bpf: Avoid unintended eviction when updating lru_hash maps

When updating an existing element in lru_hash maps, the current
implementation always calls prealloc_lru_pop() to get a new node before
checking if the key already exists. If the map is full, this triggers
LRU eviction and removes an existing element, even though the update
operation only needs to modify the value of an existing key in-place.

This is problematic because:
1. Users may unexpectedly lose entries when doing simple value updates
2. The eviction overhead is unnecessary for existing key updates

Fix this by first checking if the key exists before allocating a new
node. If the key is found, update the value in-place, refresh the LRU
reference, and return immediately without triggering any eviction.

Fixes: 29ba732 ("bpf: Add BPF_MAP_TYPE_LRU_HASH")
Signed-off-by: Leon Hwang <[email protected]>

Author: Asphaltt

kernel/bpf/hashtab.c

@@ -1207,6 +1207,27 @@ static long htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value
 	b = __select_bucket(htab, hash);
 	head = &b->head;
 
+	ret = htab_lock_bucket(b, &flags);
+	if (ret)
+		goto err_lock_bucket;
+
+	l_old = lookup_elem_raw(head, hash, key, key_size);
+
+	ret = check_flags(htab, l_old, map_flags);
+	if (ret)
+		goto err;
+
+	if (l_old) {
+		bpf_lru_node_set_ref(&l_old->lru_node);
+		copy_map_value(&htab->map, htab_elem_value(l_old, map->key_size), value);
+		check_and_free_fields(htab, l_old);
+	}
+
+	htab_unlock_bucket(b, flags);
+
+	if (l_old)
+		return 0;
+
 	/* For LRU, we need to alloc before taking bucket's
 	 * spinlock because getting free nodes from LRU may need
 	 * to remove older elements from htab and this removal