Rework prefix_insert() and prefix_remove() to properly recalculate dmetric

The med regress test triggered the fatal in prefix_set_dmetric() because
on MED priority inversion the simple check previous with next before remove
can return a negative number because that prefix is also inversed.

Adjust code so that when removing prefixes from the list wait until the
next element is checked to do the fixup. In prefix_remove() cache the
previous element and calculate the dmetric at the end of the MED check.
In prefix_insert() alter the loop to also defer the dmetric recalc by
removing most continue statements in the loop.
With and OK tb@

diff --git a/usr.sbin/bgpd/rde_decide.c b/usr.sbin/bgpd/rde_decide.c
index eb92aa4..6639dcd 100644 (file)
--- a/usr.sbin/bgpd/rde_decide.c
+++ b/usr.sbin/bgpd/rde_decide.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: rde_decide.c,v 1.93 2022/07/07 12:16:04 claudio Exp $ */
+/*     $OpenBSD: rde_decide.c,v 1.94 2022/07/07 19:46:38 claudio Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Claudio Jeker <claudio@openbsd.org>
@@ -348,7 +348,7 @@ prefix_insert(struct prefix *new, struct prefix *ep, struct rib_entry *re)
 {
        struct prefix_queue redo = TAILQ_HEAD_INITIALIZER(redo);
        struct prefix *xp, *np, *insertp = ep;
-       int testall, selected = 0;
+       int testall, preferred, selected = 0, removed = 0;
 
        /* start scan at the entry point (ep) or the head if ep == NULL */
        if (ep == NULL)
@@ -357,26 +357,25 @@ prefix_insert(struct prefix *new, struct prefix *ep, struct rib_entry *re)
        for (xp = ep; xp != NULL; xp = np) {
                np = TAILQ_NEXT(xp, entry.list.rib);
 
-               if (prefix_cmp(new, xp, &testall) > 0) {
+               if ((preferred = (prefix_cmp(new, xp, &testall) > 0))) {
                        /* new is preferred over xp */
-                       if (testall == 0)
-                               break;          /* we're done */
-                       else if (testall == 2) {
+                       if (testall == 2) {
                                /*
                                 * MED inversion, take out prefix and
                                 * put it onto redo queue.
                                 */
-                               prefix_set_dmetric(TAILQ_PREV(xp, prefix_queue,
-                                   entry.list.rib), np);
                                TAILQ_REMOVE(&re->prefix_h, xp, entry.list.rib);
                                TAILQ_INSERT_TAIL(&redo, xp, entry.list.rib);
-                       } else {
+                               removed = 1;
+                               continue;
+                       }
+
+                       if (testall == 1) {
                                /*
                                 * lock insertion point and
                                 * continue on with scan
                                 */
                                selected = 1;
-                               continue;
                        }
                } else {
                        /*
@@ -393,15 +392,26 @@ prefix_insert(struct prefix *new, struct prefix *ep, struct rib_entry *re)
                        if (!selected)
                                insertp = xp;
                }
+
+               /*
+                * If previous element(s) got removed, fixup the
+                * dmetric, now that it is clear that this element
+                * is on the list.
+                */
+               if (removed) {
+                       prefix_set_dmetric(TAILQ_PREV(xp, prefix_queue,
+                           entry.list.rib), xp);
+                       removed = 0;
+               }
+
+               if (preferred && testall == 0)
+                       break;                  /* we're done */
        }
 
        if (insertp == NULL) {
                TAILQ_INSERT_HEAD(&re->prefix_h, new, entry.list.rib);
        } else {
                TAILQ_INSERT_AFTER(&re->prefix_h, insertp, new, entry.list.rib);
-               new->dmetric = prefix_cmp(insertp, new, &testall);
-               if (new->dmetric < 0)
-                       fatalx("bad dmetric in decision process");
        }
 
        prefix_set_dmetric(insertp, new);
@@ -429,11 +439,11 @@ void
 prefix_remove(struct prefix *old, struct rib_entry *re)
 {
        struct prefix_queue redo = TAILQ_HEAD_INITIALIZER(redo);
-       struct prefix *xp, *np;
-       int testall;
+       struct prefix *xp, *np, *pp;
+       int testall, removed = 0;
 
        xp = TAILQ_NEXT(old, entry.list.rib);
-       prefix_set_dmetric(TAILQ_PREV(old, prefix_queue, entry.list.rib), xp);
+       pp = TAILQ_PREV(old, prefix_queue, entry.list.rib);
        TAILQ_REMOVE(&re->prefix_h, old, entry.list.rib);
 
        /* check if a MED inversion could be possible */
@@ -445,21 +455,36 @@ prefix_remove(struct prefix *old, struct rib_entry *re)
 
                        /* only interested in the testall result */
                        prefix_cmp(old, xp, &testall);
-                       if (testall == 0)
-                               break;          /* we're done */
-                       else if (testall == 2) {
+                       if (testall == 2) {
                                /*
                                 * possible MED inversion, take out prefix and
                                 * put it onto redo queue.
                                 */
-                               prefix_set_dmetric(TAILQ_PREV(xp, prefix_queue,
-                                   entry.list.rib), np);
                                TAILQ_REMOVE(&re->prefix_h, xp, entry.list.rib);
                                TAILQ_INSERT_TAIL(&redo, xp, entry.list.rib);
+                               removed = 1;
+                               continue;
+                       }
+                       /*
+                        * If previous element(s) got removed, fixup the
+                        * dmetric, now that it is clear that this element
+                        * is on the list.
+                        */
+                       if (removed) {
+                               prefix_set_dmetric(TAILQ_PREV(xp, prefix_queue,
+                                   entry.list.rib), xp);
+                               removed = 0;
                        }
+                       if (testall == 0)
+                               break;          /* we're done */
                }
        }
 
+       if (pp)
+               prefix_set_dmetric(pp, TAILQ_NEXT(pp, entry.list.rib));
+       else
+               prefix_set_dmetric(NULL, TAILQ_FIRST(&re->prefix_h));
+
        /* Fixup MED order again, reinsert prefixes from the start */
        while (!TAILQ_EMPTY(&redo)) {
                xp = TAILQ_FIRST(&redo);