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

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Message-ID: <CAPhsuW4WXqiK-AFP6nU1L03yXGLLuz845mFP8W_rhbyaw=Ck=w@mail.gmail.com>
Date:   Thu, 17 Sep 2020 16:21:35 -0700
From:   Song Liu <song@...nel.org>
To:     Luka Oreskovic <luka.oreskovic@...tura.hr>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Juraj Vijtiuk <juraj.vijtiuk@...tura.hr>,
        Luka Perkov <luka.perkov@...tura.hr>
Subject: Re: [PATCH bpf-next] bpf: add support for other map types to bpf_map_lookup_and_delete_elem

On Thu, Sep 17, 2020 at 7:16 AM Luka Oreskovic
<luka.oreskovic@...tura.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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