netdev - Re: [RFC PATCH 3/7] tun: allow use of BPF_PROG_TYPE_SCHED

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Date:   Mon, 18 Jan 2021 11:16:40 +0200
From:   Yuri Benditovich <yuri.benditovich@...nix.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        bpf <bpf@...r.kernel.org>
Cc:     Yan Vugenfirer <yan@...nix.com>, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Yonghong Song <yhs@...com>,
        Willem de Bruijn <willemb@...gle.com>,
        KP Singh <kpsingh@...nel.org>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Marco Elver <elver@...gle.com>, decui@...rosoft.com,
        cai@....pw, Pablo Neira Ayuso <pablo@...filter.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [RFC PATCH 3/7] tun: allow use of BPF_PROG_TYPE_SCHED_CLS program type

Hello Alexei,

Can you please answer the questions in the last email of this thread?
Your comment will be extremely helpful.

Thanks

On Tue, Jan 12, 2021 at 10:55 PM Yuri Benditovich
<yuri.benditovich@...nix.com> wrote:
>
> On Tue, Jan 12, 2021 at 10:40 PM Yuri Benditovich
> <yuri.benditovich@...nix.com> wrote:
> >
> > On Tue, Jan 12, 2021 at 9:42 PM Yuri Benditovich
> > <yuri.benditovich@...nix.com> wrote:
> > >
> > > This program type can set skb hash value. It will be useful
> > > when the tun will support hash reporting feature if virtio-net.
> > >
> > > Signed-off-by: Yuri Benditovich <yuri.benditovich@...nix.com>
> > > ---
> > >  drivers/net/tun.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > > index 7959b5c2d11f..455f7afc1f36 100644
> > > --- a/drivers/net/tun.c
> > > +++ b/drivers/net/tun.c
> > > @@ -2981,6 +2981,8 @@ static int tun_set_ebpf(struct tun_struct *tun, struct tun_prog __rcu **prog_p,
> > >                 prog = NULL;
> > >         } else {
> > >                 prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_SOCKET_FILTER);
> > > +               if (IS_ERR(prog))
> > > +                       prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_SCHED_CLS);
> > >                 if (IS_ERR(prog))
> > >                         return PTR_ERR(prog);
> > >         }
> >
> > Comment from Alexei Starovoitov:
> > Patches 1 and 2 are missing for me, so I couldn't review properly,
> > but this diff looks odd.
> > It allows sched_cls prog type to attach to tun.
> > That means everything that sched_cls progs can do will be done from tun hook?
>
> We do not have an intention to modify the packet in this steering eBPF.
> There is just one function that unavailable for BPF_PROG_TYPE_SOCKET_FILTER
> that the eBPF needs to make possible to deliver the hash to the guest
> VM - it is 'bpf_set_hash'
>
> Does it mean that we need to define a new eBPF type for socket filter
> operations + set_hash?
>
> Our problem is that the eBPF calculates 32-bit hash, 16-bit queue
> index and 8-bit of hash type.
> But it is able to return only 32-bit integer, so in this set of
> patches the eBPF returns
> queue index and hash type and saves the hash in skb->hash using bpf_set_hash().
>
> If this is unacceptable, can you please recommend a better solution?
>
> > sched_cls assumes l2 and can modify the packet.
>
> The steering eBPF in TUN module also assumes l2.
>
> > I think crashes are inevitable.
> >
> > > --
> > > 2.17.1
> > >