netdev - Re: [PATCH bpf] bpf: enforce BPF ringbuf size to be the power of 2

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Message-ID: <CAADnVQLkPPxvFV4ZftGeTNWfhtVnGR+Y8NAYZUmuyOcU_M_Y8g@mail.gmail.com>
Date:   Mon, 29 Jun 2020 21:47:10 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf] bpf: enforce BPF ringbuf size to be the power of 2

On Mon, Jun 29, 2020 at 3:19 PM Andrii Nakryiko <andriin@...com> wrote:
>
> BPF ringbuf assumes the size to be a multiple of page size and the power of
> 2 value. The latter is important to avoid division while calculating position
> inside the ring buffer and using (N-1) mask instead. This patch fixes omission
> to enforce power-of-2 size rule.
>
> Fixes: 457f44363a88 ("bpf: Implement BPF ring buffer and verifier support for it")
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
>  kernel/bpf/ringbuf.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c
> index 180414bb0d3e..dcc8e8b9df10 100644
> --- a/kernel/bpf/ringbuf.c
> +++ b/kernel/bpf/ringbuf.c
> @@ -132,7 +132,7 @@ static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node)
>  {
>         struct bpf_ringbuf *rb;
>
> -       if (!data_sz || !PAGE_ALIGNED(data_sz))
> +       if (!is_power_of_2(data_sz) || !PAGE_ALIGNED(data_sz))
>                 return ERR_PTR(-EINVAL);

What's the point checking the same value in two different places?
The check below did that already.

>  #ifdef CONFIG_64BIT
> @@ -166,7 +166,8 @@ static struct bpf_map *ringbuf_map_alloc(union bpf_attr *attr)
>                 return ERR_PTR(-EINVAL);
>
>         if (attr->key_size || attr->value_size ||
> -           attr->max_entries == 0 || !PAGE_ALIGNED(attr->max_entries))
> +           !is_power_of_2(attr->max_entries) ||
> +           !PAGE_ALIGNED(attr->max_entries))
>                 return ERR_PTR(-EINVAL);
>
>         rb_map = kzalloc(sizeof(*rb_map), GFP_USER);
> --
> 2.24.1
>