>  		memcpy(f->tap_omac, MAC_UNDEF, ETH_ALEN);
> @@ -514,22 +538,23 @@ struct flowside *flow_target(const struct ctx *c, union flow *flow,
>  		tgtpif = fwd_nat_from_host(c, proto, ini, tgt);
>  		fwd_neigh_mac_get(c, &tgt->oaddr, f->tap_omac);
>  		break;
> -
>  	default:
> -		flow_err(flow, "No rules to forward %s [%s]:%hu -> [%s]:%hu",
> -			 pif_name(f->pif[INISIDE]),
> -			 inany_ntop(&ini->eaddr, estr, sizeof(estr)),
> -			 ini->eport,
> -			 inany_ntop(&ini->oaddr, fstr, sizeof(fstr)),
> -			 ini->oport);
> +		goto nofwd;
>  	}
>  
>  	if (tgtpif == PIF_NONE)
> -		return NULL;
> +		goto nofwd;
>  
>  	f->pif[TGTSIDE] = tgtpif;
>  	flow_set_state(f, FLOW_STATE_TGT);
>  	return tgt;
> +
> +nofwd:
> +	flow_err(flow, "No rules to forward %s %s [%s]:%hu -> [%s]:%hu",
> +		 pif_name(f->pif[INISIDE]), ipproto_name(proto),
> +		 inany_ntop(&ini->eaddr, estr, sizeof(estr)), ini->eport,
> +		 inany_ntop(&ini->oaddr, ostr, sizeof(ostr)), ini->oport);

This assumes we're still using this function for inbound forwards only
(eaddr / eport -> oaddr / oport), perhaps we'll want a macro once it
starts being used for the other way around as well (if at all).

By the way, for rules, earlier in this series, you used "=>" to separate
source and target of the forward, here it's still "->", I guess we
should settle on one version (it just occurred to me while testing
stuff: it might be useful to grep in logs).

> +	return NULL;
>  }
>  
>  /**
> diff --git a/fwd.c b/fwd.c
> index 3f088fd2..633ba5db 100644
> --- a/fwd.c
> +++ b/fwd.c
> @@ -409,6 +409,39 @@ void fwd_rule_add(struct fwd_ports *fwd, uint8_t flags,
>  	}
>  }
>  
> +/**
> + * fwd_rule_match() - Does a prospective flow match a given forwarding rule

Does [...]?

> + * @rule:	Forwarding rule
> + * @ini:	Initiating side flow information
> + *
> + * Returns: true if the rule applies to the flow, false otherwise
> + */
> +static bool fwd_rule_match(const struct fwd_rule *rule,
> +			   const struct flowside *ini)
> +{
> +	return inany_matches(&ini->oaddr, fwd_rule_addr(rule)) &&
> +		ini->oport >= rule->first && ini->oport <= rule->last;

The usual alignment is:

	return inany_matches(&ini->oaddr, fwd_rule_addr(rule)) &&
	       ini->oport >= rule->first && ini->oport <= rule->last;

> +}
> +
> +/**
> + * fwd_rule_search() - Find a rule which matches a prospective flow
> + * @fwd:	Forwarding table
> + * @ini:	Initiating side flow information
> + *
> + * Returns: first matching rule, or NULL if there is none

I guess that this will eventually need to become a function matching
the most specific rule first, tie breakers could be:

1. specific address given vs. wildcard
2. specific interface given vs. no interface
3. the day we support/need it: specific port/range vs. no port
4. smallest port range
5. the day we support/need something like this: longest prefix length

and after this we should actually have an error on insertion (already
guaranteed I think).

> + */
> +const struct fwd_rule *fwd_rule_search(const struct fwd_ports *fwd,
> +				       const struct flowside *ini)
> +{
> +	unsigned i;
> +
> +	for (i = 0; i < fwd->count; i++) {
> +		if (fwd_rule_match(&fwd->rules[i], ini))
> +			return &fwd->rules[i];
> +	}

Extra newline here to clearly separate the two outcomes.

> +	return NULL;
> +}
> +
>  /**
>   * fwd_rules_print() - Print forwarding rules for debugging
>   * @fwd:	Table to print
> diff --git a/fwd.h b/fwd.h
> index f84e7c01..a10bdbb4 100644
> --- a/fwd.h
> +++ b/fwd.h
> @@ -103,6 +103,8 @@ struct fwd_ports {
>  void fwd_rule_add(struct fwd_ports *fwd, uint8_t flags,
>  		  const union inany_addr *addr, const char *ifname,
>  		  in_port_t first, in_port_t last, in_port_t to);
> +const struct fwd_rule *fwd_rule_search(const struct fwd_ports *fwd,
> +				       const struct flowside *ini);
>  void fwd_rules_print(const struct fwd_ports *fwd);
>  
>  void fwd_scan_ports_init(struct ctx *c);

-- 
Stefano