Rewrite of buggy AS path matching.

Old AS path maching supposes thath AS number appears
only once in AS path, but that is not true. It also
contains some bugs related to AS path sets.

New code does not use any assumptions about semantic
structure of AS path. It is asymptotically slower than
the old code, but on real paths it is not significant.

It also allows '?' for matching one arbitrary AS number.

1 files changed, 155 insertions, 63 deletions

diff --git a/nest/a-path.c b/nest/a-path.c
index 7ac50e1..f549987 100644
--- a/nest/a-path.c
+++ b/nest/a-path.c
@@ -280,79 +280,171 @@ as_path_is_member(struct adata *path, u32 as)
 }
 
 
-
-#define MASK_PLUS do { mask = mask->next; if (!mask) return next == q; \
-		       asterisk = mask->any; \
-                       if (asterisk) { mask = mask->next; if (!mask) { return 1; } } \
-		       } while(0)
-
-int
-as_path_match(struct adata *path, struct f_path_mask *mask)
+struct pm_pos
+{
+  u8 set;
+  u8 mark;
+  union
+  {
+    char *sp;
+    u32 asn;
+  } val;
+};
+
+static int
+parse_path(struct adata *path, struct pm_pos *pos)
 {
   int bs = bgp_as4_support ? 4 : 2;
-  int i;
-  int asterisk = 0;
   u8 *p = path->data;
-  u8 *q = p+path->length;
-  int len;
-  u8 *next;
-  u32 as;
+  u8 *q = p + path->length;
+  struct pm_pos *opos = pos;
+  int i, j, len;
 
-  if (!mask)
-    return ! path->length;
 
-  asterisk = mask->any;
-  if (asterisk)
-    { mask = mask->next; if (!mask) return 1; }
-
-  while (p<q) {
-    switch (*p++) {
-    case AS_PATH_SET:
-      len = *p++;
+  while (p < q)
+    switch (*p++)
       {
-	u8 *p_save = p;
-	next = p_save + bs * len;
-      retry:
-	p = p_save;
-	for (i=0; i<len; i++) {
-	  as = get_as(p);
-	  if (asterisk && (as == mask->val)) {
-	    MASK_PLUS;
-	    goto retry;
-	  }
-	  if (!asterisk && (as == mask->val)) {
-	    p = next;
-	    MASK_PLUS;
-	    goto okay;
+      case AS_PATH_SET:
+	pos->set = 1;
+	pos->mark = 0;
+	pos->val.sp = p;
+	len = *p;
+	p += 1 + bs * len;
+	pos++;
+	break;
+      
+      case AS_PATH_SEQUENCE:
+	len = *p++;
+	for (i = 0; i < len; i++)
+	  {
+	    pos->set = 0;
+	    pos->mark = 0;
+	    pos->val.asn = get_as(p);
+	    p += bs;
+	    pos++;
 	  }
-	  p += bs;
-	}
-	if (!asterisk)
-	  return 0;
-      okay: ;
+	break;
+
+      default:
+	bug("as_path_match: Invalid path component");
       }
-      break;
-
-    case AS_PATH_SEQUENCE:
-      len = *p++;
-      for (i=0; i<len; i++) {
-	next = p + bs;
-	as = get_as(p);
-	if (asterisk && (as == mask->val))
-	  MASK_PLUS;
-	else if (!asterisk) {
-	  if (as != mask->val)
+  
+  return pos - opos;
+}
+
+
+static int
+pm_match(struct pm_pos *pos, u32 asn)
+{
+  if (! pos->set)
+    return pos->val.asn == asn;
+
+  int bs = bgp_as4_support ? 4 : 2;
+  u8 *p = pos->val.sp;
+  int len = *p++;
+  int i;
+
+  for (i = 0; i < len; i++)
+    if (get_as(p + i * bs) == asn)
+      return 1;
+
+  return 0;
+}
+
+static void
+pm_mark(struct pm_pos *pos, int i, int plen, int *nl, int *nh)
+{
+  int j;
+
+  if (pos[i].set)
+    pos[i].mark = 1;
+  
+  for (j = i + 1; (j < plen) && pos[j].set && (! pos[j].mark); j++)
+    pos[j].mark = 1;
+  pos[j].mark = 1;
+
+  /* We are going downwards, therefore every mark is
+     new low and just the first mark is new high */
+
+  *nl = i + (pos[i].set ? 0 : 1);
+
+  if (*nh < 0)
+    *nh = j;
+}
+
+/* AS path matching is nontrivial. Because AS path can
+ * contain sets, it is not a plain wildcard matching. A set 
+ * in an AS path is interpreted as it might represent any
+ * sequence of AS numbers from that set (possibly with
+ * repetitions). So it is also a kind of a pattern,
+ * more complicated than a path mask.
+ *
+ * The algorithm for AS path matching is a variant
+ * of nondeterministic finite state machine, where
+ * positions in AS path are states, and items in
+ * path mask are input for that finite state machine.
+ * During execution of the algorithm we maintain a set
+ * of marked states - a state is marked if it can be
+ * reached by any walk through NFSM with regard to
+ * currently processed part of input. When we process
+ * next part of mask, we advance each marked state.
+ * We start with marked first position, when we
+ * run out of marked positions, we reject. When
+ * we process the whole mask, we accept iff final position
+ * (auxiliary position after last real position in AS path)
+ * is marked.
+ */
+
+int
+as_path_match(struct adata *path, struct f_path_mask *mask)
+{
+  struct pm_pos pos[2048 + 1];
+  int plen = parse_path(path, pos);
+  int l, h, i, nh, nl;
+
+  /* l and h are bound of interval of positions where
+     are marked states */
+
+  pos[plen].set = 0;
+  pos[plen].mark = 0;
+
+  l = h = 0;
+  pos[0].mark = 1;
+  
+  while (mask)
+    {
+      /* We remove this mark to not step after pos[plen] */
+      pos[plen].mark = 0;
+
+      switch (mask->kind)
+	{
+	case PM_ASTERISK:
+	  for (i = l; i <= plen; i++)
+	    pos[i].mark = 1;
+	  h = plen;
+	  break;
+
+	case PM_QUESTION:
+	case PM_ASN:
+	  nh = -1;
+	  for (i = h; i >= l; i--)
+	    if (pos[i].mark)
+	      {
+		pos[i].mark = 0;
+		if ((mask->kind == PM_QUESTION) || pm_match(pos + i, mask->val))
+		  pm_mark(pos, i, plen, &nl, &nh);
+	      }
+
+	  if (nh < 0)
 	    return 0;
-	  MASK_PLUS;
+
+	  h = nh;
+	  l = nl;
+	  break;
 	}
-	p += bs;
-      }
-      break;
 
-    default:
-      bug("as_path_match: Invalid path component");
+      mask = mask->next;
     }
-  }
-  return 0;
-}
 
+  return pos[plen].mark;
+}