On Mon, May 22, 2023 at 03:45:58PM +0000, Anton Protopopov wrote:
> The LRU and LRU_PERCPU maps allocate a new element on update before locking the
> target hash table bucket. Right after that the maps try to lock the bucket.
> If this fails, then maps return -EBUSY to the caller without releasing the
> allocated element. This makes the element untracked: it doesn't belong to
> either of free lists, and it doesn't belong to the hash table, so can't be
> re-used; this eventually leads to the permanent -ENOMEM on LRU map updates,
> which is unexpected. Fix this by returning the element to the local free list
> if bucket locking fails.
Fixes: 20b6cc34ea74 ("bpf: Avoid hashtab deadlock with map_locked")
> Signed-off-by: Anton Protopopov <aspsk@xxxxxxxxxxxxx>
> ---
> kernel/bpf/hashtab.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 00c253b84bf5..9901efee4339 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1215,7 +1215,7 @@ static long htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value
>
> ret = htab_lock_bucket(htab, b, hash, &flags);
> if (ret)
> - return ret;
> + goto err_lock_bucket;
>
> l_old = lookup_elem_raw(head, hash, key, key_size);
>
> @@ -1236,6 +1236,7 @@ static long htab_lru_map_update_elem(struct bpf_map *map, void *key, void *value
> err:
> htab_unlock_bucket(htab, b, hash, flags);
>
> +err_lock_bucket:
> if (ret)
> htab_lru_push_free(htab, l_new);
> else if (l_old)
> @@ -1338,7 +1339,7 @@ static long __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
>
> ret = htab_lock_bucket(htab, b, hash, &flags);
> if (ret)
> - return ret;
> + goto err_lock_bucket;
>
> l_old = lookup_elem_raw(head, hash, key, key_size);
>
> @@ -1361,6 +1362,7 @@ static long __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
> ret = 0;
> err:
> htab_unlock_bucket(htab, b, hash, flags);
> +err_lock_bucket:
> if (l_new)
> bpf_lru_push_free(&htab->lru, &l_new->lru_node);
> return ret;
> --
> 2.34.1
>
