[IPV4] fib: fix route replacement, fib_info is shared

fib_info can be shared by many route prefixes but we don't want
duplicate alternative routes for a prefix+tos+priority. Last change
was not correct to check fib_treeref because it accounts usage from
other prefixes. Additionally, avoid replacement without error if new
route is same, as Joonwoo Park suggests.
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_hash.c

@@ -424,19 +424,43 @@ static int fn_hash_insert(struct fib_table *tb, struct fib_config *cfg)
 
 	if (fa && fa->fa_tos == tos &&
 	    fa->fa_info->fib_priority == fi->fib_priority) {
-		struct fib_alias *fa_orig;
+		struct fib_alias *fa_first, *fa_match;
 
 		err = -EEXIST;
 		if (cfg->fc_nlflags & NLM_F_EXCL)
 			goto out;
 
+		/* We have 2 goals:
+		 * 1. Find exact match for type, scope, fib_info to avoid
+		 * duplicate routes
+		 * 2. Find next 'fa' (or head), NLM_F_APPEND inserts before it
+		 */
+		fa_match = NULL;
+		fa_first = fa;
+		fa = list_entry(fa->fa_list.prev, struct fib_alias, fa_list);
+		list_for_each_entry_continue(fa, &f->fn_alias, fa_list) {
+			if (fa->fa_tos != tos)
+				break;
+			if (fa->fa_info->fib_priority != fi->fib_priority)
+				break;
+			if (fa->fa_type == cfg->fc_type &&
+			    fa->fa_scope == cfg->fc_scope &&
+			    fa->fa_info == fi) {
+				fa_match = fa;
+				break;
+			}
+		}
+
 		if (cfg->fc_nlflags & NLM_F_REPLACE) {
 			struct fib_info *fi_drop;
 			u8 state;
 
-			if (fi->fib_treeref > 1)
+			fa = fa_first;
+			if (fa_match) {
+				if (fa == fa_match)
+					err = 0;
 				goto out;
-
+			}
 			write_lock_bh(&fib_hash_lock);
 			fi_drop = fa->fa_info;
 			fa->fa_info = fi;
@@ -459,20 +483,11 @@ static int fn_hash_insert(struct fib_table *tb, struct fib_config *cfg)
 		 * uses the same scope, type, and nexthop
 		 * information.
 		 */
-		fa_orig = fa;
-		fa = list_entry(fa->fa_list.prev, struct fib_alias, fa_list);
-		list_for_each_entry_continue(fa, &f->fn_alias, fa_list) {
-			if (fa->fa_tos != tos)
-				break;
-			if (fa->fa_info->fib_priority != fi->fib_priority)
-				break;
-			if (fa->fa_type == cfg->fc_type &&
-			    fa->fa_scope == cfg->fc_scope &&
-			    fa->fa_info == fi)
-				goto out;
-		}
+		if (fa_match)
+			goto out;
+
 		if (!(cfg->fc_nlflags & NLM_F_APPEND))
-			fa = fa_orig;
+			fa = fa_first;
 	}
 
 	err = -ENOENT;