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

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Message-ID: <CAEf4BzaMt-b=Td2A+XmjzcYF0-TrwnbosyDk3Tr+3cztpar=dw@mail.gmail.com>
Date:   Mon, 29 Jun 2020 22:11:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        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 9:47 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> 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.

I was initially treating bpf_ringbuf_alloc() as a sort of internal API
that some other code (outside of BPF map) might want to use. But I'll
drop for now, it can always be added later.

>
> >  #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
> >