Re: [PATCH bpf-next] bpf: add support for other map types to bpf_map_lookup_and_delete_elem

[Song Liu]

On Thu, Sep 17, 2020 at 7:16 AM Luka Oreskovic
<luka.oresko...@sartura.hr> wrote:
>
[...]

> +++ b/kernel/bpf/syscall.c
> @@ -1475,6 +1475,9 @@ static int map_lookup_and_delete_elem(union bpf_attr 
> *attr)
>         if (CHECK_ATTR(BPF_MAP_LOOKUP_AND_DELETE_ELEM))
>                 return -EINVAL;
>
> +       if (attr->flags & ~BPF_F_LOCK)
> +               return -EINVAL;
> +

Please explain (in comments for commit log) the use of BPF_F_LOCK in
the commit log,
as it is new for BPF_MAP_LOOKUP_AND_DELETE_ELEM.

>         f = fdget(ufd);
>         map = __bpf_map_get(f);
>         if (IS_ERR(map))
> @@ -1485,13 +1488,19 @@ static int map_lookup_and_delete_elem(union bpf_attr 
> *attr)
>                 goto err_put;
>         }
>
> +       if ((attr->flags & BPF_F_LOCK) &&
> +           !map_value_has_spin_lock(map)) {
> +               err = -EINVAL;
> +               goto err_put;
> +       }
> +
>         key = __bpf_copy_key(ukey, map->key_size);
>         if (IS_ERR(key)) {
>                 err = PTR_ERR(key);
>                 goto err_put;
>         }
>
> -       value_size = map->value_size;
> +       value_size = bpf_map_value_size(map);
>
>         err = -ENOMEM;
>         value = kmalloc(value_size, GFP_USER | __GFP_NOWARN);
> @@ -1502,7 +1511,24 @@ static int map_lookup_and_delete_elem(union bpf_attr 
> *attr)
>             map->map_type == BPF_MAP_TYPE_STACK) {
>                 err = map->ops->map_pop_elem(map, value);
>         } else {
> -               err = -ENOTSUPP;
> +               err = bpf_map_copy_value(map, key, value, attr->flags);
> +               if (err)
> +                       goto free_value;

IIUC, we cannot guarantee the value returned is the same as the value we
deleted. If this is true, I guess this may confuse the user with some
concurrency
bug.

Thanks,
Song

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