Add a comment and a check for why flag indices will be <= 63

src/or/dirvote.c

@@ -1590,10 +1590,19 @@ networkstatus_compute_consensus(smartlist_t *votes,
       unnamed_flag[i] = named_flag[i] = -1;
     chosen_named_idx = smartlist_string_pos(flags, "Named");
 
-    /* Build the flag index. */
+    /* Build the flag indexes. Note that no vote can have more than 64 members
+     * for known_flags, so no value will be greater than 63, so it's safe to
+     * do U64_LITERAL(1) << index on these values.  But note also that
+     * named_flag and unnamed_flag are initialized to -1, so we need to check
+     * that they're actually set before doing U64_LITERAL(1) << index with
+     * them.*/
     SMARTLIST_FOREACH_BEGIN(votes, networkstatus_t *, v) {
       flag_map[v_sl_idx] = tor_malloc_zero(
                            sizeof(int)*smartlist_len(v->known_flags));
+      if (smartlist_len(v->known_flags) > MAX_KNOWN_FLAGS_IN_VOTE) {
+        log_warn(LD_BUG, "Somehow, a vote has %d entries in known_flags",
+                 smartlist_len(v->known_flags));
+      }
       SMARTLIST_FOREACH_BEGIN(v->known_flags, const char *, fl) {
         int p = smartlist_string_pos(flags, fl);
         tor_assert(p >= 0);