Re: [PATCH v2 10/23] fwd_rule: Move rule conflict checking from fwd_rule_add() to caller

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 10 Apr 2026 11:02:56 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> Amongst other checks, fwd_rule_add() checks that the newly added rule
> doesn't conflict with any existing rules.  However, unlike the other things
> we verify, this isn't really required for safe operation.  Rule conflicts
> are a useful thing for the user to know about, but the forwarding logic
> is perfectly sound with conflicting rules (the first one will win).
> 
> In order to support dynamic rule updates, we want fwd_rule_add() to become
> a more low-level function, only checking the things it really needs to.
> So, move rule conflict checking to its caller via new helpers in
> fwd_rule.c.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  conf.c     |  5 +++++
>  fwd.c      | 26 +-------------------------
>  fwd_rule.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
>  fwd_rule.h |  2 ++
>  4 files changed, 53 insertions(+), 25 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index 027bbac9..b871646f 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -205,13 +205,18 @@ static void conf_ports_range_except(const struct ctx *c, char optname,
>  
>  			if (c->ifi4) {
>  				rulev.addr = inany_loopback4;
> +				fwd_rule_conflict_check(&rulev,
> +							fwd->rules, fwd->count);
>  				fwd_rule_add(fwd, &rulev);
>  			}
>  			if (c->ifi6) {
>  				rulev.addr = inany_loopback6;
> +				fwd_rule_conflict_check(&rulev,
> +							fwd->rules, fwd->count);
>  				fwd_rule_add(fwd, &rulev);
>  			}
>  		} else {
> +			fwd_rule_conflict_check(&rule, fwd->rules, fwd->count);
>  			fwd_rule_add(fwd, &rule);
>  		}
>  		base = i - 1;
> diff --git a/fwd.c b/fwd.c
> index c05107d1..c9637525 100644
> --- a/fwd.c
> +++ b/fwd.c
> @@ -341,7 +341,7 @@ void fwd_rule_add(struct fwd_table *fwd, const struct fwd_rule *new)
>  	/* Flags which can be set from the caller */
>  	const uint8_t allowed_flags = FWD_WEAK | FWD_SCAN | FWD_DUAL_STACK_ANY;
>  	unsigned num = (unsigned)new->last - new->first + 1;
> -	unsigned i, port;
> +	unsigned port;
>  
>  	assert(!(new->flags & ~allowed_flags));
>  	/* Passing a non-wildcard address with DUAL_STACK_ANY is a bug */
> @@ -354,30 +354,6 @@ void fwd_rule_add(struct fwd_table *fwd, const struct fwd_rule *new)
>  	if ((fwd->sock_count + num) > ARRAY_SIZE(fwd->socks))
>  		die("Too many listening sockets");
>  
> -	/* Check for any conflicting entries */
> -	for (i = 0; i < fwd->count; i++) {
> -		char newstr[INANY_ADDRSTRLEN], rulestr[INANY_ADDRSTRLEN];
> -		const struct fwd_rule *rule = &fwd->rules[i];
> -
> -		if (new->proto != rule->proto)
> -			/* Non-conflicting protocols */
> -			continue;
> -
> -		if (!inany_matches(fwd_rule_addr(new), fwd_rule_addr(rule)))
> -			/* Non-conflicting addresses */
> -			continue;
> -
> -		if (new->last < rule->first || rule->last < new->first)
> -			/* Port ranges don't overlap */
> -			continue;
> -
> -		die("Forwarding configuration conflict: %s/%u-%u versus %s/%u-%u",
> -		    inany_ntop(fwd_rule_addr(new), newstr, sizeof(newstr)),
> -		    new->first, new->last,
> -		    inany_ntop(fwd_rule_addr(rule), rulestr, sizeof(rulestr)),
> -		    rule->first, rule->last);
> -	}
> -
>  	fwd->rulesocks[fwd->count] = &fwd->socks[fwd->sock_count];
>  	for (port = new->first; port <= new->last; port++)
>  		fwd->rulesocks[fwd->count][port - new->first] = -1;
> diff --git a/fwd_rule.c b/fwd_rule.c
> index a034d5d1..5bc94efe 100644
> --- a/fwd_rule.c
> +++ b/fwd_rule.c
> @@ -93,3 +93,48 @@ void fwd_rules_info(const struct fwd_rule *rules, size_t count)
>  		info("    %s", fwd_rule_fmt(&rules[i], buf, sizeof(buf)));
>  	}
>  }
> +
> +/**
> + * fwd_rule_conflicts() - Test if two rules conflict with each other
> + * @a, @b:	Rules to test
> + */
> +static bool fwd_rule_conflicts(const struct fwd_rule *a, const struct fwd_rule *b)
> +{
> +	if (a->proto != b->proto)
> +		/* Non-conflicting protocols */
> +		return false;
> +
> +	if (!inany_matches(fwd_rule_addr(a), fwd_rule_addr(b)))
> +		/* Non-conflicting addresses */
> +		return false;
> +
> +	assert(a->first <= a->last && b->first <= b->last);

I expected this assert() to be gone by the end of the series, like the
ones dropped in 11/23, but it's still there. Is this something that the
client can't specifically trigger for some reason?

-- 
Stefano