bpf: Handle in-place update for full LPM trie correctly

When a LPM trie is full, in-place updates of existing elements
incorrectly return -ENOSPC.

Fix this by deferring the check of trie->n_entries. For new insertions,
n_entries must not exceed max_entries. However, in-place updates are
allowed even when the trie is full.

Fixes: b95a5c4 ("bpf: add a longest prefix match trie map implementation")
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20241206110622.1161752-5-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Hou Tao

kernel/bpf/lpm_trie.c

@@ -310,6 +310,16 @@ static struct lpm_trie_node *lpm_trie_node_alloc(const struct lpm_trie *trie,
 	return node;
 }
 
+static int trie_check_add_elem(struct lpm_trie *trie, u64 flags)
+{
+	if (flags == BPF_EXIST)
+		return -ENOENT;
+	if (trie->n_entries == trie->map.max_entries)
+		return -ENOSPC;
+	trie->n_entries++;
+	return 0;
+}
+
 /* Called from syscall or from eBPF program */
 static long trie_update_elem(struct bpf_map *map,
 			     void *_key, void *value, u64 flags)
@@ -333,20 +343,12 @@ static long trie_update_elem(struct bpf_map *map,
 	spin_lock_irqsave(&trie->lock, irq_flags);
 
 	/* Allocate and fill a new node */
-
-	if (trie->n_entries == trie->map.max_entries) {
-		ret = -ENOSPC;
-		goto out;
-	}
-
 	new_node = lpm_trie_node_alloc(trie, value);
 	if (!new_node) {
 		ret = -ENOMEM;
 		goto out;
 	}
 
-	trie->n_entries++;
-
 	new_node->prefixlen = key->prefixlen;
 	RCU_INIT_POINTER(new_node->child[0], NULL);
 	RCU_INIT_POINTER(new_node->child[1], NULL);
@@ -375,10 +377,10 @@ static long trie_update_elem(struct bpf_map *map,
 	 * simply assign the @new_node to that slot and be done.
 	 */
 	if (!node) {
-		if (flags == BPF_EXIST) {
-			ret = -ENOENT;
+		ret = trie_check_add_elem(trie, flags);
+		if (ret)
 			goto out;
-		}
+
 		rcu_assign_pointer(*slot, new_node);
 		goto out;
 	}
@@ -392,10 +394,10 @@ static long trie_update_elem(struct bpf_map *map,
 				ret = -EEXIST;
 				goto out;
 			}
-			trie->n_entries--;
-		} else if (flags == BPF_EXIST) {
-			ret = -ENOENT;
-			goto out;
+		} else {
+			ret = trie_check_add_elem(trie, flags);
+			if (ret)
+				goto out;
 		}
 
 		new_node->child[0] = node->child[0];
@@ -407,10 +409,9 @@ static long trie_update_elem(struct bpf_map *map,
 		goto out;
 	}
 
-	if (flags == BPF_EXIST) {
-		ret = -ENOENT;
+	ret = trie_check_add_elem(trie, flags);
+	if (ret)
 		goto out;
-	}
 
 	/* If the new node matches the prefix completely, it must be inserted
 	 * as an ancestor. Simply insert it between @node and *@slot.
@@ -424,6 +425,7 @@ static long trie_update_elem(struct bpf_map *map,
 
 	im_node = lpm_trie_node_alloc(trie, NULL);
 	if (!im_node) {
+		trie->n_entries--;
 		ret = -ENOMEM;
 		goto out;
 	}
@@ -445,12 +447,8 @@ static long trie_update_elem(struct bpf_map *map,
 	rcu_assign_pointer(*slot, im_node);
 
 out:
-	if (ret) {
-		if (new_node)
-			trie->n_entries--;
+	if (ret)
 		kfree(new_node);
-	}
-
 	spin_unlock_irqrestore(&trie->lock, irq_flags);
 	kfree_rcu(free_node, rcu);