Re: [PATCH 10/18] fwd_rule: Move rule conflict checking from fwd_rule_add() to caller

Stefano Brivio
2026-04-07T23:16:20Z

On Tue, 7 Apr 2026 13:16:22 +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 | 44 ++++++++++++++++++++++++++++++++++++++++++++
> fwd_rule.h | 2 ++
> 4 files changed, 52 insertions(+), 25 deletions(-)
>
> diff --git a/conf.c b/conf.c
> index c9ee8c59..a93837cc 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 abe9dfbf..4d5048f9 100644
> --- a/fwd_rule.c
> +++ b/fwd_rule.c
> @@ -87,3 +87,47 @@ 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);
> + if (a->last < b->first || b->last < a->first)
> + /* Port ranges don't overlap */
> + return false;
> +
> + return true;
> +}
> +
> +/* fwd_rule_conflict_check() - Die with errir if rule conflicts with any in list

Nit: an errir happens only when Mimir (ask Jon) makes a mistake, which
is quite rare. :)

> + * @new: New rule
> + * @rules: Existing rules against which to test
> + * @count: Number of rules in @rules
> + */
> +void fwd_rule_conflict_check(const struct fwd_rule *new,
> + const struct fwd_rule *rules, size_t count)
> +{
> + unsigned i;
> +
> + for (i = 0; i < count; i++) {
> + char newstr[FWD_RULE_STRLEN], rulestr[FWD_RULE_STRLEN];
> +
> + if (!fwd_rule_conflicts(new, &rules[i]))
> + continue;
> +
> + die("Forwarding configuration conflict: %s versus %s",
> + fwd_rule_fmt(new, newstr, sizeof(newstr)),
> + fwd_rule_fmt(&rules[i], rulestr, sizeof(rulestr)));
> + }
> +}
> diff --git a/fwd_rule.h b/fwd_rule.h
> index e92efb6d..f852be39 100644
> --- a/fwd_rule.h
> +++ b/fwd_rule.h
> @@ -52,5 +52,7 @@ struct fwd_rule {
>
> const union inany_addr *fwd_rule_addr(const struct fwd_rule *rule);
> void fwd_rules_info(const struct fwd_rule *rules, size_t count);
> +void fwd_rule_conflict_check(const struct fwd_rule *new,
> + const struct fwd_rule *rules, size_t count);
>
> #endif /* FWD_RULE_H */

I reviewed only up to here so far, the rest will come in a bit.

I had a quick look at the whole series and it all looks good to me so
far but that wasn't quite a review.

Meanwhile, I noticed some warnings that strangely appear only during the
build of passt.avx2:

cc -Wall -Wextra -Wno-format-zero-length -Wformat-security -pedantic -std=c11 -D_XOPEN_SOURCE=700 -D_GNU_SOURCE -D_FORTIFY_SOURCE=2 -pie -fPIE -DPAGE_SIZE=4096 -DVERSION=\"2026_01_20.386b5f5-84-ge87c74f\" -DDUAL_STACK_SOCKETS=1 -DHAS_GETRANDOM -fstack-protector-strong -Ofast -mavx2 -ftree-vectorize -funroll-loops \
arch.c arp.c bitmap.c checksum.c conf.c dhcp.c dhcpv6.c epoll_ctl.c flow.c fwd.c fwd_rule.c icmp.c igmp.c inany.c iov.c ip.c isolation.c lineread.c log.c mld.c ndp.c netlink.c migrate.c packet.c passt.c pasta.c pcap.c pif.c repair.c serialise.c tap.c tcp.c tcp_buf.c tcp_splice.c tcp_vu.c udp.c udp_flow.c udp_vu.c util.c vhost_user.c virtio.c vu_common.c -o passt.avx2
In file included from util.h:22,
from ip.h:12,
from fwd_rule.h:16,
from fwd_rule.c:20:
fwd_rule.c: In function ‘fwd_rules_info’:
fwd_rule.c:86:22: warning: ‘%s’ directive argument is null [-Wformat-overflow=]
86 | info(" %s", fwd_rule_fmt(&rules[i], buf, sizeof(buf)));
| ^~~~~~~~
log.h:31:66: note: in definition of macro ‘info’
31 | #define info(...) logmsg(true, false, LOG_INFO, __VA_ARGS__)
| ^~~~~~~~~~~
fwd_rule.c:86:27: note: format string is defined here
86 | info(" %s", fwd_rule_fmt(&rules[i], buf, sizeof(buf)));
| ^~

...but I don't think I looked at the code changes causing them, yet (and
didn't bisect the series either). This is with gcc 13.3.0.

--
Stefano