[PATCH bpf-next v1] bpf: Use hlist_nulls_replace_rcu() when updating htab elements

[xuanqiang.luo@linux.dev]

From: Xuanqiang Luo <luoxuanqiang@kylinos.cn>

When updating an existing element, the current code does
hlist_nulls_add_head_rcu() followed by hlist_nulls_del_rcu(), exposing
a transient state where both old and new elements are visible to
concurrent RCU traversals.

Use hlist_nulls_replace_rcu() to atomically replace the old element in
place, so RCU readers always see a consistent hash chain.

Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
---
 kernel/bpf/hashtab.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index 3dd9b4924ae4f..b20e82bd253dc 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -1246,18 +1246,16 @@ static long htab_map_update_elem(struct bpf_map *map, void *key, void *value,
 		goto err;
 	}
 
-	/* add new element to the head of the list, so that
-	 * concurrent search will find it before old elem
-	 */
-	hlist_nulls_add_head_rcu(&l_new->hash_node, head);
 	if (l_old) {
-		hlist_nulls_del_rcu(&l_old->hash_node);
+		hlist_nulls_replace_rcu(&l_old->hash_node, &l_new->hash_node);
 
 		/* l_old has already been stashed in htab->extra_elems, free
 		 * its special fields before it is available for reuse.
 		 */
 		if (htab_is_prealloc(htab))
 			check_and_free_fields(htab, l_old);
+	} else {
+		hlist_nulls_add_head_rcu(&l_new->hash_node, head);
 	}
 	htab_unlock_bucket(b, flags);
 	if (l_old && !htab_is_prealloc(htab))
@@ -1319,13 +1317,11 @@ static long htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value
 	if (ret)
 		goto err;
 
-	/* add new element to the head of the list, so that
-	 * concurrent search will find it before old elem
-	 */
-	hlist_nulls_add_head_rcu(&l_new->hash_node, head);
 	if (l_old) {
 		bpf_lru_node_set_ref(&l_new->lru_node);
-		hlist_nulls_del_rcu(&l_old->hash_node);
+		hlist_nulls_replace_rcu(&l_old->hash_node, &l_new->hash_node);
+	} else {
+		hlist_nulls_add_head_rcu(&l_new->hash_node, head);
 	}
 	ret = 0;
 
-- 
2.43.0