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stp: compare the whole Bridge Identifier, not the priority byte and the MAC

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A Bridge Identifier is two priority octets followed by the MAC, compared
as one unsigned number. The test here read the first priority octet and
then went straight to the MAC, so the system ID extension in between was
never looked at and two bridges differing only in it were ranked by MAC
instead. The field is stored, sent and printed, just not compared.

Ordinary single instance RSTP leaves the extension zero on both sides,
which is why this has not shown up. Where it is not zero the ranking is
simply wrong: same priority octet, extension 0x0a against 0x00, and the
worse bridge wins if its MAC happens to be lower.

cmpMAC becomes cmpBytes with a length, since the identifier is eight
contiguous bytes in both the packet overlay and root_bridge, and the
loop was already doing the right thing for six of them. sdcc lays the
struct out with no padding, checked, so the eight byte compare is the
standard's rule written directly.
This commit is contained in:
d00f
2026-08-18 23:30:35 +02:00
committed by d00f
parent 4264856109
commit 988bddacf7
1 changed files with 11 additions and 6 deletions
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rtl837x_stp.c
+11 -6
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@@ -208,9 +208,12 @@ static void stp_record_designated(uint8_t port) __reentrant
}
signed char cmpMAC(__xdata uint8_t *m1, __xdata uint8_t *m2) __reentrant
/* Lexicographic compare of n bytes. A MAC is 6 of them; a Bridge Identifier
* is 8, the two priority octets ahead of the MAC, compared as one unsigned
* number per 802.1D. */
signed char cmpBytes(__xdata uint8_t *m1, __xdata uint8_t *m2, uint8_t n) __reentrant
{
for (uint8_t i = 0; i < 6; i++) {
for (uint8_t i = 0; i < n; i++) {
if (m1[i] == m2[i])
continue;
if (m1[i] < m2[i])
@@ -499,7 +502,7 @@ void stp_in(void) __banked
* still receiving. Pull the cable and the re-arming stops, so the loser
* comes back on its own after a forward delay - and the link
* supervision above gets there first anyway. */
if (cmpMAC(STP_I->bridge.mac, uip_ethaddr.addr) == 0) {
if (cmpBytes(STP_I->bridge.mac, uip_ethaddr.addr, 6) == 0) {
/* Equal means the frame came back on the port it left: a loop
* further out, behind an unmanaged switch. There is no pair to
* pick from, so that port holds itself down - and since it can
@@ -527,9 +530,11 @@ void stp_in(void) __banked
stp_record_designated(port);
/* Better root than the one we know? */
if (STP_I->root.prio < root_bridge.prio
|| ((STP_I->root.prio == root_bridge.prio) && cmpMAC(STP_I->root.mac, root_bridge.mac) < 0)) {
/* Better root than the one we know? The identifier is priority, system
* ID extension and MAC in that order: comparing the priority byte and
* then jumping to the MAC skipped the twelve bits in between, so two
* bridges differing only in the extension were ranked by MAC. */
if (cmpBytes((__xdata uint8_t *)&STP_I->root, (__xdata uint8_t *)&root_bridge, 8) < 0) {
/* Root guard: this port must never become our path to the root. */
if (stp_pflags[port] & STP_PF_ROOTGUARD) {
print_string("STP: root guard blocking port ");