linux-kernel - Re: [PATCH bpf-next 1/2] bpf: fix setting return values for htab batch ops

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Date:   Tue, 18 Jul 2023 17:52:38 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Anton Protopopov <aspsk@...valent.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Brian Vazquez <brianvv@...gle.com>,
        Hou Tao <houtao1@...wei.com>, Joe Stringer <joe@...valent.com>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf: fix setting return values for htab
 batch ops

On Mon, Jul 17, 2023 at 4:42 AM Anton Protopopov <aspsk@...valent.com> wrote:
>
> The map_lookup{,_and_delete}_batch operations are expected to set the
> output parameter, counter, to the number of elements successfully copied
> to the user space. This is also expected to be true if an error is
> returned and the errno is set to a value other than EFAULT. The current
> implementation can return -EINVAL without setting the counter to zero, so
> some userspace programs may confuse this with a [partially] successful
> operation. Move code which sets the counter to zero to the top of the
> function so that we always return a correct value.
>
> Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> Signed-off-by: Anton Protopopov <aspsk@...valent.com>
> ---
>  kernel/bpf/hashtab.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index a8c7e1c5abfa..fa8e3f1e1724 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -1692,6 +1692,13 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>         struct bucket *b;
>         int ret = 0;
>
> +       max_count = attr->batch.count;
> +       if (!max_count)
> +               return 0;
> +
> +       if (put_user(0, &uattr->batch.count))
> +               return -EFAULT;
> +
>         elem_map_flags = attr->batch.elem_flags;
>         if ((elem_map_flags & ~BPF_F_LOCK) ||
>             ((elem_map_flags & BPF_F_LOCK) && !btf_record_has_field(map->record, BPF_SPIN_LOCK)))
> @@ -1701,13 +1708,6 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
>         if (map_flags)
>                 return -EINVAL;
>
> -       max_count = attr->batch.count;
> -       if (!max_count)
> -               return 0;
> -
> -       if (put_user(0, &uattr->batch.count))
> -               return -EFAULT;
> -

I hear your concern, but I don't think it's a good idea
to return 0 when flags were incorrect.
That will cause more suprises to user space.
I think the code is fine as-is.